ALBERT

Description: On teaching styles of water educators and the impact of didactic training Hydrology and Earth System Sciences Discussions, 9, 2959-2986, 2012 Author(s): A. Pathirana, J. H. Koster, E. de Jong, and S. Uhlenbrook Solving today's complex hydrological problems requires originality, creative thinking and trans-disciplinary approaches. Hydrological education that was traditionally teacher centred, where the students look up to the teacher for expertise and information, should change to better prepare hydrologists to develop new knowledge and apply it in new contexts. An important first step towards this goal is to change the concept of education in the educators' minds. The results of an investigation to find out whether didactic training influences the beliefs of hydrology educators about their teaching styles is presented. Faculty of UNESCO-IHE has been offered a didactic certification program named University Teaching Qualification (UTQ). The hypothesis that UTQ training will significantly alter the teaching style of faculty at UNESCO-IHE from expert/formal authority traits towards facilitator/delegator traits was tested. A first survey was conducted among the entire teaching staff (total 101, response rate 58%). The results indicated that there are significantly higher traits of facilitator and delegator teaching styles among UTQ graduates compared to faculty who were not significantly trained in didactics. The second survey which was conducted among UTQ graduates (total 20, response rate 70%), enquiring after their teaching styles before and after UTQ, corroborated these findings.

Description: The Nexus Land-Use model version 1.0, an approach articulating biophysical potentials and economic dynamics to model competition for land-use Geoscientific Model Development Discussions, 5, 571-638, 2012 Author(s): F. Souty, T. Brunelle, P. Dumas, B. Dorin, P. Ciais, R. Crassous, C. Müller, and A. Bondeau Interactions between food demand, biomass energy and forest preservation are driving both food prices and land-use changes, regionally and globally. This study presents a new model called Nexus Land-Use version 1.0 which describes these interactions through a generic representation of agricultural intensification mechanisms. The Nexus Land-Use model equations combine biophysics and economics into a single coherent framework to calculate crop yields, food prices, and resulting pasture and cropland areas within 12 regions inter-connected with each other by international trade. The representation of cropland and livestock production systems in each region relies on three components: (i) a biomass production function derived from the crop yield response function to inputs such as industrial fertilisers; (ii) a detailed representation of the livestock production system subdivided into an intensive and an extensive component, and (iii) a spatially explicit distribution of potential (maximal) crop yields prescribed from the Lund-Postdam-Jena global vegetation model for managed Land (LPJmL). The economic principles governing decisions about land-use and intensification are adapted from the Ricardian rent theory, assuming cost minimisation for farmers. The land-use modelling approach described in this paper entails several advantages. Firstly, it makes it possible to explore interactions among different types of biomass demand for food and animal feed, in a consistent approach, including indirect effects on land-use change resulting from international trade. Secondly, yield variations induced by the possible expansion of croplands on less suitable marginal lands are modelled by using regional land area distributions of potential yields, and a calculated boundary between intensive and extensive production. The model equations and parameter values are first described in details. Then, idealised scenarios exploring the impact of forest preservation policies or rising energy price on agricultural intensification are described, and their impacts on pasture and cropland areas are investigated.

Description: Modelling sub-grid wetland in the ORCHIDEE global land surface model: evaluation against river discharges and remotely sensed data Geoscientific Model Development Discussions, 5, 683-735, 2012 Author(s): B. Ringeval, B. Decharme, S. L. Piao, P. Ciais, F. Papa, N. de Noblet-Ducoudré, C. Prigent, P. Friedlingstein, I. Gouttevin, C. Koven, and A. Ducharne The quality of the global hydrological simulations performed by Land Surface Models (LSMs) strongly depends on processes that occur at unresolved spatial scales. Approaches such as TOPMODEL have been developed, which allow soil moisture redistribution within each grid-cell, based upon sub-grid scale topography. Moreover, the coupling between TOPMODEL and a LSM appears as a potential way to simulate wetland extent dynamic and its sensitivity to climate, a recently identified research problem for biogeochemical modelling, including methane emissions. Global evaluation of the coupling between TOPMODEL and an LSM is difficult, and prior attempts have been indirect, based on the evaluation of the simulated river flow. This study presents a new way to evaluate this coupling, within the ORCHIDEE LSM, using remote sensing data of inundated areas. Because of differences in nature between the satellite derived information – inundation extent – and the variable diagnosed by TOPMODEL/ORCHIDEE – area at maximum soil water content –, the evaluation focuses on the spatial distribution of these two quantities as well as on their temporal variation. Despite some difficulties in exactly matching observed localized inundated events, we obtain a rather good agreement in the distribution of these two quantities at a global scale. Floodplains are not accounted for in the model, and this is a major limitation. The difficulty of reproducing the year-to-year variability of the observed inundated area (for instance, the decreasing trend by the end of 90s) is also underlined. Classical indirect evaluation based on comparison between simulated and observed riverflow is also performed and underlines difficulties to simulate riverflow after coupling with TOPMODEL. The relationship between inundation and river flow at the basin scale in the model is analyzed, using both methods (evaluation against remote sensing data and riverflow). Finally, we discuss the potential of the TOPMODEL/LSM coupling to simulate wetland areas. A major limitation of the coupling for this purpose is linked to its ability to simulate a global wetland coverage consistent with the commonly used datasets. However, it seems to be a good opportunity to account for the wetland areas sensitivity to the climate and thus to simulate its temporal variability.

Description: River monitoring from satellite radar altimetry in the Zambezi River Basin Hydrology and Earth System Sciences Discussions, 9, 3203-3235, 2012 Author(s): C. I. Michailovsky, S. McEnnis, P. A. M. Berry, R. Smith, and P. Bauer-Gottwein Satellite radar altimetry can be used to monitor surface water levels from space. While current and past altimetry missions were designed to study oceans, retracking the waveforms returned over land allows data to be retrieved for smaller water bodies or narrow rivers. In this study, retracked Envisat altimetry data was extracted over the Zambezi River Basin using a detailed river mask based on Landsat imagery. This allowed for stage measurements to be obtained for rivers down to 80 m wide with an RMSE relative to in situ levels of 0.32 to 0.72 m at different locations. The altimetric levels were then converted to discharge using three different methods adapted to different data-availability scenarios: first with an in situ rating curve available, secondly with one simultaneous field measurement of cross-section and discharge, and finally with only historical discharge data available. For the two locations at which all three methods could be applied the accuracies of the different methods were found to be comparable, with RMSE values ranging from 5.5 to 7.4 % terms of high flow estimation relative to in situ gauge measurements. The precision obtained with the different methods was analyzed by running Monte Carlo simulations and also showed comparable values for the three approaches with standard deviations found between 8.2 and 25.8 % of the high flow estimates.

Description: Water management simulation games and the construction of knowledge Hydrology and Earth System Sciences Discussions, 9, 3063-3085, 2012 Author(s): M. Rusca, J. Heun, and K. Schwartz In recent years simulations have become an important part of teaching activities. The reasons behind the popularity of simulation games are twofold. On the one hand, emerging theories on how people learn have called for an experienced-based learning approach. On the other hand, the demand for water management professionals has changed. Three important developments are having considerable consequences for water management programmes, which educate and train these professionals. These developments are the increasing emphasis on integration in water management, the characteristics and speed of reforms in the public sector and the shifting state-society relations in many countries. In response to these developments, demand from the labour market is oriented toward water professionals who need to have both a specialist in-depth knowledge in their own field, as well as the ability to understand and interact with other disciplines and interests. In this context, skills in negotiating, consensus building and working in teams are considered essential for all professionals. In this paper we argue that simulation games have an important role to play in (actively) educating students and training the new generation of water professionals to respond to the above-mentioned challenges. At the same time, simulations are not a panacea for learners and teachers. Challenges of using simulations games include the demands it places on the teacher. Setting up the simulation game, facilitating the delivery and ensuring that learning objectives are achieved requires considerable knowledge and experience as well as considerable time-inputs of the teacher. Moreover, simulation games usually incorporate a case-based learning model, which may neglect or underemphasize theories and conceptualization. For simulations to be effective they have to be embedded in this larger theoretical and conceptual framework. Simulations, therefore, complement rather than substitute traditional teaching methods.

Description: Evaluation of a complementary based model for mapping land surface evapotranspiration Hydrology and Earth System Sciences Discussions, 9, 3029-3062, 2012 Author(s): Z. Sun, Q. Wang, Z. Ouyang, and Y. Yang A modified Priestley-Taylor (P-T) equation was proposed by Venturini et al. (2008) to map actual evapotranspiration (ET) based solely on satellite remote sensing data, involving a parameter based on a scaled temperature between dew point temperature and surface temperature. In this study, however, theoretical analyses and field experimental evidence show that it is hard to obtain this scaled temperature using dew point temperature and surface temperature. This study also presents a new parameterization method using air temperature, surface temperature, and surface temperature of a reference dry surface. The actual ET estimates obtained by means of our proposed parameterization method are validated at a site scale, and a case study is conducted to map actual ET from Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) images using our proposed method. Results of ground-based validation and a case study of mapping ET using ASTER images indicate that the improvement on the modified P-T equation proposed by Venturini et al. (2008) can contribute to generating reliable actual ET.

Description: Precipitation fields interpolated from gauge stations versus a merged radar-gauge precipitation product: influence on modelled soil moisture at local scale and at SMOS scale Hydrology and Earth System Sciences Discussions, 9, 3385-3413, 2012 Author(s): J. T. dall'Amico, W. Mauser, F. Schlenz, and H. Bach For the validation of coarse resolution soil moisture products from missions such as the Soil Moisture and Ocean Salinity (SMOS) mission, hydrological modelling of soil moisture is an important tool. The spatial distribution of precipitation is among the most crucial input data for such models. Thus, reliable time series of precipitation fields are required, but these often need to be interpolated from data delivered by scarcely distributed gauge station networks. In this study, a commercial precipitation product derived by Meteomedia AG from merging radar and gauge data is introduced as a novel means of adding the promising area-distributed information given by a radar network to the more accurate, but point-like measurements from a gauge station network. This precipitation product is first validated against an independent gauge station network. Further, the novel precipitation product is assimilated into the hydrological land surface model PROMET for the Upper Danube Catchment in southern Germany, one of the major SMOS calibration and validation sites in Europe. The modelled soil moisture fields are compared to those obtained when the operational interpolation from gauge station data is used to force the model. The results suggest that the assimilation of the novel precipitation product can lead to deviations of modelled soil moisture in the order of 0.15 m 3 m −3 on small spatial (∼1 km 2 ) and short temporal resolutions (∼1 day). As expected, after spatial aggregation to the coarser grid on which SMOS data are delivered (~195 km 2 ), these differences are reduced to the order of 0.04 m 3 m −3 , which is the accuracy benchmark for SMOS. The results of both model runs are compared to brightness temperatures measured by the airborne L-band radiometer EMIRAD during the SMOS Validation Campaign 2010. Both comparisons yield equally good correlations, confirming the model's ability to realistically model soil moisture fields in the test site. The fact that the two model runs perform similarly in the comparison is likely associated with the lack of substantial rain events before the days on which EMIRAD was flown.

Description: Advancing data assimilation in operational hydrologic forecasting: progresses, challenges, and emerging opportunities Hydrology and Earth System Sciences Discussions, 9, 3415-3472, 2012 Author(s): Y. Liu, A. H. Weerts, M. Clark, H.-J. Hendricks Franssen, S. Kumar, H. Moradkhani, D.-J. Seo, D. Schwanenberg, P. Smith, A. I. J. M. van Dijk, N. van Velzen, M. He, H. Lee, S. J. Noh, O. Rakovec, and P. Restrepo Data assimilation (DA) holds considerable potential for improving hydrologic predictions as demonstrated in numerous research studies. However, advances in hydrologic DA research have not been adequately or timely implemented into operational forecast systems to improve the skill of forecasts to better inform real-world decision making. This is due in part to a lack of mechanisms to properly quantify the uncertainty in observations and forecast models in real-time forecasting situations and to conduct the merging of data and models in a way that is adequately efficient and transparent to operational forecasters. The need for effective DA of useful hydrologic data into the forecast process has become increasingly recognized in recent years. This motivated a hydrologic DA workshop in Delft, The Netherlands in November 2010, which focused on advancing DA in operational hydrologic forecasting and water resources management. As an outcome of the workshop, this paper reviews, in relevant detail, the current status of DA applications in both hydrologic research and operational practices, and discusses the existing or potential hurdles and challenges in transitioning hydrologic DA research into cost-effective operational forecasting tools, as well as the potential pathways and newly emerging opportunities for overcoming these challenges. Several related aspects are discussed, including (1) theoretical or mathematical considerations in DA algorithms, (2) the estimation of different types of uncertainty, (3) new observations and their objective use in hydrologic DA, (4) the use of DA for real-time control of water resources systems, and (5) the development of community-based, generic DA tools for hydrologic applications. It is recommended that cost-effective transition of hydrologic DA from research to operations should be helped by developing community-based, generic modelling and DA tools or frameworks, and through fostering collaborative efforts among hydrologic modellers, DA developers, and operational forecasters.

Description: Impact of climate change on sediment yield in the Mekong River Basin: a case study of the Nam Ou Basin, Lao PDR Hydrology and Earth System Sciences Discussions, 9, 3339-3384, 2012 Author(s): B. Shrestha, M. S. Babel, S. Maskey, A. van Griensven, S. Uhlenbrook, A. Green, and I. Akkharath This paper evaluates the impact of climate change on sediment yield in the Nam Ou Basin located in Northern Laos. The Soil and Water Assessment Tool (SWAT) is used to assess future changes in sediment flux attributable to climate change. Future precipitation and temperature series are constructed through a delta change approach. As per the results, in general, temperature as well as precipitation show increasing trends in both scenarios, A2 and B2. However, monthly precipitation shows both increasing and decreasing trends. The simulation results exhibit that the wet and dry seasonal and annual stream discharges are likely to increase (by up to 15, 17 and 14% under scenario A2; and 11, 5 and 10% under scenario B2 respectively) in the future, which will lead to increased wet and dry seasonal and annual sediment yields (by up to 39, 28 and 36% under scenario A2; and 23, 12 and 22% under scenario B2 respectively). A higher discharge and more sediment flux are expected during the wet seasons, although the changes, percentage-wise, are observed to be higher during the dry months. In conclusion, the sediment yield from the Nam Ou Basin is likely to increase with climate change, which strongly suggests the need for basin-wide sediment management strategies in order to reduce the negative impact of this change.

Description: Correcting the radar rainfall forcing of a hydrological model with data assimilation: application to flood forecasting in the Lez Catchment in Southern France Hydrology and Earth System Sciences Discussions, 9, 3527-3579, 2012 Author(s): E. Harader, V. Borrell Estupina, S. Ricci, M. Coustau, O. Thual, A. Piacentini, and C. Bouvier The present study explores the application of a data assimilation (DA) procedure to correct the radar rainfall inputs of an event-based, distributed, parsimonious hydrological model. A simplified Kalman filter algorithm was built on top of a rainfall-runoff model in order to assimilate discharge observations at the catchment outlet. The study site is the 114 km 2 Lez Catchment near Montpellier, France. This catchment is subject to heavy orographic rainfall and characterized by a karstic geology, leading to flash flooding events. The hydrological model uses a derived version of the SCS method, combined with a Lag and Route transfer function. Because it depends on geographical features and cloud structures, the radar rainfall input to the model is particularily uncertain and results in significant errors in the simulated discharges. The DA analysis was applied to estimate a constant correction to each event hyetogram. The analysis was carried out for 19 events, in two different modes: re-analysis and pseudo-forecast. In both cases, it was shown that the reduction of the uncertainty in the rainfall data leads to a reduction of the error in the simulated discharge. The resulting correction of the radar rainfall data was then compared to the mean field bias (MFB), a corrective coefficient determined using ground rainfall measurements, which are more accurate than radar but have a decreased spatial resolution. It was shown that the radar rainfall corrected using DA leads to improved discharge simulations and Nash criteria compared to the MFB correction.

Description: Numerical modelling of climate change impacts on freshwater lenses on the North Sea Island of Borkum Hydrology and Earth System Sciences Discussions, 9, 3473-3525, 2012 Author(s): H. Sulzbacher, H. Wiederhold, B. Siemon, M. Grinat, J. Igel, T. Burschil, T. Günther, and K. Hinsby A numerical variable-density groundwater model is set up for the North Sea Island of Borkum to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea. The database includes information from boreholes, a seismic survey, a helicopter-borne electromagnetic survey (HEM), monitoring of the freshwater-saltwater boundary by vertical electrode chains in two boreholes, measurements of groundwater table, pumping and slug tests, as well as water samples. Based on a statistical analysis of borehole columns, seismic sections and HEM, a hydrogeological model is set up. The groundwater model is developed using the finite-element programme FEFLOW. The variable-density groundwater model is calibrated on the basis of hydraulic, hydrological and geophysical data, in particular spatial HEM and local monitoring data. Verification runs with the calibrated model show good agreement between measured and computed hydraulic heads. A good agreement is also obtained between measured and computed density or total dissolved solids data for both the entire freshwater lens on a large scale and in the area of the well fields on a small scale. For simulating future changes in this coastal groundwater system until the end of the current century we use the climate scenario A2, specified by the Intergovernmental Panel on Climate Change and in particular the data for the German North Sea coast. Simulation runs show proceeding salinization with time beneath the well fields of the two waterworks Waterdelle and Ostland. The modelling study shows that spreading of well fields is an appropriate protection measure against excessive salinization of the water supply until the end of the current century.

Description: Moving beyond traditional model calibration or how to better identify realistic model parameters: sub-period calibration Hydrology and Earth System Sciences Discussions, 9, 1885-1918, 2012 Author(s): S. Gharari, M. Hrachowitz, F. Fenicia, and H. H. G. Savenije Conceptual hydrological models often rely on calibration for the identification of their parameters. As these models are typically designed to reflect real catchment processes, a key objective of an appropriate calibration strategy is the determination of parameter sets that reflect a "realistic" model behavior. Previous studies have shown that parameter estimates for different calibration periods can be significantly different. This questions model transposability in time, which is one of the key conditions for the set-up of a "realistic" model. This paper presents a new approach that selects parameter sets that provide a consistent model performance in time. The approach consists of confronting model performance in different periods, and selecting parameter sets that are as close as possible to the optimum of each individual sub-period. While aiding model calibration, the approach is also useful as a diagnostic tool, illustrating tradeoffs in the identification of time consistent parameter sets. The approach is demonstrated in a case study where we illustrate the multi-objective calibration of the HyMod hydrological model to a Luxembourgish catchment.

Description: Impacts of inhomogeneous landscapes in oasis interior on the oasis self-maintaining mechanism by integrating numerical model with satellite data Hydrology and Earth System Sciences Discussions, 9, 1979-2004, 2012 Author(s): X. Meng, S. Lu, T. Zhang, Y. Ao, S. Li, Y. Bao, L. Wen, and S. Luo Mesoscale meteorological modeling is an important tool to help understand the energy budget of the oasis. While basic dynamic and thermodynamic processes for oasis self-maintaining in the desert environment is well investigated, influence of heterogeneous landscapes of oasis interior on the processes are still important and remain to be investigated. In this study, two simulations are designed for investigating the influence of inhomogeneity. In the first case, land surface parameters including land-use types, vegetation cover fraction, and surface layer soil moisture are derived by satellite remote sensing data from EOS/MODIS, and then be used specify the respective options in the MM5 model, to describe a real inhomogeneity for the oasis interior. In the other run, land use types are set to MM5 default, in which landscapes in the oasis interior is relative uniform, and then surface layer soil moisture and vegetation fraction is set to be averages of the first case for the respective oasis and desert surface lying, to represent a relative homogeneity. Results show that the inhomogeneity leads to a weaker oasis "cold-wet island" effect and a stronger turbulence over the oasis interior, both of which will reduce the oasis-desert secondary circulation and increase the evaporation over the oasis, resulting in a negative impact on the oasis self-protecting mechanism. The simulation of homogeneity indicates that the oasis may be more stable even with relative lower soil moisture if landscapes in the oasis interior are comparatively uniform.

Description: Calibration and evaluation of a semi-distributed watershed model of sub-Saharan Africa using GRACE data Hydrology and Earth System Sciences Discussions, 9, 2071-2120, 2012 Author(s): H. Xie, L. Longuevergne, C. Ringler, and B. Scanlon Irrigation development is rapidly expanding in mostly rainfed Sub-Saharan Africa. This expansion underscores the need for a more comprehensive understanding of water resources beyond surface water. Gravity Recovery and Climate Experiment (GRACE) satellites provide valuable information on spatio-temporal variability of water storage. The objective of this study was to calibrate and evaluate a semi-distributed regional-scale hydrological model, or a large-scale application of the Soil and Water Assessment Tool (SWAT) model, for basins in Sub-Saharan Africa using seven-year (2002–2009) 10-day GRACE data. Multi-site river discharge data were used as well, and the analysis was conducted in a multi-criteria framework. In spite of the uncertainty arising from the tradeoff in optimizing model parameters with respect to two non-commensurable criteria defined for two fluxes, it is concluded that SWAT can perform well in simulating total water storage variability in most areas of Sub-Saharan Africa, which have semi-arid and sub-humid climates, and that among various water storages represented in SWAT, the water storage variations from soil, the vadose zone, and groundwater are dominant. On the other hand, the study also showed that the simulated total water storage variations tend to have less agreement with the GRACE data in arid and equatorial humid regions, and the model-based partition of total water storage variations into different water storage compartments could be highly uncertain. Thus, future work will be needed for model enhancement in these areas with inferior model fit and for uncertainty reduction in component-wise estimation of water storage variations.

Description: Evaluation of a near-global eddy-resolving ocean model Geoscientific Model Development Discussions, 5, 4305-4354, 2012 Author(s): P. R. Oke, D. A. Griffin, A. Schiller, R. J. Matear, R. Fiedler, J. Mansbridge, A. Lenton, M. Cahill, M. A. Chamberlain, and K. Ridgway Analysis of the variability in an 18-yr run of a near-global, eddy-resolving ocean general circulation model coupled with biogeochemistry is presented. Comparisons between modelled and observed mean sea level (MSL), mixed-layer depth (MLD), sea-level anomaly (SLA), sea-surface temperature (SST), and Chlorophyll a indicate that the model variability is realistic. We find some systematic errors in the modelled MLD, with the model generally deeper than observations, that results in errors in the Chlorophyll a , owing to the strong biophysical coupling. We evaluate several other metrics in the model, including the zonally-averaged seasonal cycle of SST, meridional overturning, volume transports through key Straits and passages, zonal averaged temperature and salinity, and El Nino-related SST indices. We find that the modelled seasonal cycle in SST is 0.5–1.5 °C weaker than observed; volume transports of the Antarctic Circumpolar Current, the East Australian Current, and Indonesian Throughflow are in good agreement with observational estimates; and the correlation between the modelled and observed NINO SST indices exceed 0.91. Most aspects of the model circulation are realistic. We conclude that the model output is suitable for broader analysis to better understand ocean dynamics and ocean variability.

Description: Elusive drought: uncertainty in observed trends and short- and long-term CMIP5 projections Hydrology and Earth System Sciences Discussions, 9, 13773-13803, 2012 Author(s): B. Orlowsky and S. I. Seneviratne Recent years have seen a number of severe droughts in different regions around the world, causing agricultural and economic losses, famines and migration. Despite their devastating consequences, the Standardised Precipitation Index (SPI) of these events lies within the range of internal climate variability, which we estimate from simulations from the 5th phase of the Coupled Model Intercomparison Project (CMIP5). In terms of drought magnitude, regional trends of SPI over the last decades remain mostly inconclusive in observations and CMIP5 simulations, although Soil Moisture Anomalies (SMAs) in CMIP5 simulations hint at increased drought in a few regions (e.g. the Mediterranean, Central America/Mexico, the Amazon, North-East Brazil and South Africa). Also for the future, projections of meteorological (SPI) and agricultural (SMA) drought in CMIP5 display large uncertainties over all time frames, generally impeding trend detection. Analogue analyses of the frequencies rather than magnitudes of future drought display, however, more robust signal-to-noise ratios with detectable trends towards more frequent drought until the end of the 21st century in the Mediterranean, South Africa and Central America/Mexico. Other present-day hot spots are projected to become less drought-prone, or to display unsignificant changes in drought occurrence. A separation of different sources of uncertainty in drought projections reveals that for the near term, internal climate variability is the dominant source, while the formulation of Global Climate Models (GCMs) generally becomes the dominant source of uncertainty by the end of the 21st century, especially for agricultural (soil moisture) drought. In comparison, the uncertainty in Green-House Gas (GHG) concentrations scenarios is negligible for most regions. These findings stand in contrast to respective analyses for a heat wave indicator, for which GHG concentrations scenarios constitute the main source of uncertainty. Our results highlight the inherent difficulty of drought quantification and the uncertainty of drought projections. However, high uncertainty should not be equated with low drought risk, since potential scenarios include large drought increases in key agricultural and ecosystem regions.

Description: Simulation of hydrological processes in the Zhalong Wetland within a river basin, Northeast China Hydrology and Earth System Sciences Discussions, 9, 14035-14063, 2012 Author(s): X. Q. Feng, G. X. Zhang, and Y. Jun Xu Zhalong National Nature Preserve is a large wetland reserve on the Songnen Plain in Northeast China. Wetlands in the preserve play a key role in maintaining regional ecosystem function and integrity. Global climate change and intensified anthropogenic activities in the region have raised great concerns over the change of natural flow regime, wetland degradation and losses. In this study, two key hydrologic components in the preserve, open water area and storage, as well as their variations during the period 1985–2006 were investigated with a spatially-distributed hydrologic modeling system, SWAT. A wetland module was incorporated into the SWAT model to represent hydrological linkages between the wetland and adjacent upland areas. The modified modeling system was calibrated with streamflow measurements from 1987 to 1989, in a Nash efficiency coefficient ( E ns ) of 0.86, and was validated for the period 2005–2006, in an E ns of 0.66. In the past 20 yr, open water area in the Zhalong Wetland fluctuated from approximately 200 km 2 to 1145 km 2 with a rapid decreasing trend through the early 2000s. Consequently, open water storage in the preserve decreased largely, especially in the dry seasons. The situation changed following the implementation of a river diversion in 2001. Overall, the modeling yielded plausible estimates of hydrologic changes in this large wetland reserve, building a foundation for assessing ecological water requirements and developing strategies and plans for water resources management within the river basin.

Description: On the use of spring baseflow recession for a more accurate parameterization of aquifer transit time distribution functions Hydrology and Earth System Sciences Discussions, 9, 14109-14128, 2012 Author(s): J. Farlin and P. Maloszewski Baseflow recession analysis and groundwater dating have up to now developed as two distinct branches of hydrogeology and were used to solve entirely different problems. We show that by combining two classical models, namely Boussinesq's Equation describing spring baseflow recession and the exponential piston-flow model used in groundwater dating studies, the parameters describing the transit time distribution of an aquifer can be in some cases estimated to a far more accurate degree than with the latter alone. Under the assumption that the aquifer basis is sub-horizontal, the mean residence time of water in the saturated zone can be estimated from spring baseflow recession. This provides an independent estimate of groundwater residence time that can refine those obtained from tritium measurements. This approach is demonstrated in a case study predicting atrazine concentration trend in a series of springs draining the fractured-rock aquifer known as the Luxembourg Sandstone. A transport model calibrated on tritium measurements alone predicted different times to trend reversal following the nationwide ban on atrazine in 2005 with different rates of decrease. For some of the springs, the best agreement between observed and predicted time of trend reversal was reached for the model calibrated using both tritium measurements and the recession of spring discharge during the dry season. The agreement between predicted and observed values was however poorer for the springs displaying the most gentle recessions, possibly indicating the stronger influence of continuous groundwater recharge during the dry period.

Description: Development of a method of robust rain gauge network optimization based on intensity-duration-frequency results Hydrology and Earth System Sciences Discussions, 9, 14205-14230, 2012 Author(s): A. Chebbi, Z. K. Bargaoui, and M. da Conceição Cunha Based on rainfall intensity-duration-frequency (IDF) curves, a robust optimization approach is proposed to identify the best locations to install new rain gauges. The advantage of robust optimization is that the resulting design solutions yield networks which behave acceptably under hydrological variability. Robust optimisation can overcome the problem of selecting representative rainfall events when building the optimization process. This paper reports an original approach based on Montana IDF model parameters. The latter are assumed to be geostatistical variables and their spatial interdependence is taken into account through the adoption of cross-variograms in the kriging process. The problem of optimally locating a fixed number of new monitoring stations based on an existing rain gauge network is addressed. The objective function is based on the mean spatial kriging variance and rainfall variogram structure using a variance-reduction method. Hydrological variability was taken into account by considering and implementing several return periods to define the robust objective function. Variance minimization is performed using a simulated annealing algorithm. In addition, knowledge of the time horizon is needed for the computation of the robust objective function. A short and a long term horizon were studied, and optimal networks are identified for each. The method developed is applied to north Tunisia (area = 21 000 km 2 ). Data inputs for the variogram analysis were IDF curves provided by the hydrological bureau and available for 14 tipping bucket type rain gauges. The recording period was from 1962 to 2001, depending on the station. The study concerns an imaginary network augmentation based on the network configuration in 1973, which is a very significant year in Tunisia because there was an exceptional regional flood event in March 1973. This network consisted of 13 stations and did not meet World Meteorological Organization (WMO) recommendations for the minimum spatial density. So, it is proposed to virtually augment it by 25, 50, 100 and 160% which is the rate that would meet WMO requirements. Results suggest that for a given augmentation robust networks remain stable overall for the two time horizons.

Description: Evaluating a lightning parameterization based on cloud-top height for mesoscale numerical model simulations Geoscientific Model Development Discussions, 5, 3493-3531, 2012 Author(s): J. Wong, M. C. Barth, and D. Noone The Price and Rind lightning parameterization based on cloud-top height is a commonly used method for predicting flash rate in global chemistry models. As mesoscale simulations begin to implement flash rate predictions at resolutions that partially resolve convection, it is necessary to validate and understand the behavior of this method within such regime. In this study, we tested the flash rate parameterization, intra-cloud/cloud-to-ground (IC:CG) partitioning parameterization, and the associated resolution dependency "calibration factor" by Price and Rind using the Weather Research and Forecasting (WRF) model running at 36 km, 12 km, and 4 km grid spacings within the continental United States. Our results show that while the integrated flash count is consistent with observation when model biases in convection are taken into account, an erroneous frequency distribution is simulated. When the spectral characteristics of lightning flash rate is a concern, we recommend the use of prescribed IC:CG values. In addition, using cloud-top from convective parameterization, the "calibration factor" is also shown to be insufficient in reconciling the resolution dependency at the tested grid spacing used in this study. We recommend scaling by areal ratio relative to a base-case grid spacing determined by convective core density.

Description: Using model reduction to predict the soil-surface C 18 OO flux: an example of representing complex biogeochemical dynamics in a computationally efficient manner Geoscientific Model Development Discussions, 5, 3469-3491, 2012 Author(s): W. J. Riley Earth System Models (ESMs) must calculate large-scale interactions between the land and atmosphere while accurately characterizing fine-scale spatial heterogeneity in water, carbon, and nutrient dynamics. We present here a high-dimensional model representation (HDMR) approach that allows detailed process representation of a coupled carbon and water tracer (the δ 18 O value of the soil-surface CO 2 flux (δ F s )) in a computationally tractable manner. δ F s depends on the δ 18 O value of soil water, soil moisture, soil temperature, and soil CO 2 production (all of which are depth-dependent), and the δ 18 O value of above-surface CO 2 . We tested the HDMR approach over a growing season in a C 4 -dominated pasture using two vertical soil discretizations. The difference between the HDMR approach and the full model solution in the three-month integrated isoflux was less than 0.2% (0.5 mol m −2 ‰), and the approach is up to 100 times faster than the full numerical solution. This type of model reduction approach allows representation of complex coupled biogeochemical processes in regional and global climate models and can be extended to characterize subgrid-scale spatial heterogeneity.

Description: Technical Note: An open source library for processing weather radar data ( wradlib ) Hydrology and Earth System Sciences Discussions, 9, 12333-12356, 2012 Author(s): M. Heistermann, S. Jacobi, and T. Pfaff The potential of weather radar observations for hydrological and meteorological research and applications is undisputed, particularly with increasing world-wide radar coverage. However, several barriers impede the use of weather radar data. These barriers are of both scientific and technical nature. The former refers to inherent measurement errors and artefacts, the latter to aspects such as reading specific data formats, geo-referencing, visualisation. The radar processing library wradlib is intended to lower these barriers by providing a free and open source tool for the most important steps in processing weather radar data for hydro-meteorological and hydrological applications. Moreover, the community-based development approach of wradlib allows scientists to share their knowledge about efficient processing algorithms and to make this knowledge available to the weather radar community in a transparent, structured and well-documented way.

Description: GloFAS – global ensemble streamflow forecasting and flood early warning Hydrology and Earth System Sciences Discussions, 9, 12293-12332, 2012 Author(s): L. Alfieri, P. Burek, E. Dutra, B. Krzeminski, D. Muraro, J. Thielen, and F. Pappenberger Anticipation and preparedness for large-scale flood events have a key role in mitigating their impact and optimizing the strategic planning of water resources. Although several developed countries have well-established systems for river monitoring and flood early warning, figures of population affected every year by floods in developing countries are unsettling. This paper presents the Global Flood Awareness System, which has been set up to provide an overview on upcoming floods in large world river basins. The Global Flood Awareness System is based on distributed hydrological simulation of numerical ensemble weather predictions with global coverage. Streamflow forecasts are compared statistically to climatological simulations to detect probabilistic exceedance of warning thresholds. In this article, the system setup is described, together with an evaluation of its performance over a two-year test period and a qualitative analysis of a case study for the Pakistan flood, in summer 2010. It is shown that hazardous events in large river basins can be skilfully detected with a forecast horizon of up to 1 month. In addition, results suggest that an accurate simulation of initial model conditions and an improved parameterization of the hydrological model are key components to reproduce accurately the streamflow variability in the many different runoff regimes of the Earth.

Description: ECOCLIMAP-II/Europe: a twofold database of ecosystems and surface parameters at 1-km resolution based on satellite information for use in land surface, meteorological and climate models Geoscientific Model Development Discussions, 5, 3573-3620, 2012 Author(s): S. Faroux, A. T. Kaptué Tchuenté, J.-L. Roujean, V. Masson, E. Martin, and P. Le Moigne The overall objective of the present study is to introduce the new ECOCLIMAP-II database for Europe, which is an upgrade for this region of the former initiative, ECOCLIMAP-I, already implemented at global scale. The ECOCLIMAP programme is a dual database at 1-km resolution that includes an ecosystem classification and a coherent set of land surface parameters that are primarily mandatory in meteorological modelling (notably leaf area index and albedo). Hence, the aim of this innovative physiography is to enhance the quality of initialisation and impose some surface attributes within the scope of weather forecasting and climate related studies. The strategy for implementing ECOCLIMAP-II is to depart from prevalent land cover products such as CLC2000 (Corine Land Cover) and GLC2000 (Global Land Cover) by splitting existing classes into new classes that possess a better regional character by virtue of the climatic environment (latitude, proximity to the sea, topography). The leaf area index (LAI) from MODIS and NDVI from SPOT/Vegetation yield the two proxy variables that were considered here in order to perform a multi-year trimmed analysis between 1999 and 2005 using the K-means method. Further, meteorological applications require each land cover type to appear as a partition of fractions of 4 main surface types or tiles (nature, water bodies, sea, urban areas) and, inside the nature tile, fractions of 12 Plant Functional Types (PFTs) representing generic vegetation types – principally broadleaf forest, needleleaf forest, C3 and C4 crops, grassland and bare land – as incorporated by the SVAT model ISBA developed at Météo France. This landscape division also forms the cornerstone of a validation exercise. The new ECOCLIMAP-II can be verified with auxiliary land cover products at very fine and coarse resolutions by means of versatile land occupation nomenclatures.

Description: Flood discharge measurement of mountain rivers Hydrology and Earth System Sciences Discussions, 9, 12655-12690, 2012 Author(s): Y.-C. Chen An efficient method that accounts for personal safety, accuracy and reliability for measuring flood discharge of mountain rivers is proposed. It is composed of new measurement method, tools, and techniques. Measuring flood discharge from mountain rivers by using conventional method is costly, time-consuming, and dangerous. Thus previous discharge measurements for mountainous areas were typically based on estimated precipitation, which alone cannot generate accurate measurements. This study applies a novel flood discharge measurement system composed of an Acoustic Doppler Profiler and crane system to accurately and quickly measure velocity distributions and water depths. Moreover a novel and efficient method for measuring discharge, which is based on the relationship between mean and maximum velocities and the relationship between cross-sectional area and gauge height is applied to estimate flood discharge. Flood discharge from mountain rivers can be estimated easily and rapidly by measuring maximum velocity in the river crosssection and the gauge height. The measured flood discharges can be utilized to create a reliable stage-discharge relationship for continuous estimations of discharge using records of water stage. The proposed method was applied to the Nanshih River, Taiwan. Results of measured discharges and estimated discharges only slightly differed from each other, demonstrating the efficiency and accuracy of the proposed method.

Description: New climate change scenarios reveal uncertain future for Central Asian glaciers Hydrology and Earth System Sciences Discussions, 9, 12691-12727, 2012 Author(s): A. F. Lutz, W. W. Immerzeel, A. Gobiet, F. Pellicciotti, and M. F. P. Bierkens Central Asian water resources largely depend on (glacier) melt water generated in the Pamir and Tien Shan mountain ranges, located in the basins of the Amu and Syr Darya rivers, important life lines in Central Asia and the prominent water source of the Aral Sea. To estimate future water availability in the region, it is thus necessary to project the future glacier extent and volume in the Amu and Syr Darya river basins. The aim of this study is to quantify the impact of uncertainty in climate change projections on the future glacier extent in the Amu and Syr Darya river basins. The latest climate change projections provided by the fifth Coupled Model Intercomparison Project (CMIP5) generated for the upcoming fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) are used to model future glacier extent in the Central Asian region for the two large river basins. The outcomes are compared to model results obtained with the climate change projections used for the fourth IPCC assessment (CMIP3). We use a regionalized glacier mass balance model to estimate changes in glacier extent as a function of glacier size and projections of temperature and precipitation. The model is developed for implementation in (large scale) hydrological models, when the spatial model resolution does not allow for modelling of individual glaciers and data scarcity is an issue. Both CMIP3 and CMIP5 model simulations point towards a strong decline in glacier extent in Central Asia. However, compared to the CMIP3 projections, the CMIP5 projections of future glacier extent in Central Asia provide a wider range of outcomes, mostly owing to greater variability in precipitation projections among the latest suite of climate models. These findings have great impact on projections of the timing and quantity of water availability in glacier melt dominated rivers in the region. Uncertainty about the size of the decline in glacier extent remains large, making estimates of future Central Asian glacier extent and downstream water availability uncertain.

Description: Intercomparison of temperature trends in IPCC CMIP5 simulations with observations, reanalyses and CMIP3 models Geoscientific Model Development Discussions, 5, 3621-3645, 2012 Author(s): J. Xu and A. M. Powell Jr. On the basis of the fifth Coupled Model Intercomparison Project (CMIP5) and the climate model simulations covering 1979 through 2005, the temperature trends and their uncertainties have been examined to note the similarities or differences compared to the radiosonde observations, reanalyses and the third Coupled Model Intercomparison Project (CMIP3) simulations. The results show noticeable discrepancies for the estimated temperature trends in the four data groups (Radiosonde, Reanalysis, CMIP3 and CMIP5) although similarities can be observed. Compared to the CMIP3 model simulations, the simulation in some of CMIP5 models were improved. The CMIP5 models displayed a negative temperature trend in the stratosphere closer to the strong negative trend seen in the observations. However, the positive tropospheric trend in the tropics is overestimated by the CMIP5 models relative to CMIP3 models. While some of the models produce temperature trend patterns more highly correlated with the observed patterns in CMIP5, the other models (such as CCSM4 and IPSL_CM5A-LR) exhibit the reverse tendency. The CMIP5 temperature trend uncertainty was significantly reduced in most areas, especially in the Arctic and Antarctic stratosphere, compared to the CMIP3 simulations. Similar to the CMIP3, the CMIP5 simulations overestimated the tropospheric warming in the tropics and Southern Hemisphere and underestimated the stratospheric cooling. The crossover point where tropospheric warming changes into stratospheric cooling occurred near 100 hPa in the tropics, which is higher than in the radiosonde and reanalysis data. The result is likely related to the overestimation of convective activity over the tropical areas in both the CMIP3 and CMIP5 models. Generally, for the temperature trend estimates associated with the numerical models including the reanalyses and global climate models, the uncertainty in the stratosphere is much larger than that in the troposphere, and the uncertainty in the Antarctic is the largest. In addition, note that the reanalyses show the largest uncertainty in the lower tropical stratosphere, and the CMIP3 simulations show the largest uncertainty in both the south and north polar regions.

Description: Basin-wide water accounting using remote sensing data: the case of transboundary Indus Basin Hydrology and Earth System Sciences Discussions, 9, 12921-12958, 2012 Author(s): P. Karimi, W. G. M. Bastiaanssen, D. Molden, and M. J. M. Cheema The paper describes the application of a new Water Accounting Plus (WA+) framework to produce spatial information on water flows, sinks, uses, storages and assets, in the Indus Basin, South Asia. It demonstrates how satellite-derived estimates of land use, land cover, rainfall, evaporation ( E ), transpiration ( T ), interception ( I ) and biomass production can be used in the context of WA+. The results for one selected year showed that total annual water depletion in the basin (502 km 3 ) plus outflows (21 km 3 ) exceeded total precipitation (482 km 3 ). The deficit in supply was augmented through abstractions beyond actual capacity, mainly from groundwater storage (30 km 3 ). The "landscape ET" (depletion directly from rainfall) was 344 km 3 (69% of total consumption). "Blue water" depletion ("utilized flow") was 158 km 3 (31%). Agriculture was the biggest water consumer and accounted for 59% of the total depletion (297 km 3 ), of which 85% (254 km 3 ) was through irrigated agriculture and the remaining 15% (44 km 3 ) through rainfed systems. While the estimated basin irrigation efficiency was 0.84, due to excessive evaporative losses in agricultural areas, half of all water consumption in the basin was non-beneficial. Average rainfed crop yields were 0.9 t ha −1 and 7.8 t ha −1 for two irrigated crop growing seasons combined. Water productivity was low due to a lack of proper agronomical practices and poor farm water management. The paper concludes that the opportunity for a food-secured and sustainable future for the Indus Basin lies in focusing on reducing soil evaporation. Results of future scenario analyses suggest that by implementing techniques to convert soil evaporation to crop transpiration will not only increase production but can also result in significant water savings that would ease the pressure on the fast declining storage.

Description: Water Accounting Plus (WA+) – a water accounting procedure for complex river basins based on satellite measurements Hydrology and Earth System Sciences Discussions, 9, 12879-12919, 2012 Author(s): P. Karimi, W. G. M. Bastiaanssen, and D. Molden Coping with the issue of water scarcity and growing competition for water among different sectors requires proper water management strategies and decision processes. A pre-requisite is a clear understanding of the basin hydrological processes, manageable and unmanageable water flows, the interaction with land use and opportunities to mitigate the negative effects and increase the benefits of water depletion on society. Currently, water professionals do not have a common framework that links hydrological flows to user groups of water and their benefits. The absence of a standard hydrological and water management summary is causing confusion and wrong decisions. The non-availability of water flow data is one of the underpinning reasons for not having operational water accounting systems for river basins in place. In this paper we introduce Water Accounting Plus (WA+), which is a new framework designed to provide explicit spatial information on water depletion and net withdrawal processes in complex river basins. The influence of land use on the water cycle is described explicitly by defining land use groups with common characteristics. Analogous to financial accounting, WA+ presents four sheets including (i) a resource base sheet , (ii) a consumption sheet , (iii) a productivity sheet , and (iv) a withdrawal sheet . Every sheet encompasses a set of indicators that summarize the overall water resources situation. The impact of external (e.g. climate change) and internal influences (e.g. infrastructure building) can be estimated by studying the changes in these WA+ indicators. Satellite measurements can be used for 3 out of the 4 sheets, but is not a precondition for implementing WA+ framework. Data from hydrological models and water allocation models can also be used as inputs to WA+.

Description: Multi-variable evaluation of hydrological model predictions for a headwater basin in the Canadian Rocky Mountains Hydrology and Earth System Sciences Discussions, 9, 12825-12877, 2012 Author(s): X. Fang, J. W. Pomeroy, C. R. Ellis, M. K. MacDonald, C. M. DeBeer, and T. Brown One of the purposes of the Cold Regions Hydrological Modelling platform (CRHM) is to diagnose inadequacies in the understanding of the hydrological cycle and its simulation. A physically based hydrological model including a full suite of snow and cold regions hydrology processes as well as warm season, hillslope and groundwater hydrology was developed in CRHM for application in the Marmot Creek Research Basin (~ 9.4km 2 ), located in the Front Ranges of Canadian Rocky Mountains. Parameters were selected from digital elevation model, forest, soil and geological maps, and from the results of many cold regions hydrology studies in the region and elsewhere. Non-calibrated simulations were conducted for six hydrological years during 2005–2011 and were compared with detailed field observations of several hydrological cycle components. Results showed good model performance for snow accumulation and snowmelt compared to the field observations for four seasons during 2007–2011, with a small bias and normalized root mean square difference (NRMSD) ranging from 40 to 42% for the subalpine conifer forests and from 31 to 67% for the alpine tundra and tree-line larch forest environments. Overestimation or underestimation of the peak SWE ranged from 1.6 to 29%. Simulations matched well with the observed unfrozen moisture fluctuation in the top soil layer at a lodgepole pine site during 2006–2011, with a NRMSD ranging from 17% to 39%, but with consistent overestimation of 7 to 34%. Evaluations of seasonal streamflow during 2006–2011 revealed the model generally predicted well compared to observations at the basin scale, with a NRMSD of 77% and small model bias (6%), but at the sub-basin scale NRMSD were larger, ranging from 86 to 106%; though overestimation or underestimation for the cumulative seasonal discharge was within 24%. Timing of discharge was better predicted at the Marmot Creek basin outlet having a Nash-Sutcliffe efficiency (NSE) of 0.31 compared to the outlets of the sub-basins where NSE ranged from −0.03 to −0.76. The Pearson product-moment correlation coefficient of 0.12 and 0.17 for comparisons between the simulated groundwater storage and observed groundwater level fluctuation at two wells indicate weak but positive correlations. The model results are encouraging for uncalibrated prediction and indicate research priorities to improve simulations of snow accumulation at treeline, groundwater dynamics and small-scale runoff generation processes in this environment. The study shows that improved hydrological cycle model prediction can be derived from improved hydrological understanding and therefore is a model that can be applied for prediction in ungauged basins.

Description: Is bias correction of Regional Climate Model (RCM) simulations possible for non-stationary conditions? Hydrology and Earth System Sciences Discussions, 9, 12765-12795, 2012 Author(s): C. Teutschbein and J. Seibert In hydrological climate-change impact studies, Regional Climate Models (RCMs) are commonly used to transfer large-scale Global Climate Model (GCM) data to smaller scales and to provide more detailed regional information. However, there are often considerable biases in RCM simulations, which have led to the development of a number of bias correction approaches to provide more realistic climate simulations for impact studies. Bias correction procedures rely on the assumption that RCM biases do not change over time, because correction algorithms and their parameterizations are derived for current climate conditions and assumed to apply also for future climate conditions. This underlying assumption of bias stationarity is the main concern when using bias correction procedures. It is in principle not possible to test whether this assumption is actually fulfilled for future climate conditions. In this study, however, we demonstrate that it is possible to evaluate how well bias correction methods perform for conditions different from those used for calibration. For five Swedish catchments, several time series of RCM simulated precipitation and temperature were obtained from the ENSEMBLES data base and different commonly-used bias correction methods were applied. We then performed a differential split-sample test by dividing the data series into cold and warm respective dry and wet years. This enabled us to evaluate the performance of different bias correction procedures under systematically varying climate conditions. The differential split-sample test resulted in a large spread and a clear bias for some of the correction methods during validation years. More advanced correction methods such as distribution mapping performed relatively well even in the validation period, whereas simpler approaches resulted in the largest deviations and least reliable corrections for changed conditions. Therefore, we question the use of simple bias correction methods such as the widely used delta-change approach and linear scaling for RCM-based climate-change impact studies and recommend using higher-skill bias correction methods.

Description: Estimation of debris flow critical rainfall thresholds by a physically-based model Hydrology and Earth System Sciences Discussions, 9, 12797-12824, 2012 Author(s): M. N. Papa, V. Medina, F. Ciervo, and A. Bateman Real time assessment of debris flow hazard is fundamental for setting up warning systems that can mitigate its risk. A convenient method to assess the possible occurrence of a debris flow is the comparison of measured and forecasted rainfall with rainfall threshold curves (RTC). Empirical derivation of the RTC from the analysis of rainfall characteristics of past events is not possible when the database of observed debris flows is poor or when the environment changes with time. For landslides triggered debris flows, the above limitations may be overcome through the methodology here presented, based on the derivation of RTC from a physically based model. The critical RTC are derived from mathematical and numerical simulations based on the infinite-slope stability model in which land instability is governed by the increase in groundwater pressure due to rainfall. The effect of rainfall infiltration on landside occurrence is modelled trough a reduced form of the Richards equation. The simulations are performed in a virtual basin, representative of the studied basin, taking into account the uncertainties linked with the definition of the characteristics of the soil. A large number of calculations are performed combining different values of the rainfall characteristics (intensity and duration of event rainfall and intensity of antecedent rainfall). For each combination of rainfall characteristics, the percentage of the basin that is unstable is computed. The obtained database is opportunely elaborated to derive RTC curves. The methodology is implemented and tested on a small basin of the Amalfi Coast (South Italy).

Description: Expected changes in future temperature extremes and their elevation dependency over the Yellow River source region Hydrology and Earth System Sciences Discussions, 9, 13609-13634, 2012 Author(s): Y. Hu, S. Maskey, and S. Uhlenbrook Using the Statistical DownScaling Model (SDSM) and the outputs from two global climate models we investigate possible changes in mean and extreme temperature indices and their elevation dependency over the Yellow River source region for the period 2081–2100 under the IPCC SRES A2, A1B and B1 emission scenarios. Changes in interannual variability of mean and extreme temperature indices are also analyzed. The validation results show that SDSM performs better in reproducing the maximum temperature-related indices than the minimum temperature-related indices. The projections show that by the end of the 21st century all parts of the study region may experience increases in both mean and extreme temperature in all seasons, along with an increase in the frequency of hot days and warm nights and with a decrease in frost days. Interannual variability increases in all seasons for the frequency of hot days and warm nights and in spring for frost days while it decreases for frost days in summer. Autumn demonstrates pronounced elevation-dependent changes in which six out of eight indices show significant increasing changes with elevation.

Description: Do probabilistic forecasts lead to better decisions? Hydrology and Earth System Sciences Discussions, 9, 13569-13607, 2012 Author(s): M. H. Ramos, S. J. van Andel, and F. Pappenberger The last decade has seen growing research in producing probabilistic hydro-meteorological forecasts and increasing their reliability. This followed the promise that, supplied with information about uncertainty, people would take better risk-based decisions. In recent years, therefore, research and operational developments have also start putting attention to ways of communicating the probabilistic forecasts to decision makers. Communicating probabilistic forecasts includes preparing tools and products for visualization, but also requires understanding how decision makers perceive and use uncertainty information in real-time. At the EGU General Assembly 2012, we conducted a laboratory-style experiment in which several cases of flood forecasts and a choice of actions to take were presented as part of a game to participants, who acted as decision makers. Answers were collected and analyzed. In this paper, we present the results of this exercise and discuss if indeed we make better decisions on the basis of probabilistic forecasts.

Description: Present state of global wetland extent and wetland methane modelling: methodology of a model intercomparison project (WETCHIMP) Geoscientific Model Development Discussions, 5, 4071-4136, 2012 Author(s): R. Wania, J. R. Melton, E. L. Hodson, B. Poulter, B. Ringeval, R. Spahni, T. Bohn, C. A. Avis, G. Chen, A. V. Eliseev, P. O. Hopcroft, W. J. Riley, Z. M. Subin, H. Tian, V. Brovkin, P. M. van Bodegom, T. Kleinen, Z. C. Yu, J. S. Singarayer, S. Zürcher, D. P. Lettenmaier, D. J. Beerling, S. N. Denisov, C. Prigent, F. Papa, and J. O. Kaplan The Wetland and Wetland CH 4 Intercomparison of Models Project (WETCHIMP) was created to evaluate our present ability to simulate large-scale wetland characteristics and corresponding methane (CH 4 ) emissions. A multi-model comparison is essential to evaluate the key uncertainties in the mechanisms and parameters leading to methane emissions. Ten modelling groups joined WETCHIMP to run eight global and two regional models with a common experimental protocol using the same climate and atmospheric carbon dioxide (CO 2 ) forcing datasets. We reported the main conclusions from the intercomparison effort in a companion paper (Melton et al., 2012). Here we provide technical details for the six experiments, which included an equilibrium, a transient, and an optimized run plus three sensitivity experiments (temperature, precipitation, and atmospheric CO 2 concentration). The diversity of approaches used by the models is summarized through a series of conceptual figures, and is used to evaluate the wide range of wetland extents and CH 4 fluxes predicted by the models in the equilibrium run. We discuss relationships among the various approaches and patterns in consistencies of these model predictions. Within this group of models, there are three broad classes of methods used to estimate wetland extent: prescribed based on wetland distribution maps, prognostic relationships between hydrological states based on satellite observations, and explicit hydrological mass balances. A larger variety of approaches was used to estimate the net CH 4 fluxes from wetland systems. Even though modelling of wetland extents and CH 4 emissions has progressed significantly over recent decades, large uncertainties still exist when estimating CH 4 emissions: there is little consensus on model structure or complexity due to knowledge gaps, different aims of the models, and the range of temporal and spatial resolutions of the models.

Description: Improving the representation of secondary organic aerosol (SOA) in the MOZART-4 global chemical transport model Geoscientific Model Development Discussions, 5, 4187-4232, 2012 Author(s): A. Mahmud and K. C. Barsanti The secondary organic aerosol (SOA) module in the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4) has been updated by replacing existing two-product (2p) parameters with those obtained from two-product volatility basis set (2p-VBS) fits, and by treating SOA formation from the following volatile organic compounds (VOCs): isoprene, propene and lumped alkenes. Strong seasonal and spatial variations in global SOA distributions were demonstrated, with significant differences in the predicted concentrations between the base-case and updated model versions. The base-case MOZART-4 predicted annual average SOA of 0.36 ± 0.50 μg m −3 in South America, 0.31 ± 0.38 μg m −3 in Indonesia, 0.09 ± 0.05 μg m −3 in the USA, and 0.12 ± 0.07 μg m −3 in Europe. Concentrations from the updated versions of the model showed a~marked increase in annual average SOA. Using the updated set of parameters alone (MZ4-v1) increased annual average SOA by ~8%, ~16%, ~56%, and ~108% from the base-case in South America, Indonesia, USA, and Europe, respectively. Treatment of additional parent VOCs (MZ4-v2) resulted in an even more dramatic increase of ~178–406% in annual average SOA for these regions over the base-case. The increases in predicted SOA concentrations further resulted in increases in corresponding SOA contributions to annual average total aerosol optical depth (AOD) by

Description: Modeling agriculture in the Community Land Model Geoscientific Model Development Discussions, 5, 4137-4185, 2012 Author(s): B. Drewniak, J. Song, J. Prell, V. R. Kotamarthi, and R. Jacob The potential impact of climate change on agriculture is uncertain. In addition, agriculture could influence above- and below-ground carbon storage. Development of models that represent agriculture is necessary to address these impacts. We have developed an approach to integrate agriculture representations for three crop types – maize, soybean, and spring wheat – into the coupled carbon-nitrogen version of the Community Land Model (CLM), to help address these questions. Here we present the new model, CLM-Crop, validated against observations from two AmeriFlux sites in the United States, planted with maize and soybean. Seasonal carbon fluxes compared well with field measurements. CLM-Crop yields were comparable with observations in some regions, although the generality of the crop model and its lack of technology and irrigation made direct comparison difficult. CLM-Crop was compared against the standard CLM3.5, which simulates crops as grass. The comparison showed improvement in gross primary productivity in regions where crops are the dominant vegetation cover. Crop yields and productivity were negatively correlated with temperature and positively correlated with precipitation. In case studies with the new crop model looking at impacts of residue management and planting date on crop yield, we found that increased residue returned to the litter pool increased crop yield, while reduced residue returns resulted in yield decreases. Using climate controls to signal planting date caused different responses in different crops. Maize and soybean had opposite reactions: when low temperature threshold resulted in early planting, maize responded with a loss of yield, but soybean yields increased. Our improvements in CLM demonstrate a new capability in the model – simulating agriculture in a realistic way, complete with fertilizer and residue management practices. Results are encouraging, with improved representation of human influences on the land surface and the potentially resulting climate impacts.

Description: An opportunity of application of excess factor in hydrology Hydrology and Earth System Sciences Discussions, 9, 13635-13649, 2012 Author(s): V. Kovalenko, E. Gaidukova, and A. Kachalova In last few years in hydrology an interest to excess factor has appeared as a reaction to unsuccessful attempts to simulate and predict evolving hydrological processes, which attributive property is statistical instability. The article shows, that the latter has a place at strong relative multiplicative noises of probabilistic stochastic model of a river flow formation, phenomenological display of which are "the thick tails" and polymodality, for which the excess factor "answers", by being ignored by a modern hydrology in connection to the large error of its calculation because of insufficient duration of lines of observation over a flow. However, it is found out, that the duration of observation of several decades practically stabilizes variability of the excess factor, the error of which definition appears commensurable with an error of other calculated characteristics used in engineering hydrology.

Description: A test of numerical instability and stiffness in the parametrizations of the ARPÉGE and ALADIN models Geoscientific Model Development Discussions, 5, 4233-4268, 2012 Author(s): M. Tudor Meteorological numerical weather prediction (NWP) models solve a system of partial differential equations in time and space. Semi-lagrangian advection scheme in the model dynamics allows for long time-steps. These longer time-steps can result in instabilities occurring in the model physics. A system of differential equations in which some solution components decay more rapidly than others is stiff. In this case it is stability rather than accuracy that restricts the time-step. The vertical diffusion parametrization can cause fast non-meteorological oscillations around the slowly evolving true solution (fibrillations). These are treated with an anti-fibrillation scheme. But small oscillations remain in an operational weather forecasts using ARPÉGE and ALADIN models. It is needed to test of the complete model formulation, as implemented in the operational forecast. In this paper, a simple test is designed. The test reveals if the formulation of particular physical parametrization is a stiff problem or potentially numerically unstable in combination with any other part of the model. When the test is applied to a stable scheme, the solution remains stable. But, applying the test to a potentially unstable scheme yields a solution with fibrillations of substantial amplitude. The parametrizations of a NWP model ARPÉGE were tested one by one to see which one may be the source of unstable model behaviour. The test has identified the stratiform precipitation scheme (a diagnostic Kessler type scheme) as a stiff problem, particularly the term that describes the evaporation of snow.

Description: Spatial distribution of solute leaching with snowmelt and irrigation: measurements and simulations Hydrology and Earth System Sciences Discussions, 9, 13451-13490, 2012 Author(s): D. Schotanus, M. J. van der Ploeg, and S. E. A. T. M. van der Zee Transport of a tracer and a degradable solute in a heterogeneous soil was measured in the field, and simulated with several transient and steady state infiltration rates. Leaching surfaces were used to investigate the solute leaching in space and time simultaneously. In the simulations, a random field for the scaling factor in the retention curve was used for the heterogeneous soil, which was based on the spatial distribution of drainage in an experiment with a multi-compartment sampler. As a criterion to compare the results from simulations and observations, the sorted and cumulative total drainage in a cell was used. The effect of the ratio of the infiltration rate over the degradation rate on leaching of degradable solutes was investigated. Furthermore, the spatial distribution of the leaching of degradable and non-degradable solutes was compared. The infiltration rate determines the amount of leaching of the degradable solute. This can be partly explained by a decreasing travel time with an increasing infiltration rate. The spatial distribution of the leaching also depends on the infiltration rate. When the infiltration rate is high compared to the degradation rate, the leaching of the degradable solute is similar as for the tracer. The fraction of the soil that contributes to solute leaching increases with an increasing infiltration rate. This fraction is similar for a tracer and a degradable solute. With increasing depth, the leaching becomes more homogeneous, as a result of dispersion. The spatial distribution of the solute leaching is different under different transient infiltration rates, therefore also the amount of leaching is different. With independent stream tube approaches, this effect would be ignored.

Description: MESMO 2: a mechanistic marine silica cycle and coupling to a simple terrestrial scheme Geoscientific Model Development Discussions, 5, 2999-3033, 2012 Author(s): K. Matsumoto, K. S. Tokos, A. Huston, and H. Joy-Warren Here we describe the second version of Minnesota Earth System Model for Ocean biogeochemistry (MESMO 2), an earth system model of intermediate complexity, which consists of a dynamical ocean, dynamic-thermodynamic sea ice, and energy moisture balanced atmosphere. The new version has more realistic land ice masks and is driven by seasonal winds. A major aim in version 2 is representing the marine silica cycle mechanistically in order to investigate climate-carbon feedbacks involving diatoms, a critically important class of phytoplankton in terms of carbon export production. This is achieved in part by including iron, on which phytoplankton uptake of silicic acid depends. Also, MESMO 2 is coupled to an existing terrestrial model, which allows for the exchange of carbon, water, and energy between land and the atmosphere. The coupled model, called MESMO 2E, is appropriate for more complete earth system simulations. The new version was calibrated with the goal of preserving reasonable interior ocean ventilation and various biological production rates in the ocean and land, while simulating key features of the marine silica cycle.

Description: On selection of the optimal data time interval for real-time hydrological forecasting Hydrology and Earth System Sciences Discussions, 9, 10829-10875, 2012 Author(s): J. Liu and D. Han With the advancement in modern telemetry and communication technologies, hydrological data can be collected with an increasingly higher sampling rate. An important issue deserving attention from the hydrological community is what suitable time interval of the model input data should be chosen in hydrological forecasting. Such a problem has long been recognised in the control engineering community but is a largely ignored topic in operational applications of hydrological forecasting. In this study, the intrinsic properties of rainfall-runoff data with different time intervals are first investigated from the perspectives of the sampling theorem and the information loss using the discrete wavelet decomposition tool. It is found that rainfall signals with very high sampling rates may not always improve the accuracy of rainfall-runoff modelling due to the catchment low-pass filtering effect. To further investigate the impact of data time interval in real-time forecasting, a real-time forecasting system is constructed by incorporating the Probability Distributed Model (PDM) with a real-time updating scheme, the autoregressive-moving average (ARMA) model. Case studies are then carried out on four UK catchments with different concentration times for real-time flow forecasting using data with different time intervals of 15 min, 30 min, 45 min, 60 min, 90 min and 120 min. A positive relation is found between the forecast lead time and the optimal choice of the data time interval, which is also highly dependent on the catchment concentration time. Finally, based on the conclusions from the case studies, a hypothetical pattern is proposed in three-dimensional coordinates to describe the general impact of the data time interval and to provide implications on the selection of the optimal time interval in real-time hydrological forecasting. Although nowadays most operational hydrological systems still have low data sampling rates (daily or hourly), the trend in the future is that higher sampling rates will become widespread and there is an urgent need for both academic and practising hydrologists to realise the significance of the data time interval issue. It is important that more case studies in different catchments with various hydrological forecasting models should be explored in the future to further verify and improve the proposed hypothetical pattern.

Description: Catchment classification based on characterisation of streamflow and precipitation time-series Hydrology and Earth System Sciences Discussions, 9, 10805-10828, 2012 Author(s): E. Toth The formulation of objective procedures for the delineation of homogeneous groups of catchments is a fundamental issue in both operational and research hydrology. For assessing catchment similarity, a variety of hydrological information may be considered; in this paper, gauged sites are characterised by a set of streamflow signatures that include a representation, albeit simplified, of the properties of fine time-scale flow series and in particular of the dynamic components of the data, in order to keep into account the sequential order and the stochastic nature of the streamflow process. The streamflow signatures are provided in input to a clustering algorithm based on unsupervised SOM neural networks, providing an overall reasonable grouping of catchments on the basis of their hydrological response. In order to assign ungauged sites to such groups, the catchments are represented through a parsimonious set of morphometric and pluviometric variables, including also indexes that attempt to synthesize the variability and correlation properties of the precipitation time-series, thus providing information on the type of weather forcing that is specific to each basin. Following a principal components analysis, needed for synthesizing and better understanding the morpho-pluviometric catchment properties, a discriminant analysis finally classifies the ungauged catchments, through a leave-one-out cross-validation, to one of the above identified hydrologic response classes. The approach delivers quite satisfactory results for ungauged catchments, since the comparison of the two cluster sets shows an acceptable overlap. Overall results indicate that the inclusion of information on the properties of the fine time-scale streamflow and rainfall time-series may be a promising way for better representing the hydrologic and climatic character of the study catchments.

Description: Modeling postfire water erosion mitigation strategies Hydrology and Earth System Sciences Discussions, 9, 10877-10916, 2012 Author(s): M. C. Rulli, L. Offeddu, and M. Santini Severe wildfires are often followed by significant increase in runoff and erosion, due to vegetation damages and changes in physical and chemical soil properties. Peak flows and sediment yields can increase up to two orders of magnitude becoming dangerous for human lives and ecosystem, especially in the wildland-urban interface. Watershed post fire rehabilitation measures are usually used to mitigate the effects of fire on runoff and erosion, by protecting soil from splash and shear stress detachment and enhancing its infiltration capacity. Modeling post fire erosion and erosion mitigation strategies can be useful in selecting the effectiveness of rehabilitation method. In this paper a distributed model based on Revised Universal Soil Loss Equation (RUSLE), properly parameterized for a Mediterranean basin located in Sardinia, is used to determine soil losses for six different scenarios describing both natural and post-fire basin condition, the last accounting also for the single and combined effect of different erosion mitigation measures. Fire effect on vegetation and soil properties have been mimed by changing soil drainage capacity and organic matter content, and RUSLE factors related to soil cover and protection measures. Model results show for the analyzed rehabilitation treatments their effect in reducing the amount of soil losses with the peculiar characteristics of the spatial distribution of such changes.

Description: Seasonal forecasts of drought indices in African basins Hydrology and Earth System Sciences Discussions, 9, 11093-11129, 2012 Author(s): E. Dutra, F. Di Giuseppe, F. Wetterhall, and F. Pappenberger Vast parts of Africa rely on the rainy season for livestock and agriculture. Droughts can have a severe impact in these areas which often have a very low resilience and limited capabilities to mitigate their effects. This paper tries to assess the predictive capabilities of an integrated drought monitoring and forecasting system based on the Standard precipitation index (SPI). The system is firstly constructed by temporally extending near real-time precipitation fields (ECMWF ERA-Interim reanalysis and the Climate Anomaly Monitoring System-Outgoing Longwave Radiation Precipitation Index, CAMS-OPI) with forecasted fields as provided by the ECMWF seasonal forecasting system and then is evaluated over four basins in Africa: the Blue Nile, Limpopo, Upper Niger, and Upper Zambezi. There are significant differences in the quality of the precipitation between the datasets depending on the catchments, and a general statement regarding the best product is difficult to make. All the datasets show similar patterns in the South and North West Africa, while there is a low correlation in the tropical region which makes it difficult to define ground truth and choose an adequate product for monitoring. The Seasonal forecasts have a higher reliability and skill in the Blue Nile, Limpopo and Upper Niger in comparison with the Zambezi. This skill and reliability depends strongly on the SPI time-scale, and more skill is observed at larger time-scales. The ECMWF seasonal forecasts have predictive skill which is higher than using climatology for most regions. In regions where no reliable near real-time data is available, the seasonal forecast can be used for monitoring (first month of forecast). Furthermore, poor quality precipitation monitoring products can reduce the potential skill of SPI seasonal forecasts in two to four months lead time.

Description: A new method to diagnose the contribution of anthropogenic activities to temperature: temperature tagging Geoscientific Model Development Discussions, 5, 3183-3215, 2012 Author(s): V. Grewe This study presents a new methodology, called temperature tagging. It keeps track of the contributions of individual processes to temperature within a climate model simulation. As a first step and as a test bed a simple box climate model is regarded. The model consists of an atmosphere, which absorbs and emits radiation and of a surface, which reflects, absorbs and emits radiation. The tagging methodology is used to investigate the impact of the atmosphere on surface temperature. Four processes are investigated in more detail and their contribution to the surface temperature quantified: (i) shortwave influx and shortwave atmospheric absorption ("sw"), (ii) longwave atmospheric absorption due to non-CO 2 greenhouse gases ("nC"), (iii) due to a base case CO 2 concentration ("bC"), and (iv) due to an enhanced CO 2 concentration ("eC"). The differential equation for the temperature in the box climate model is decomposed into four equations for the tagged temperatures. This method is applied to investigate the contribution of longwave absorption to the surface temperature (greenhouse effect), which is calculated to be 68 K. This estimate contrasts an alternative calculation of the greenhouse effect of slightly more than 30 K based on the difference of the surface temperature with and without an atmosphere. The difference of the two estimates is due to a shortwave cooling effect and a reduced contribution of the shortwave to the total downward flux: The shortwave absorption of the atmosphere results in a reduced net shortwave flux at the surface of 192 W m −2 , leading to a cooling of the surface by 14 K. Introducing an atmosphere results in a downward longwave flux at the surface due to atmospheric absorption of 189 W m −2 , which roughly equals the net shortwave flux of 192 W m −2 . This longwave flux is a result of both, the radiation due to atmospheric temperatures and its longwave absorption. Hence the longwave absorption roughly accounts for 91 W m −2 out of a total of 381 W m −2 (roughly 25%) and therefore accounts for a temperature of 68 K. In a second experiment, the CO 2 concentration is doubled, which leads to an increase in surface temperature of 1.2 K, resulting from an temperature increase due to CO 2 of 1.9 K, due to non-CO 2 greenhouse gases of 0.6 K and a cooling of 1.3 K due to a reduced importance of the solar heating for the surface and atmospheric temperatures. These two experiments show the feasibility of temperature tagging and its potential as a diagnostic for climate simulations.

Description: Multi-satellite rainfall sampling error estimates – a comparative study Hydrology and Earth System Sciences Discussions, 9, 11677-11706, 2012 Author(s): M. Itkin and A. Loew This study focus is set on quantifying sampling related uncertainty in the satellite rainfall estimates. We conduct observing system simulation experiment to estimate sampling error for various constellations of Low-Earth orbiting and geostationary satellites. There are two types of microwave instruments currently available: cross track sounders and conical scanners. We evaluate the differences in sampling uncertainty for various satellite constellations that carry instruments of the common type as well as in combination with geostationary observations. A precise orbital model is used to simulate realistic satellite overpasses with orbital shifts taken into account. With this model we resampled rain gauge timeseries to simulate satellites rainfall estimates free of retrieval and calibration errors. We concentrate on two regions, Germany and Benin, areas with different precipitation regimes. Our results show that sampling uncertainty for all satellite constellations does not differ greatly depending on the area despite the differences in local precipitation patterns. Addition of 3 hourly geostationary observations provides equal performance improvement in Germany and Benin, reducing rainfall undersampling by 20–25% of the total rainfall amount. Authors do not find a significant difference in rainfall sampling between conical imager and cross-track sounders.

Description: Soil moisture controls on patterns of grass green-up in Inner Mongolia: an index based approach Hydrology and Earth System Sciences Discussions, 9, 11641-11675, 2012 Author(s): H. Liu, F. Tian, H. Hu, H. Hu, and M. Sivapalan Water availability is one of the most important environmental controls on vegetation phenology, especially in semi-arid regions, and is often represented in terms of soil moisture in small-scale studies whereas it tends to be represented by precipitation in large-scale (e.g. regional) studies. Clearly, soil moisture is the more appropriate indicator for root water uptake and vegetation growth/phenology and therefore its potential advantage and applicability needs to be demonstrated at regional scales. This paper represents a data-based regional study of the effectiveness of alternative indices based on water and energy availability on space-time patterns of spring vegetation green-up onset dates estimated from Normalized Difference Vegetation Index (NDVI) datasets in the grasslands of Inner Mongolia, China. The macro-scale hydrological model, VIC, is employed to generate a soil moisture database across the region. In addition to standard index based on temperature, two potential hydrology based indices for prediction of spring onset dates are defined based on the simulated soil moisture data as well as on observed precipitation data. Results indicate that the correspondence between the NDVI-derived green-up onset date and the soil moisture derived potential onset date exhibits a significantly better correlation as a function of increasing aridity, compared to that based on precipitation. In this way the soil moisture based index is demonstrated to be superior to the precipitation based index in terms of capturing grassland spring phenology. The results also showed that both of the hydrological (water based) indices were superior to the thermal (temperature based) index in determining the patterns of grass green-up in the Inner Mongolia region, indicating water availability to be the dominant control, on average. The understanding about the relative controls on grassland phenology, and the effectiveness of alternative indices to capture these controls, are important for future studies and predictions of vegetation phenology change under climate change.

Description: Acid-base characteristics of the Grass Pond watershed in the Adirondack Mountains of New York State, USA: interactions between soil, vegetation and surface waters Hydrology and Earth System Sciences Discussions, 9, 10775-10803, 2012 Author(s): K. M. McEathron, M. J. Mitchell, and L. Zhang Grass Pond watershed is located within the Southwestern Adirondack Mountain region of New York State, USA. This region receives some of the highest rates of acidic deposition in North America and is particularly sensitive to acidic inputs due to many of its soils having shallow depths and being generally base-poor. Differences in soil chemistry and tree species between seven subwatersheds were examined in relation to acid-base characteristics of the seven major streams that drain into Grass Pond. Mineral soil pH, stream water BCS and pH exhibited a positive correlation with sugar maple basal area ( p = 0.055; 0.48 and 0.39, respectively). Black cherry basal area was inversely correlated with stream water BCS, ANC c and NO 3 – ( p = 0.23; 0.24 and 0.20, respectively). Sugar maple basal areas were positively correlated with watershed characteristics associated with the neutralization of atmospheric acidic inputs while in contrast, black cherry basal areas showed opposite relationships to these same watershed characteristics. Canonical Correspondence Analysis indicated that black cherry had a distinctive relationship with forest floor chemistry apart from the other tree species, specifically a strong positive association with forest floor NH 4 while sugar maple had a distinctive relationship with stream chemistry variables, specifically a strong positive association with stream water ANC c , BCS and pH. Our results provide evidence that sugar maple is acid-intolerant or calciphilic tree species and also demonstrate that black cherry is likely an acid-tolerant tree species.

Description: Estimation of evapotranspiration from TOA radiances in the Poyang Lake Basin, China Hydrology and Earth System Sciences Discussions, 9, 10963-11003, 2012 Author(s): J. Peng, Y. Liu, X. Zhao, and A. Loew Routine and rapid estimation of ET (evapotranspiration) at regional scale is of great significance for agricultural, hydrological and climatic studies. A simplified single-source energy balance parameterization scheme, known as the LST/NDVI (Land Surface Temperature/Vegetation Index) triangle method, has been applied successfully to estimate regional clear sky ET in many studies. Based on the triangle method, we proposed a new method in this study to estimate daily ET directly using the TOA (Top of Atmosphere) radiances without performing atmospheric correction and other complicated processes. Firstly, the EF (Evaporative Fraction, defined as the ratio of latent heat flux to surface available energy) was estimated by interpolation in the LST/NDVI triangular-shaped scatter space, which was constructed using the MODIS (Moderate Resolution Imaging Spectro radiometer) TOA radiances over a heterogeneous area of the Poyang Lake Basin in China. Then the net radiation over the same study area was derived based entirely on MODIS TOA radiances as well. Finally, daily ET maps were estimated from these EF maps and net radiation maps by using a sinusoidal temporal interpolation model. The estimated EF, net radiation and ET have been validated against field observations collected for the period October 2007–July 2008. The results indicate comparable accuracy to results of other current widely used satellite-based methods. In addition, intercomparisons between the proposed method-based estimates and MODIS products-based estimates were also carried out over the validation site. The results suggest that the proposed method performed slightly better than the MODIS products-based triangle method. Overall, the proposed algorithm requires fewer assumptions and can avoid uncertainties associated with the satellite derived products, while its accuracy is slightly higher. It should facilitate direct use of satellite data for determining ET and relevant applications as well. Nonetheless, more validation work needs to be carried out with more integration of satellite data and ground-based measurements over various climatic regions and under different surface conditions in the future.

Description: Data expansion: the potential of grey literature for understanding floods Hydrology and Earth System Sciences Discussions, 9, 11049-11092, 2012 Author(s): S. Uhlemann, R. Bertelmann, and B. Merz Sophisticated methods have been developed and become standard in analysing floods as well as for assessing the flood risk. However, increasingly critique of the current standards and scientific practice can be found both in the flood hydrology community as well as in the risk community who argue that the considerable amount of information already available on natural disasters has not been adequately deployed and brought to effective use. We describe this phenomenon as a failure to synthesize knowledge that results from barriers and ignorance in awareness, use and management of the entire spectrum of relevant content, that is, data, information and knowledge. In this paper we argue that the scientific community in flood risk research ignores event specific analysis and documentations as another source of data. We present results from a systematic search that includes an intensive study on sources and ways of information dissemination of flood relevant publications. We obtain 183 documents that contain information on the sources, pathways, receptors and/or consequences for any of the 40 strongest trans-basin floods in Germany in the period 1952–2002. This study therefore provides the most comprehensive meta-data collection of flood documentations for the considered geographical space and period. 87.5% of all events have been documented and especially the most severe floods have received extensive coverage. Only 30% of the material has been produced in the scientific/academic environment and the majority of all documents (about 80%) can be considered grey literature. Therefore, ignoring grey sources in flood research also means ignoring the largest part of knowledge available on single flood events (in Germany). Further, the results of this study underpin the rapid changes in information dissemination of flood event literature over the last decade. We discuss the options and obstacles of incorporating this data in the knowledge building process in the light of the current technological developments and international, interdisciplinary debates for data curation.

Description: A Bayesian joint probability post-processor for reducing errors and quantifying uncertainty in monthly streamflow predictions Hydrology and Earth System Sciences Discussions, 9, 11199-11225, 2012 Author(s): P. Pokhrel, D. E. Robertson, and Q. J. Wang Hydrological post-processors refer here to statistical models that are applied to hydrological model predictions to further reduce prediction errors and to quantify remaining uncertainty. For streamflow predictions, post-processors are generally applied to daily or sub-daily time scales. For many applications such as seasonal streamflow forecasting and water resources assessment, monthly volumes of streamflows are of primary interest. While it is possible to aggregate post-processed daily or sub-daily predictions to monthly time scales, the monthly volumes so produced may not have the least errors achievable and may not be reliable in uncertainty distributions. Post-processing directly at the monthly time scale is likely to be more effective. In this study, we investigate the use of a Bayesian joint probability modelling approach to directly post-process model predictions of monthly streamflow volumes. We apply the BJP post-processor to 18 catchments located in eastern Australia and demonstrate its effectiveness in reducing prediction errors and quantifying prediction uncertainty.

Description: Gains from modelling dependence of rainfall variables into a stochastic model: application of the copula approach at several sites Hydrology and Earth System Sciences Discussions, 9, 11227-11266, 2012 Author(s): P. Cantet and P. Arnaud Since the last decade, copulas have become more and more widespread in the construction of hydrological models. Unlike the multivariate statistics which are traditionally used, this tool enables scientists to model different dependence structures without drawbacks. The authors propose to apply copulas to improve the performance of an existing model. The hourly rainfall stochastic model SHYPRE is based on the simulation of descriptive variables. It generates long series of hourly rainfall and enables the estimation of distribution quantiles for different climates. The paper focuses on the relationship between two variables describing the rainfall signal. First, Kendall's tau is estimated on each of the 217 rain gauge stations in France, then the False Discovery Rate procedure is used to define stations for which the dependence is significant. Among three usual archimedean copulas, a unique 2-copula is chosen to model this dependence for any station. Modelling dependence leads to an obvious improvement in the reproduction of the standard and extreme statistics of maximum rainfall, especially for the sub-daily rainfall. An accuracy test for the extreme values shows the good asymptotic behaviour of the new rainfall generator version and the impacts of the copula choice on extreme quantile estimation.

Description: Streamflow input to Lake Athabasca, Canada Hydrology and Earth System Sciences Discussions, 9, 9065-9093, 2012 Author(s): K. Rasouli, M. A. Hernández-Henríquez, and S. J. Déry The 271 000 km 2 Lake Athabasca drainage in Northern Canada encompasses ecologically-rich and sensitive ecosystems, intensive agricultural lands, vast forests, glacier-clad mountains, and abundant oil reserves in the form of tar-sands. In this study, streamflow variability and trends in eight rivers feeding the 7800 km 2 Lake Athabasca are investigated over the period 1960–2010. Hydrological regimes and trends are established using a robust regime shift detection method and the Mann-Kendall (MK) test, respectively. Results show that the Athabasca River, which provides ~ 57% of the total annual lake inflow of 34.06 km 3 yr −1 , experiences marked declines in recent decades impacting lake levels and its ecosystem. The Fond du Lac River, which contributes ~ 30% of total Lake Athabasca inflow, has an increasing trend of 0.021 km 3 yr −1 over 1970–2010 according to the MK test, equating to a 0.86 km 3 discharge increase from Fond du Lac River to the lake. From 1960 to 2010 there has been approximately a 21.2% reduction of average discharge equivalent to a 7.22 km 3 recession in the Lake Athabasca causing lake levels to drop. The lake level has a trend of −0.008 m yr −1 which is equivalent to a 0.39 m decline in the lake level over 1960–2010. The total lake inflow trend over 1977–2010 is −0.207 km 3 yr −1 or a reduction of 25.67 km 3 by 2100 by linear extrapolation. This may imply a further reduction of 2 m to 3 m in lake level that is in the range of a 5200-yr historical minimum inferred from proxy data in nearby sediment cores.

Description: Balancing energy and environmental concerns: the case of the Kayraktepe dam, Turkey Hydrology and Earth System Sciences Discussions, 9, 11769-11789, 2012 Author(s): Ö. Sever, Ş. Tiğrek, and N. Şarlak In this study, an alternative solution for a large dam, namely the Kayraktepe Dam in Turkey, is investigated. The dam was planned for flood control, energy generation and flow regulation for a downstream irrigation project more than 30 yr ago, but until now the project has not begun due to it receiving severe criticism about environmental and social considerations. The project formulation was redeveloped several times in the past but the options were not found to be feasible. In this study, a detailed analysis of the available feasibility studies is provided and then a new formulation, consisting of the proposed one medium dam and five run-of-river type hydropower stations instead of a large scale dam, is evaluated. The new formulation is equivalent to the existing project in terms of energy production and flood control. On the other hand, there are some benefits relative to other configurations as solutions to some of the environmental and social problems being addressed.

Description: Implementation of the Fast-JX Photolysis scheme into the UKCA component of the MetUM chemistry climate model Geoscientific Model Development Discussions, 5, 3217-3260, 2012 Author(s): P. J. Telford, N. L. Abraham, A. T. Archibald, P. Braesicke, M. Dalvi, O. Morgenstern, F. M. O'Connor, N. A. D. Richards, and J. A. Pyle Atmospheric chemistry is driven by photolytic reactions, making their modelling a crucial component of atmospheric models. We describe the implementation and validation of Fast-JX, a state of the art model of interactive photolysis, into the MetUM chemistry climate model. This allows for interactive photolysis frequencies to be calculated in the troposphere and augments the calculation of the frequencies in the stratosphere by accounting for clouds and aerosols in addition to ozone. In order to demonstrate the effectiveness of this new photolysis scheme we employ new methods of validating the model, including techniques for sampling the model to compare to flight track and satellite data.

Description: COSTRICE – three model online coupling using OASIS: problems and solutions Geoscientific Model Development Discussions, 5, 3261-3310, 2012 Author(s): H. T. M. Ho, B. Rockel, H. Kapitza, B. Geyer, and E. Meyer The coupled system COSTRICE is developed for the first time in order to reproduce the interactions and feedbacks between atmosphere, ocean and sea-ice in a two-way online coupled model system containing three component models for regional climate simulations over Baltic Sea and North Sea regions. The regional climate model CCLM 1 is coupled to the regional ocean model TRIMNP 1 and the sea ice model CICE 1 via the coupler OASIS3. In this study, CCLM is setup with a horizontal grid mesh size of 50 km and 32 vertical atmosphere layers and driven by the 6-h ERA-interim reanalysis data as initial and boundary conditions. TRIMNP is setup with a horizontal grid mesh size of 12.8 km and 50 vertical ocean levels. CICE calculates ice in 5 categories and runs with the same horizontal resolution as TRIMNP but only over the Baltic Sea and the Kattegat Bay of the North Sea. In a two-way online coupling process, CCLM is linked to TRIMNP through sea surface temperature (SST) as lower boundary condition every 3 h and TRIMNP is driven by 1-h atmospheric state variables and fluxes of CCLM. The data exchange processes between TRIMNP and CICE as well as from CCLM to CICE take place with an interval of 3 h. The coupled model is applied in a study for climate simulations over Baltic Sea and North Sea regions in 1997. The coupled system is set up to run in parallel on the super computing system IBM-power 6 at the German Climate Computing Center (DKRZ). 1 See Table A1.

Description: A generalized tagging method Geoscientific Model Development Discussions, 5, 3311-3324, 2012 Author(s): V. Grewe The understanding of causes of changes in climate-chemistry simulations is an important, but often challenging task. In atmospheric chemistry, one approach is to tag species according to their origin (e.g. emission categories) and to inherit these tags to other species during subsequent reactions. This concept was recently employed to calculate the contribution of atmospheric processes to temperature. Here a new concept for tagging any state variable is presented. This generalized tagging method results from a sensitivity analysis of the forcing terms of the right hand side of the governing differential equations. In a couple of examples, the consistency with previous approaches is shown. Since the method is based on a ratio describing relative sensitivities, singularities occur where the method is not applicable. For some applications, like in atmospheric chemistry, these singularities can easily be removed. However, one theoretical example is given, where this method is not applicable at all.

Description: Technical Note: Improving computational efficiency in large linear inverse problems: an example from carbon dioxide flux estimation Geoscientific Model Development Discussions, 5, 3325-3342, 2012 Author(s): V. Yadav and A. M. Michalak Addressing a variety of questions within Earth science disciplines entails the inference of the spatio-temporal distribution of parameters of interest based on observations of related quantities. Such estimation problems often represent inverse problems that are formulated as linear optimization problems. Computational limitations arise when the number of observations and/or the size of the discretized state space become large, especially if the inverse problem is formulated in a probabilistic framework and therefore aims to assess the uncertainty associated with the estimates. This work proposes two approaches to lower the computational costs and memory requirements for large linear space-time inverse problems, taking the Bayesian approach for estimating carbon dioxide (CO 2 ) emissions and uptake (a.k.a. fluxes) as a prototypical example. The first algorithm can be used to efficiently multiply two matrices, as long as one can be expressed as a Kronecker product of two smaller matrices, a condition that is typical when multiplying a sensitivity matrix by a covariance matrix in the solution of inverse problems. The second algorithm can be used to compute a posteriori uncertainties directly at aggregated spatio-temporal scales, which are the scales of most interest in many inverse problems. Both algorithms have significantly lower memory requirements and computational complexity relative to direct computation of the same quantities (O( n 2.5 ) vs. O( n 3 )). For an examined benchmark problem, the two algorithms yielded a three and six order of magnitude increase in computational efficiency, respectively, relative to direct computation of the same quantities. Sample computer code is provided for assessing the computational and memory efficiency of the proposed algorithms for matrices of different dimensions.

Description: Global runoff over 1993–2009 estimated from coupled land-ocean-atmosphere water budgets and its relation with climate variability Hydrology and Earth System Sciences Discussions, 9, 4633-4665, 2012 Author(s): S. Munier, H. Palanisamy, P. Maisongrande, A. Cazenave, and E. F. Wood Whether the global runoff (or freshwater discharge from land to the ocean) is currently increasing and the global water cycle is intensifying is still a controversial issue. Here we compute land-atmosphere and ocean-atmosphere water budgets and derive two independent estimates of the global runoff over the period 1993–2009. Water storage variations in the land, ocean and atmosphere reservoirs are estimated from different types of datasets: atmospheric reanalyses, land surface models, satellite altimetry and in situ ocean temperature data (the difference between altimetry based global mean sea level and ocean thermal expansion providing an estimate of the ocean mass component). Results for the global runoff from the two methods show a very good correlation between both estimates. More importantly, no significant trend is observed over the whole period. Besides, the global runoff appears to be clearly impacted by large-scale climate phenomena such as major ENSO events. To infer this, we compute the zonal runoff over four latitudinal bands and set up for each band a new index (Combined Runoff Index) obtained by optimization of linear combinations of various climate indices. Results show that, in particular, the intertropical and northern mid-latitude runoffs are mainly driven by ENSO and the Atlantic Multidecadal Oscillation (AMO) with opposite behavior. Indeed, the zonal runoff in the intertropical zone decreases during major El Niño events whereas it increases in the northern mid-latitudes, suggesting that water masses over land are shifted northward/southward during El Niño/La Niña. In addition to this study, we propose an innovative method to estimate the global ocean thermal expansion. The method is based on the assumption that the difference between both runoff estimates is mainly due the thermal expansion term not accounted for in the estimation of the ocean mass. Comparison of our reconstructed thermal expansion with two existing datasets shows the relevance of this new method.

Description: Joint statistical correction of clutters, spokes and beam height for a radar climatology in Southern Germany Hydrology and Earth System Sciences Discussions, 9, 4703-4746, 2012 Author(s): A. Wagner, J. Seltmann, and H. Kunstmann Extensive corrections of radar data are a crucial prerequisite for radar derived climatology. This kind of climatology demands a high level of data quality. Little deviations or minor systematic underestimations or overestimations in single radar images become a major cause of error in statistical analysis. First results of radar derived climatology have emerged over the last years, as data sets of appropriate extent are becoming available. Usually, these statistics are based on time series lasting up to ten years as storage of radar data was not achieved before. We present a new statistical post-correction scheme, which is based on seven years of radar data of the Munich weather radar (2000–2006) that is operated by DWD (German Weather Service). The typical correction algorithms for single radar images, such as clutter corrections, are used. Then an additional statistical post-correction based on the results of a climatological analysis from radar images follows. The aim of this statistical correction is to correct systematic errors caused by clutter effects or measuring effects but to conserve small-scale natural variations in space. The statistical correction is based on a thorough analysis of the different causes of possible errors for the Munich weather radar. This robust analysis revealed the following basic effects: the decrease of rain rate in relation to height and distance from the radar, clutter effects such as remaining clutter, eliminated clutter or shading effects from obstacles near the radar, visible as spokes, as well as the influence of the Bright Band. The correction algorithm is correspondingly based on these results. It consists of three modules. The first one is an altitude correction, which minimizes measuring effects. The second module corrects clutter effects and the third one realizes a mean adjustment to selected rain gauges. Two different radar products are used. The statistical analysis as well as module one and module two of the correction algorithm are based on frequencies of occurrence of the so-called PX-product with six reflectivity levels. For correction module 3 and for the validation of the correction algorithm rain rates are calculated from the 8-bit-depth so-called DX-product. An application (2004–2006) and a validation (2007–2009) of this correction algorithm with rain gauges show a much higher conformity for radar climatology after the statistical correction. In the years 2004 to 2006 the Root-Mean-Square-Error (RMSE) decreases from 262 mm to 118 mm excluding those pair of values where the rain gauges are situated in areas of obviously corrupted radar data. The results for the validation period 2007 to 2009 are based on all pairs of values and show a decline of the RMSE from 322 mm to 174 mm.

Description: Quality assessment concept of the World Data Center for Climate and its application to CMIP5 data Geoscientific Model Development Discussions, 5, 781-802, 2012 Author(s): M. Stockhause, H. Höck, F. Toussaint, and M. Lautenschlager The preservation of data in a high state of quality and suitable for interdisciplinary use is one of the most pressing and challenging current issues in long-term archiving. For high volume data such as climate model data, the data and data replica are no longer stored centrally but distributed over several local data repositories, e.g. the data of the Climate Model Intercomparison Project No. 5 (CMIP5). The most important part of the data is to be published as DOI according to the World Data Center for Climate's (WDCC) application of the DataCite regulations. The integrated part of WDCC's data publication process, the data quality assessment, was adapted to the requirements of a federated data infrastructure. A concept of a distributed and federated quality assessment procedure was developed, in which the work load and responsibility for quality control is shared between the three primary CMIP5 data centers: Program for Climate Model Diagnosis and Intercomparison (PCMDI), British Atmospheric Data Centre (BADC), and WDCC. This distributed quality control concept, its pilot implementation for CMIP5, and first experiences are presented.

Description: Record extension for short-gauged water quality parameters using a newly proposed robust version of the line of organic correlation technique Hydrology and Earth System Sciences Discussions, 9, 4667-4702, 2012 Author(s): B. Khalil and J. Adamowski In many situations the extension of hydrological or water quality time series at short-gauged stations is required. Ordinary least squares regression (OLS) of any hydrological or water quality variable is a traditional and commonly used record extension technique. However, OLS tends to underestimate the variance in the extended records, which leads to underestimation of high percentiles and overestimation of low percentiles, given that the data is normally distributed. The development of the line of organic correlation (LOC) technique is aimed at correcting this bias. On the other hand, the Kendall-Theil robust line (KTRL) method has been proposed as an analogue of OLS with the advantage of being robust in the presence of outliers. Given that water quality data are characterised by the presence of outliers, positive skewness and non-normal distribution of data, a robust record extension technique is more appropriate. In this paper, four record-extension techniques are described, and their properties are explored. These techniques are OLS, LOC, KTRL and a new technique proposed in this paper, the robust line of organic correlation technique (RLOC). RLOC includes the advantage of the LOC in reducing the bias in estimating the variance, but at the same time it is also robust to the presence of outliers. A Monte Carlo study and empirical experiment were conducted to examine the four techniques for the accuracy and precision of the estimate of statistical moments and over the full range of percentiles. Results of the Monte Carlo study showed that the OLS and KTRL techniques have serious deficiencies as record-extension techniques, while the LOC and RLOC techniques are nearly similar. However, RLOC outperforms OLS, KTRL and LOC when using real water quality records.

Description: Multi-criteria parameter estimation for the unified land model Hydrology and Earth System Sciences Discussions, 9, 4417-4463, 2012 Author(s): B. Livneh and D. P. Lettenmaier We describe a parameter estimation framework for the Unified Land Model (ULM) that utilizes multiple independent data sets over the Continental United States. These include a satellite-based evapotranspiration (ET) product based on MODerate resolution Imaging Spectroradiometer (MODIS) and Geostationary Operation Environmental Satellites (GOES) imagery, an atmospheric-water balance based ET estimate that utilizes North American Regional Reanalysis (NARR) atmospheric fields, terrestrial water storage content (TWSC) data from the Gravity Recovery and Climate Experiment (GRACE), and streamflow ( Q ) primarily from the United States Geological Survey (USGS) stream gauges. The study domain includes 10 large-scale (≥10 5 km 2 ) river basins and 250 smaller-scale (

Description: Towards an integrated soil moisture drought monitor for East Africa Hydrology and Earth System Sciences Discussions, 9, 4587-4631, 2012 Author(s): W. B. Anderson, B. F. Zaitchik, C. R. Hain, M. C. Anderson, M. T. Yilmaz, J. Mecikalski, and L. Schultz Drought in East Africa is a recurring phenomenon with significant humanitarian impacts. Given the steep climatic gradients, topographic contrasts, general data scarcity, and, in places, political instability that characterize the region, there is a need for spatially distributed, remotely derived monitoring systems to inform national and international drought response. At the same time, the very diversity and data scarcity that necessitate remote monitoring also make it difficult to evaluate the reliability of these systems. Here we apply a suite of remote monitoring techniques to characterize the temporal and spatial evolution of the 2010–2011 Horn of Africa drought. Diverse satellite observations allow for evaluation of meteorological, agricultural, and hydrological aspects of drought, each of which is of interest to different stakeholders. Focusing on soil moisture, we apply triple collocation analysis (TCA) to three independent methods for estimating soil moisture anomalies to characterize relative error between products and to provide a basis for objective data merging. The three soil moisture methods evaluated include microwave remote sensing using the Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) sensor, thermal remote sensing using the Atmosphere-Land Exchange Inverse (ALEXI) surface energy balance algorithm, and physically-based land surface modeling using the Noah land surface model. It was found that the three soil moisture monitoring methods yield similar drought anomaly estimates in areas characterized by extremely low or by moderate vegetation cover, particularly during the below-average 2011 long rainy season. Systematic discrepancies were found, however, in regions of moderately low vegetation cover and high vegetation cover, especially during the failed 2010 short rains. The merged, TCA-weighted soil moisture composite product takes advantage of the relative strengths of each method, as judged by the consistency of anomaly estimates across independent methods. This approach holds potential as a remote soil moisture-based drought monitoring system that is robust across the diverse climatic and ecological zones of East Africa.

Description: Modeling water resources trends in Middle East and North Africa towards 2050 Hydrology and Earth System Sciences Discussions, 9, 4381-4416, 2012 Author(s): P. Droogers, W. W. Immerzeel, W. Terink, J. Hoogeveen, M. F. P. Bierkens, L. P. H. van Beek, and B. D. Negewo Changes in water resources availability can be expected as consequences of climate change, population growth, economic development and environmental considerations. A two-stage modeling approach is used to explore the impact of these changes in the Middle East and North Africa (MENA) region. An advanced physical based distributed hydrological model is applied to determine the internal and external renewable water resources for the current situation and under future changes. Subsequently, a water allocation model is used to combine the renewable water resources with sectorial water demands. Results show that total demand in the region will increase to 132 km 3 yr −1 in 2050, while total water shortage will grow to 199 km 3 yr −1 in 2050 for the average climate change projection; an increase of 157 km 3 . This increase in shortage is the combined impact of an increase in water demand by 50% with a decrease in water supply by 12%. Uncertainty based on the output of the nine GCMs applied, reveals that expected water shortage ranges from 85 km 3 to 283 km 3 in 2050. The analysis shows that 22% of the water shortage can be attributed to climate change and 78% to changes in socio-economic factors.

Description: Fluorescent particle tracers for surface flow measurements: a proof of concept in a semi-natural hillslope Hydrology and Earth System Sciences Discussions, 9, 4465-4503, 2012 Author(s): F. Tauro, S. Grimaldi, A. Petroselli, M. C. Rulli, and M. Porfiri In this paper, a proof of concept experiment is conducted to assess the feasibility of tracing overland flow on a semi-natural hillslope plot via a novel fluorescent particle tracer. Runoff on the experimental plot is artificially simulated by using a custom-built rainfall system. Experiments are performed by using beads of diameters ranging from 75 to 1180 μm that are sensed through an experimental apparatus comprising a light source and a video acquisition unit. Particles' transit is detected through an unsupervised methodology based on image analysis techniques and compared to results from supervised visual inspection. Average flow velocity estimations are obtained from travel time measurements of the particles as they are dragged by the overland flow on the hillslope. Velocities are compared to flow measurements obtained using rhodamine dye. Experimental findings demonstrate the potential of the methodology for understanding overland flow dynamics in complex natural settings. In addition, considerations for optimizing the particle size are presented based on the visibility of the beads and their accuracy in flow tracing.

Description: Integration of SRTM and TRMM date into the GIS-based hydrological model for the purpose of flood modelling Hydrology and Earth System Sciences Discussions, 9, 4747-4775, 2012 Author(s): A. Akbari, A. Abu Samah, and F. Othman Due to land use and climate changes, more severe and frequent floods occur worldwide. Flood simulation as the first step in flood risk management can be robustly conducted with integration of GIS, RS and flood modeling tools. The primary goal of this research is to examine the practical use of public domain satellite data and GIS-based hydrologic model. Firstly, database development process is described. GIS tools and techniques were used in the light of relevant literature to achieve the appropriate database. Watershed delineation and parameterizations were carried out using cartographic DEM derived from digital topography at a scale of 1:25 000 with 30 m cell size and SRTM elevation data at 30 m cell size. The SRTM elevation dataset is evaluated and compared with cartographic DEM. With the assistance of statistical measures such as Correlation coefficient ( r ), Nash-Sutcliffe efficiency (NSE), Percent Bias (PBias) or Percent of Error (PE). According to NSE index, SRTM-DEM can be used for watershed delineation and parameterization with 87% similarity with Topo-DEM in a complex and underdeveloped terrains. Primary TRMM (V6) data was used as satellite based hytograph for rainfall-runoff simulation. The SCS-CN approach was used for losses and kinematic routing method employed for hydrograph transformation through the reaches. It is concluded that TRMM estimates do not give adequate information about the storms as it can be drawn from the rain gauges. Event-based flood modeling using HEC-HMS proved that SRTM elevation dataset has the ability to obviate the lack of terrain data for hydrologic modeling where appropriate data for terrain modeling and simulation of hydrological processes is unavailable. However, TRMM precipitation estimates failed to explain the behavior of rainfall events and its resultant peak discharge and time of peak.

Description: Dryland ecohydrology and climate change: critical issues and technical advances Hydrology and Earth System Sciences Discussions, 9, 4777-4825, 2012 Author(s): L. Wang, P. D'Odorico, J. P. Evans, D. Eldridge, M. F. McCabe, K. K. Caylor, and E. G. King Drylands cover about 40% of the terrestrial land surface and account for approximately 40% of global net primary productivity. Water is fundamental to the biophysical processes that sustain ecosystem function and food production, particularly in drylands, where a tight coupling exists between water resource availability and ecosystem productivity, surface energy balance, and biogeochemical cycles. Currently, drylands support at least 2 billion people and comprise both natural and managed ecosystems. In this synthesis, we identify some current critical issues in the understanding of dryland systems and discuss how arid and semiarid environments are responding to the changes in climate and land use. Specifically, we focus on dryland agriculture and food security, dryland population growth, desertification, shrub encroachment and dryland development issues as factors of change requiring increased understanding and management. We also review recent technical advances in the quantitative assessment of human versus climate change related drivers of desertification, evapotranspiration partitioning using field deployable stable water isotope systems and the remote sensing of key ecohydrological processes. These technological advances provide new tools that assist in addressing major critical issues in dryland ecohydrology under climate change

Description: Quantifying heterogeneous transport of a tracer and a degradable contaminant in the field, under two infiltration rates Hydrology and Earth System Sciences Discussions, 9, 4827-4868, 2012 Author(s): D. Schotanus, M. J. van der Ploeg, and S. E. A. T. M. van der Zee To examine the persistence of preferential flow paths in a field soil, and to compare the leaching of a degradable contaminant with the leaching of a non-degradable tracer, we did two field experiments, using a multicompartment sampler. The first experiment was done during the snowmelt period in early spring, characterized by high infiltration fluxes from snowmelt. The second experiment was done in early summer with irrigation to mimic homogeneous rainfall. In the second experiment, the soil was warmer and degradation of the degradable contaminant was observed. For both experiments, the highest tracer concentrations were found in the same area of the sampler, but the leached tracer masses of the individual locations were not highly correlated. Thus, the preferential flow paths were stable between seasons. With a lower infiltration rate, in the second experiment, more isolated peaks in the drainage and the leached masses were found than in the first experiment. Therefore it is concluded that the soil heterogeneity is mainly caused by local differences in the soil hydraulic properties, and not by macropores. With higher infiltration rates, the clustering of high and low leaching cells was higher. The leached masses of the degradable contaminant were lower than the leached masses of the non-degradable tracer, but the masses were highly correlated. The first-order degradation rate was 0.02 d −1 . The dispersivity varied between 1.9 and 7.1 cm. Soil heterogeneity is the main reason for the heterogeneous water flow and solute transport in this soil. Heterogeneous melting of snow does not influence the heterogeneous flow in the soil much at this scale.

Description: SPITFIRE-2: an improved fire module for Dynamic Global Vegetation Models Geoscientific Model Development Discussions, 5, 2347-2443, 2012 Author(s): M. Pfeiffer and J. O. Kaplan Fire is the primary disturbance factor in many terrestrial ecosystems. Wildfire alters vegetation structure and composition, affects carbon storage and biogeochemical cycling, and results in the release of climatically relevant trace gases, including CO 2 , CO, CH 4 , NO x , and aerosols. Assessing the impacts of global wildfire on centennial to multi-millennial timescales requires the linkage of process-based fire modeling with vegetation modeling using Dynamic Global Vegetation Models (DGVMs). Here we present a new fire module, SPITFIRE-2, and an update to the LPJ-DGVM that includes major improvements to the way in which fire occurrence, behavior, and the effect of fire on vegetation is simulated. The new fire module includes explicit calculation of natural ignitions, the representation of multi-day burning and coalescence of fires and the calculation of rates of spread in different vegetation types, as well as a simple scheme to model crown fires. We describe a new representation of anthropogenic biomass burning under preindustrial conditions that distinguishes the way in which the relationship between humans and fire are different between hunter-gatherers, obligate pastoralists, and farmers. Where and when available, we evaluate our model simulations against remote-sensing based estimates of burned area. While wildfire in much of the modern world is largely influenced by anthropogenic suppression and ignitions, in those parts of the world where natural fire is still the dominant process, e.g. in remote areas of the boreal forest, our results demonstrate a significant improvement in simulated burned area over previous models. With its unique properties of being able to simulate preindustrial fire, the new module we present here is particularly well suited for the investigation of climate-human-fire relationships on multi-millennial timescales.

Description: The chemical signature of a livestock farming catchment: synthesis from a high-frequency multi-element long term monitoring Hydrology and Earth System Sciences Discussions, 9, 9715-9741, 2012 Author(s): A. H. Aubert, C. Gascuel-Odoux, G. Gruau, J. Molénat, M. Faucheux, Y. Fauvel, C. Grimaldi, Y. Hamon, A. Jaffrézic, M. Lecoz-Boutnik, P. Petitjean, L. Ruiz, and Ph. Merot Assessing the impact of human pressures on water quality is difficult. First, there is a high temporal and spatial variability of climate and human activity. Second, chemical elements have their own characteristics mixing short and long term dynamics. High frequency, long-term and multi-element measurements are required. But, such data series are scarce. This paper aims at determining what the hydro-chemical particularities of a livestock farming catchment are in a temperate climatic context. It is based on an original and never published time series, from Kervidy-Naizin headwater catchment. Stream chemistry was monitored daily and shallow groundwater roughly every four month, for 10 yr and five elements (nitrate, sulphate, chloride, and dissolved organic and inorganic carbon). The five elements present strong but different seasonal patterns. Nitrate and chloride present a seasonal flush, all along or at the beginning of the wet season, respectively. Sulphate, organic and inorganic carbon present storm flushes, with constant or decreasing peaks throughout the wet season. These depicted nitrate and chloride patterns are typical of a livestock farming catchment. There, nitrate and chloride coming from organic fertilisation have been accumulating over years in the shallow groundwater. They are seasonally flushed when the groundwater connects to the stream. Sulphate, organic and inorganic carbon patterns do not seem specific to agricultural catchments. These elements are produced each year and flushed by storms. Finally, a generic classification of temporal patterns and elements is established for agricultural catchments. It is based on the distance of the source component to the stream and the dominant controlling process (accumulation versus production). This classification could be applied to any chemical element and help assessing the level of water disturbances.

Description: Natural vs. artificial groundwater recharge, quantification through inverse modeling Hydrology and Earth System Sciences Discussions, 9, 9767-9807, 2012 Author(s): H. Hashemi, R. Berndtsson, M. Kompani-Zare, and M. Persson Estimating the change in groundwater recharge from an introduced artificial recharge system is important in order to evaluate future water availability. This paper presents an inverse modeling approach to quantify the recharge contribution from both an ephemeral river channel and an introduced artificial recharge system based on floodwater spreading in arid Iran. The study used the MODFLOW-2000 to estimate recharge for both steady and unsteady-state conditions. The model was calibrated and verified based on the observed hydraulic head in observation wells and model precision, uncertainty, and model sensitivity were analyzed in all modeling steps. The results showed that in a normal year without extreme events the floodwater spreading system is the main contributor to recharge with 80% and the ephemeral river channel with 20% of total recharge in the studied area. Uncertainty analysis revealed that the river channel recharge estimation represents relatively more uncertainty in comparison to the artificial recharge zones. The model is also less sensitive to the river channel. The results show that by expanding the artificial recharge system the recharge volume can be increased even for small flood events while the recharge through the river channel increases only for major flood events.

Description: The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP): overview and description of models, simulations and climate diagnostics Geoscientific Model Development Discussions, 5, 2445-2502, 2012 Author(s): J.-F. Lamarque, D. T. Shindell, B. Josse, P. J. Young, I. Cionni, V. Eyring, D. Bergmann, P. Cameron-Smith, W. J. Collins, R. Doherty, S. Dalsoren, G. Faluvegi, G. Folberth, S. J. Ghan, L. W. Horowitz, Y. H. Lee, I. A. MacKenzie, T. Nagashima, V. Naik, D. Plummer, M. Righi, S. Rumbold, M. Schulz, R. B. Skeie, D. S. Stevenson, S. Strode, K. Sudo, S. Szopa, A. Voulgarakis, and G. Zeng The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) consists of a series of timeslice experiments targeting the long-term changes in atmospheric composition between 1850 and 2100, with the goal of documenting radiative forcing and the associated composition changes. Here we introduce the various simulations performed under ACCMIP and the associated model output. The ACCMIP models have a wide range of horizontal and vertical resolutions, vertical extent, chemistry schemes and interaction with radiation and clouds. While anthropogenic and biomass burning emissions were specified for all time slices in the ACCMIP protocol, it is found that the natural emissions lead to a significant range in emissions, mostly for ozone precursors. The analysis of selected present-day climate diagnostics (precipitation, temperature, specific humidity and zonal wind) reveals biases consistent with state-of-the-art climate models. The model-to-model comparison of changes in temperature, specific humidity and zonal wind between 1850 and 2000 and between 2000 and 2100 indicates mostly consistent results, but with outliers different enough to possibly affect their representation of climate impact on chemistry.

Description: Effects of rating-curve uncertainty on probabilistic flood mapping Hydrology and Earth System Sciences Discussions, 9, 9809-9845, 2012 Author(s): A. Domeneghetti, S. Vorogushyn, A. Castellarin, B. Merz, and A. Brath Comprehensive flood risk assessment studies should quantify the global uncertainty in flood hazard estimation, for instance by mapping inundation extents together with their confidence intervals. This appears of particular importance in case of flood hazard assessments along dike-protected reaches where the possibility of occurrence of dike failures may considerably enhance the uncertainty. We present a methodology to derive probabilistic flood maps in dike-protected flood prone areas, where several sources of uncertainty are taken into account. In particular, this paper focuses on a 50 km reach of River Po (Italy) and three major sources of uncertainty in hydraulic modelling and flood mapping: uncertainties in the (i) upstream and (ii) downstream boundary conditions, and (iii) uncertainties in dike failures. Uncertainties in the definition of upstream boundary conditions (i.e. design-hydrographs) are assessed by applying different bivariate copula families to model the frequency regime of flood peaks and volumes. Uncertainties in the definition of downstream boundary conditions are characterised by associating the rating-curve used as downstream boundary condition with confidence intervals which reflect discharge measurements errors and interpolation errors. The effects of uncertainties in boundary conditions and randomness of dike failures are assessed by means of the Inundation Hazard Assessment Model (IHAM), a recently proposed hybrid probabilistic-deterministic model that considers three different failure mechanisms: overtopping, piping and micro-instability due to seepage. The results of the study show that the IHAM-based analysis enables probabilistic flood hazard mapping and provides decision makers with a fundamental piece of information for devising and implementing flood risk mitigation strategies in the presence of various sources of uncertainty.

Description: Conceptual and numerical modeling of the Guaraní Aquifer System Hydrology and Earth System Sciences Discussions, 9, 9885-9930, 2012 Author(s): L. Rodríguez, L. Vives, and A. Gomez In large aquifers relevant for their considerable size, regional groundwater modeling remains challenging given geologic complexity and data scarcity in space and time. The Guaraní Aquifer System is the largest transboundary aquifer in South America. It contains an enormous volume of water, however, it is not well known being difficult to assess the impact of exploitation currently used to supply over 25 million inhabitants. This is a sensitive issue because the aquifer is shared by four countries. Moreover, an integrated groundwater model, and therefore, a global water balance were not available. In this work, a transient regional scale model for the entire aquifer based upon five simplified, equally plausible conceptual models represented by different hydraulic conductivity parametrizations, is used to analyze the flow system and water balance components. Combining an increasing number of hydraulic conductivity zones and an appropriate set of boundary conditions, the hypothesis of a continuous sedimentary unit yielded errors within the calibration target in a regional sense. The magnitude of the water budget terms resulted very similar for all parametrizations. Recharge and stream/aquifer fluxes were the dominant components representing, on average, 84.2% of total inflows and 61.4% of total outflows, respectively. However, leakage was small compared to stream discharges of main rivers. For instance, the simulated average leakage for the Uruguay river was 8 m 3 s −1 while the observed absolute minimum discharge was 382 m 3 s −1 . Streams located in heavily pumped regions switched from a gaining condition on early years to a losing condition over time. Water is discharged through the aquifer boundaries, except at the eastern boundary. On average, pumping represented 16.2% of inflows while aquifer storage experienced a small overall increment. The model water balance indicates that the current rate of groundwater withdrawals does not exceed the rate of recharge on a regional sense.

Description: Trends in timing and magnitude of flow in the Upper Indus Basin Hydrology and Earth System Sciences Discussions, 9, 9931-9966, 2012 Author(s): M. Sharif, D. R. Archer, H. J. Fowler, and N. Forsythe River flow is a reflection of the input of moisture and its transformation in storage and transmission over the catchment. In the Upper Indus Basin (UIB), since high altitude climate measurement and observations of glacier mass balance are weak or absent, analysis of trends in magnitude and timing in river flow provides a window on trends and fluctuations in climate and glacier outflow. Trend analysis is carried out using a Mann-Kendall nonparametric trend test on records extending from 1960 to 1998. High level glacial catchments show a falling trend in runoff magnitude and a declining proportion of glacial contribution to the main stem of the Indus. Elsewhere annual flow has predominantly increased with several stations exhibiting statistically significant positive trends. Analysis of timing using spring onset date (SOT) and centre of volume date (CoV) indicated no clear trends – in direct contrast to what has been observed in Western North America. There is, however, a consistent relationship between CoV and annual runoff volume. A consistently positive correlation was also found between SOT and CoV for all the stations implying that initial snowpack conditions before the onset of runoff influence timing throughout the season. The results of the analysis presented here indicate that the magnitude and timing of streamflow hydrograph is influenced both by the initial snowpack and by seasonally varied trends in temperature. The study contributes to the understanding of the links between climate trends and variability and river runoff and glacier mass balance and runoff. The Upper Indus Basin is predominantly influenced by winter precipitation; similar trend analysis applied to summer monsoon dominated catchments of the Central Himalaya is recommended.

Description: On the nature of rainfall intermittency as revealed by different metrics and sampling approaches Hydrology and Earth System Sciences Discussions, 9, 9967-10009, 2012 Author(s): G. Mascaro, R. Deidda, and M. Hellies A general consensus on the concept of rainfall intermittency has not yet been reached and intermittency is often referred to different aspects of rainfall variability, including the fragmentation of the wet-dry rainfall support, the strength of intensity fluctuations and of rainfall bursts, and sometimes also to the memory/correlation of the process. To explore these different aspects, a systematic analysis of rainfall intermittency properties in the time domain is presented using high-resolution (1-min) data recorded by a network of 201 tipping-bucket gages covering the entire island of Sardinia (Italy). Four techniques, including spectral and scale invariance analysis, and computation of clustering and intermittency exponents, are applied to quantify the contribution to the overall intermittency due to the alternation of dry and wet periods (i.e. the rainfall support fragmentation), and to the fluctuations of intensity amplitudes. The presence of three ranges of scaling regimes in the time interval from 1 min to ~45 days is first demonstrated. In accordance with past studies, these regimes can be associated with a range dominated by single storms, a regime typical of frontal systems, and a transition zone. The position of the breaking points separating these regimes change with the considered technique, suggesting that such tools explain different aspects of rainfall variability. Results indicate that the intermittency properties of rainfall support are fairly similar across the island, while metrics related to rainfall intensity fluctuations are characterized by significant spatial variability, implying that the local climate has a significant effect on the fluctuations of the rainfall amplitudes and minimal influence on the process of rainfall occurrence. Next, evidence is shown of spatial patterns of the scaling exponents computed, for each analysis tool, in the range of frontal systems. These patterns resemble the main pluviometric regimes observed in the island and, thus, can be associated with the corresponding synoptic circulation patterns. Last but not least, it is demonstrated how the methodology adopted to sample the rainfall signal from the records of the tipping instants can significantly affect the intermittency analysis, especially at smaller scales. The multifractal scale invariance analysis is the only tool that is insensitive to the sampling approach. Results of this work may be useful to improve the calibration of stochastic algorithms used to downscale coarse rainfall predictions of climate and weather forecasting models, as well as the parameterization of intensity-duration-frequency curves, adopted for land planning and design of civil infrastructures.

Description: Calibration of a transient transport model to tritium measurements in rivers and streams in the Western Lake Taupo catchment, New Zealand Hydrology and Earth System Sciences Discussions, 9, 9743-9765, 2012 Author(s): M. A. Gusyev, M. Toews, U. Morgenstern, M. Stewart, C. Daughney, and J. Hadfield Here we present a general approach of calibrating transient transport models to tritium concentrations in river waters developed for the MT3DMS/MODFLOW model of the Western Lake Taupo catchment, New Zealand. Tritium is a time-dependent tracer with radioactive half-life of 12.32 yr. In the transport model, the tritium input (measured in rain) passes through the groundwater system, and the modelled tritium concentrations are compared to the measured tritium concentrations in the river outlets for the Waihaha, Whanganui, Whareroa, Kuratau and Omori river catchments from 2000–2007. For the Kuratau River, tritium was also measured between 1960 and 1970, which allowed us to fine-tune the transport model. In order to incorporate all surface flows from rivers to small streams, an 80 m uniform grid cell size was selected in the steady-state MODFLOW model for the model area of 1072 km 2 . The groundwater flow model was first calibrated to groundwater levels and stream flow observations. Then, the transport model was calibrated to the measured tritium concentrations in the river waters. The MT3DMS model results show good agreement with the measured tritium values in all five river catchments. Finally, the calibrated MT3DMS model is applied to simulate groundwater ages that are used to construct groundwater age distributions for the river catchments.

Description: Setup of the PMIP3 paleoclimate experiments conducted using an Earth System Model, MIROC-ESM Geoscientific Model Development Discussions, 5, 2527-2569, 2012 Author(s): T. Sueyoshi, R. Ohgaito, A. Yamamoto, M. O. Chikamoto, T. Hajima, H. Okajima, M. Yoshimori, M. Abe, R. O'ishi, F. Saito, S. Watanabe, M. Kawamiya, and A. Abe-Ouchi The importance of climate model evaluation using paleoclimate simulations for better future climate projections has been recognized by the Intergovernmental Panel on Climate Change. In recent years, Earth System Models (ESMs) were developed to investigate carbon-cycle climate feedback, as well as to project the future climate. Paleoclimate events, especially those associated with the variations in atmospheric CO 2 level or land vegetation, provide suitable benchmarks to evaluate ESMs. Here we present implementations of the paleoclimate experiments proposed by the Coupled Model Intercomparison Project phase 5/Paleoclimate Modelling Intercomparison Project phase 3 (CMIP5/PMIP3) using an Earth System Model, MIROC-ESM. In this paper, experimental settings and procedures of the mid-Holocene, the Last Glacial Maximum, and the Last Millennium experiments are explained. The first two experiments are time slice experiments and the last one is a transient experiment. The complexity of the model requires various steps to correctly configure the experiments. Several basic outputs are also shown.

Description: Comparing dynamical, stochastic and combined downscaling approaches – lessons from a case study in the Mediterranean region Hydrology and Earth System Sciences Discussions, 9, 9847-9884, 2012 Author(s): N. Guyennon, E. Romano, I. Portoghese, F. Salerno, S. Calmanti, A. B. Petrangeli, G. Tartari, and D. Copetti Various downscaling techniques have been developed to bridge the scale gap between global climate models (GCMs) and finer scales required to assess hydrological impacts of climate change. Such techniques may be grouped into two downscaling approaches: the deterministic dynamical downscaling (DD) and the stochastic statistical downscaling (SD). Although SD has been traditionally seen as an alternative to DD, recent works on statistical downscaling have aimed to combine the benefits of these two approaches. The overall objective of this study is to examine the relative benefits of each downscaling approach and their combination in making the GCM scenarios suitable for basin scale hydrological applications. The case study presented here focuses on the Apulia region (South East of Italy, surface area about 20 000 km 2 ), characterized by a typical Mediterranean climate; the monthly cumulated precipitation and monthly mean of daily minimum and maximum temperature distribution were examined for the period 1953–2000. The fifth-generation ECHAM model from the Max-Planck-Institute for Meteorology was adopted as GCM. The DD was carried out with the Protheus system (ENEA), while the SD was performed through a monthly quantile-quantile transform. The SD resulted efficient in reducing the mean bias in the spatial distribution at both annual and seasonal scales, but it was not able to correct the miss-modeled non-stationary components of the GCM dynamics. The DD provided a partial correction by enhancing the trend spatial heterogeneity and time evolution predicted by the GCM, although the comparison with observations resulted still underperforming. The best results were obtained through the combination of both DD and SD approaches.

Description: Aerosol-climate interactions in the Norwegian Earth System Model – NorESM Geoscientific Model Development Discussions, 5, 2599-2685, 2012 Author(s): A. Kirkevåg, T. Iversen, Ø. Seland, C. Hoose, J. E. Kristjánsson, H. Struthers, A. M. L. Ekman, S. Ghan, J. Griesfeller, E. D. Nilsson, and M. Schulz The objective of this study is to document and evaluate recent changes and updates to the module for aerosols and aerosol-cloud-radiation interactions in the atmospheric module CAM4-Oslo of the Norwegian Earth System Model (NorESM). Particular attention is paid to the role of natural organics, sea salt, and mineral dust in determining the gross aerosol properties as well as the anthropogenic contribution to these properties and the associated direct and indirect radiative forcing. The aerosol module is extended from earlier versions that have been published, and includes life-cycling of sea-salt, mineral dust, particulate sulphate, black carbon, and primary and secondary organics. The impacts of most of the numerous changes since previous versions are thoroughly explored by sensitivity experiments. The most important changes are: modified prognostic sea salt emissions; updated treatment of precipitation scavenging and gravitational settling; inclusion of biogenic primary organics and methane sulphonic acid (MSA) from oceans; almost doubled production of land-based biogenic secondary organic aerosols (SOA); and increased ratio of organic matter to organic carbon (OM / OC) for biomass burning aerosols from 1.4 to 2.6. Compared with in-situ measurements and remotely sensed data, the new treatments of sea salt and dust aerosols give smaller biases in near surface mass concentrations and aerosol optical depth than in the earlier model version. The model biases for mass concentrations are approximately unchanged for sulphate and BC. The enhanced levels of modeled OM yield improved overall statistics, even though OM is still underestimated in Europe and over-estimated in North America. The global direct radiative forcing (DRF) at the top of the atmosphere has changed from a small positive value to −0.08 W m −2 in CAM4-Oslo. The sensitivity tests suggest that this change can be attributed to the new treatment of biomass burning aerosols and gravitational settling. Although it has not been a goal in this study, the new DRF estimate is closer both to the median model estimate from the AeroCom inter-comparison and the best estimate in IPCC AR4. Estimated DRF at the ground surface has increased by ca. 60%, to −1.89 W m −2 . We show that this can be explained by new emission data and omitted mixing of constituents between updrafts and downdrafts in convective clouds. The increased abundance of natural OM and the introduction of a cloud droplet spectral dispersion formulation are the most important contributions to a considerably decreased estimate of the indirect radiative forcing (IndRF). The IndRF is also found to be sensitive to assumptions about the coating of insoluble aerosols by sulphate and OM. The IndRF of −1.2 W m −2 , which is closer to the IPCC AR4 estimates than the previous estimate of −1.9 W m −2 , has thus been obtained without imposing unrealistic artificial lower bounds on cloud droplet number concentrations.

Description: Inclusion of Ash and SO 2 emissions from volcanic eruptions in WRF-CHEM: development and some applications Geoscientific Model Development Discussions, 5, 2571-2597, 2012 Author(s): M. Stuefer, S. R. Freitas, G. Grell, P. Webley, S. Peckham, and S. A. McKeen We describe a new functionality within the Weather Research and Forecasting model with coupled Chemistry (WRF-Chem) that allows simulating emission, transport, dispersion, transformation and sedimentation of pollutants released during volcanic activities. Emissions from both an explosive eruption case and relatively calm degassing situation are considered using the most recent volcanic emission databases. A preprocessor tool provides emission fields and additional information needed to establish the initial three-dimensional cloud umbrella/vertical distribution within the transport model grid, as well as the timing and duration of an eruption. From this source condition, the transport, dispersion and sedimentation of the ash-cloud can be realistically simulated by WRF-Chem using its own dynamics, physical parameterization as well as data assimilation. Examples of model validation include a comparison of tephra fall deposits from the 1989 eruption of Mount Redoubt (Alaska), and the dispersion of ash from the 2010 Eyjafjallajökull eruption in Iceland. Both model applications show good coincidence between WRF-Chem and observations.

Description: Assessing student understanding of physical hydrology Hydrology and Earth System Sciences Discussions, 9, 10095-10113, 2012 Author(s): J. A. Marshall, A. J. Castillo, and M. B. Cardenas Our objective is to characterize and assess upper division and graduate student thinking in hydrology. We accomplish this through development and testing of an assessment tool for a physical hydrology class. Students were asked to respond to two questions that probed understanding and one question that assessed their ability to apply their knowledge. Student and expert responses to the questions were then used to develop a rubric to score responses. Using the rubric, three researchers independently blind-coded the full set of pre and post artifacts, resulting in 89% inter-rater agreement on the pre-tests and 83% agreement on the post-tests. This result has limitations, including the small number of participants who were all from one institution and the fact that the rubric was still under development. Nevertheless, the high inter-rater agreement from a group of experts is significant; the rubric we developed is a potentially useful tool for assessment of learning and understanding physical hydrology.

Description: An effective depression filling algorithm for DEM-based 2-dimensional surface flow modelling Hydrology and Earth System Sciences Discussions, 9, 10011-10051, 2012 Author(s): D. Zhu, Q. Ren, Y. Xuan, Y. Chen, and I. Cluckie The surface runoff process in fluvial/pluvial flood modelling is often simulated employing a two-dimensional (2-D) diffusive wave approximation to described by grid based digital elevation models (DEMs). However, a serious problem of this approach may arise when using a 2-D surface flow model which exchanges flows through adjacent cells, or conventional rink removal algorithms which also allow flow to be exchanged along diagonal directions, due to the existence of artificial depression in DEMs. This study firstly analyses the two types of depressions in DEMs and reviews the current depression filling algorithms with a medium sized basin in South-East England, the Upper Medway Catchment (220 km 2 ) used to demonstrate the depression issue in 2-D surface runoff simulation by MIKE SHE with different DEM resolutions (50 m, 100 m and 200 m). An alternative depression-filling algorithm for 2-D overland flow modelling is developed and evaluated by comparing the simulated flows at the outlet of the catchment. This result suggests that the depression estimates at different grid resolution of DEM highly influences overland flow estimation and the new depression filling algorithm is shown to be effective in tackling this issue when comparing simulations in sink-dominated and sink-free digital elevation models, especially for depressions in relatively flat areas on digital land surface models.

Description: A web-based software tool to estimate unregulated daily streamflow at ungauged rivers Geoscientific Model Development Discussions, 5, 2503-2526, 2012 Author(s): S. A. Archfield, P. A. Steeves, J. D. Guthrie, and K. G. Ries III Streamflow information is critical for solving any number of hydrologic problems. Often times, streamflow information is needed at locations which are ungauged and, therefore, have no observations on which to base water management decisions. Furthermore, there has been increasing need for daily streamflow time series to manage rivers for both human and ecological functions. To facilitate negotiation between human and ecological demands for water, this paper presents the first publically-available, map-based, regional software tool to interactively estimate daily streamflow time series at any user-selected ungauged river location. The map interface allows users to locate and click on a river location, which then returns estimates of daily streamflow for the location selected. For the demonstration region in the northeast United States, daily streamflow was shown to be reliably estimated by the software tool, with efficiency values computed from observed and estimated streamflows ranging from 0.69 to 0.92. The software tool provides a general framework that can be applied to other regions for which daily streamflow estimates are needed.

Description: Flood-initiating catchment conditions: a spatio-temporal analysis of large-scale soil moisture patterns in the Elbe river basin Hydrology and Earth System Sciences Discussions, 9, 10053-10094, 2012 Author(s): M. Nied, Y. Hundecha, and B. Merz Floods are the result of a complex interaction between meteorological event characteristics and pre-event catchment conditions. While the large-scale meteorological conditions have been classified and successfully linked to floods, this is lacking for the large-scale pre-event catchment conditions. Therefore, we propose to classify soil moisture as a key variable of pre-event catchment conditions and to investigate the link between soil moisture patterns and flood occurrence in the Elbe river basin. Soil moisture is simulated using a semi-distributed conceptual rainfall-runoff model over the period 1951–2003. Principal component analysis (PCA) and cluster analysis are applied successively to identify days of similar soil moisture patterns. The results show that PCA considerably reduced the dimensionality of the soil moisture data. The first principal component (PC) explains 75.71% of the soil moisture variability and represents the large-scale seasonal wetting and drying. The successive PCs express the spatial heterogeneous antecedent catchment conditions. By clustering the leading PCs, we detected large-scale soil moisture patterns which frequently occur before the onset of floods. In winter floods are initiated by overall high soil moisture content whereas in summer the flood initiating soil moisture patterns are diverse and less stable in time. The results underline the importance of large-scale pre-event catchment conditions in flood initiation.

Description: Circulation pattern based parameterization of a multiplicative random cascade for disaggregation of daily rainfall under nonstationary climatic conditions Hydrology and Earth System Sciences Discussions, 9, 10115-10149, 2012 Author(s): D. Lisniak, J. Franke, and C. Bernhofer The use of multiplicative random cascades (MRCs) for temporal rainfall disaggregation has been extensively studied in the past. MRCs are appealing for rainfall disaggregation due to their formal simplicity and the possibility to extract the model parameters directly from observed high resolution rainfall data. These parameters, however, represent the rainfall characteristics of the observation period. Since rainfall characteristics of different time slices are changing due to climate variability, we propose a parameterization approach for MRCs to adjust the parameters according to past (observed) or future (projected) time series. This is done on the basis of circulation patterns (CPs) by extracting a distinct MRC parameterization from high resolution rainfall data, as observed on days governed by each individual CP. The parameterization approach is tested by comparing the statistical properties of disaggregated rainfall time series of two time slices, 1969–1979 and 1989–1999, to the results obtained by two other disaggregation methods (a conceptually similar MRC without CP-based parameterization and a recombination approach) and to the statistical properties of observed hourly rainfall data. In this context, all three approaches use rainfall data of the time slice 1989–1999 for parameterization. We found that the inclusion of CPs into the parameterization of a MRC yields hourly time series that better reproduce the properties of observed rainfall in time slice 1989–1999, as compared to the simple MRC. Despite similar properties of both MRCs for the time slice 1969–1979, we can conclude that the CP-based parameterization approach is applicable for temporal rainfall disaggregation in time slices distinct from the parameterization period. This approach accounts for changes in rainfall characteristics due to changes in the frequency of occurrence of the CPs and allows generating hourly rainfall from daily data, as often provided by a statistical downscaling of global climate change.

Description: Complexity versus simplicity: an example of groundwater model ranking with the Akaike Information Criterion Hydrology and Earth System Sciences Discussions, 9, 9687-9714, 2012 Author(s): I. Engelhardt, J. G. De Aguinaga, H. Mikat, C. Schüth, O. Lenz, and R. Liedl A groundwater model characterized by a lack of field data to estimate hydraulic model parameters and boundary conditions combined with many piezometric head observations was investigated concerning model uncertainty. Different conceptual models with a stepwise increase from 0 to 30 adjustable parameters were calibrated using PEST. Residuals, sensitivities, the Akaike Information Criterion (AIC), and the likelihood of each model were computed. As expected, residuals and standard errors decreased with an increasing amount of adjustable model parameters. However, the model with only 15 adjusted parameters was evaluated by AIC as the best option with a likelihood of 98%, while the uncalibrated model obtained the worst AIC value. Computing of the AIC yielded the most important information to assess the model likelihood. Comparing only residuals of different conceptual models was less valuable and would result in an overparameterization of the conceptual model approach. Sensitivities of piezometric heads were highest for the model with five adjustable parameters reflecting also changes of extracted groundwater volumes. With increasing amount of adjustable parameters piezometric heads became less sensitive for the model calibration and changes of pumping rates were no longer displayed by the sensitivity coefficients. Therefore, when too many model parameters were adjusted, these parameters lost their impact on the model results. Additionally, using only sedimentological data to derive hydraulic parameters resulted in a large bias between measured and simulated groundwater level.

Description: A framework for global river flood risk assessments Hydrology and Earth System Sciences Discussions, 9, 9611-9659, 2012 Author(s): H. C. Winsemius, L. P. H. Van Beek, B. Jongman, P. J. Ward, and A. Bouwman There is an increasing need for strategic global assessments of flood risks in current and future conditions. In this paper, we propose a framework for global flood risk assessment for river floods, which can be applied in current conditions, as well as in future conditions due to climate and socio-economic changes. The framework's goal is to establish flood hazard and impact estimates at a high enough resolution to allow for their combination into a risk estimate. The framework estimates hazard at high resolution (~1 km 2 ) using global forcing datasets of the current (or in scenario mode, future) climate, a global hydrological model, a global flood routing model, and importantly, a flood extent downscaling routine. The second component of the framework combines hazard with flood impact models at the same resolution (e.g. damage, affected GDP, and affected population) to establish indicators for flood risk (e.g. annual expected damage, affected GDP, and affected population). The framework has been applied using the global hydrological model PCR-GLOBWB, which includes an optional global flood routing model DynRout, combined with scenarios from the Integrated Model to Assess the Global Environment (IMAGE). We performed downscaling of the hazard probability distributions to 1 km 2 resolution with a new downscaling algorithm, applied on Bangladesh as a first case-study application area. We demonstrate the risk assessment approach in Bangladesh based on GDP per capita data, population, and land use maps for 2010 and 2050. Validation of the hazard and damage estimates has been performed using the Dartmouth Flood Observatory database and damage estimates from the EM-DAT database and World Bank sources. We discuss and show sensitivities of the estimated risks with regard to the use of different climate input sets, decisions made in the downscaling algorithm, and different approaches to establish impact models.

Description: Hydrogeological settings of a volcanic island (San Cristóbal, Galapagos) from joint interpretation of airborne electromagnetics and geomorphological observations Hydrology and Earth System Sciences Discussions, 9, 9661-9686, 2012 Author(s): A. Pryet, N. d'Ozouville, S. Violette, B. Deffontaines, and E. Auken Many volcanic islands face freshwater stress and the situation may worsen with climate change and sea level rise. In this context, an optimum management of freshwater resources becomes crucial, but is often impeded by the lack of data. With the aim of investigating the hydrogeological settings of Southern San Cristóbal Island (Galapagos), we conducted an helicopter-borne, transient electromagnetic survey with the SkyTEM system. It provided unprecedented insights in the 3-D resistivity structure of this extinct basaltic shield. Combined with remote sensing and fieldwork, it allowed the definition of the first hydrogeological conceptual model of the island. Springs are fed by a series of perched aquifers overlying a regional basal aquifer subject to seawater intrusion. Dykes, evidenced by alignments of eruptive cones at the surface, correspond to sharp sub-vertical contrasts in resistivity in the subsurface, and impound groundwater in a summit channel. Combined with geomorphological observations, airborne electromagnetics is shown to be a useful tool for hydrogeological exploratory studies in complex, poorly known environments. It allows optimal development of land-based geophysical surveys and drilling campaigns.

Description: Using multi-model averaging to improve the reliability of catchment scale nitrogen predictions Geoscientific Model Development Discussions, 5, 2289-2310, 2012 Author(s): J.-F. Exbrayat, N. R. Viney, H.-G. Frede, and L. Breuer Hydro-biogeochemical models are used to foresee the impact of mitigation measures on water quality. Usually, scenario-based studies rely on single model applications. This is done in spite of the widely acknowledged advantage of ensemble approaches to cope with structural model uncertainty issues. As an attempt to demonstrate the reliability of such multi-model efforts in the hydro-biogeochemical context, this methodological contribution proposes an adaptation of the Reliability Ensemble Averaging (REA) philosophy to nitrogen losses predictions. A total of 4 models are used to predict the total nitrogen (TN) losses from the well-monitored Ellen Brook catchment in Western Australia. Simulations include re-predictions of current conditions and a set of straightforward management changes targeting fertilization scenarios. Results show that, in spite of good calibration metrics, one of the models provides a very different response to management changes. This behaviour leads the simple average of the ensemble members to also predict reductions in TN export that are not in agreement with the other models. However, considering the convergence of model predictions in the more sophisticated REA approach assigns more weight to previously less well calibrated models that are more in agreement with each other. This method also avoids having to disqualify any of the ensemble members, which is always sensible.

Description: Development of high resolution land surface parameters for the Community Land Model Geoscientific Model Development Discussions, 5, 1435-1481, 2012 Author(s): Y. Ke, L. R. Leung, M. Huang, A. M. Coleman, H. Li, and M. S. Wigmosta There is a growing need for high-resolution land surface parameters as land surface models are being applied at increasingly higher spatial resolution offline as well as in regional and global models. The default land surface parameters for the most recent version of the Community Land Model (i.e. CLM 4.0) are at 0.5° or coarser resolutions, released with the model from the National Center for Atmospheric Research (NCAR). Plant Functional Types (PFTs), vegetation properties such as Leaf Area Index (LAI), Stem Area Index (SAI), and non-vegetated land covers were developed using remotely-sensed datasets retrieved in late 1990's and the beginning of this century. In this study, we developed new land surface parameters for CLM 4.0, specifically PFTs, LAI, SAI and non-vegetated land cover composition, at 0.05° resolution globally based on the most recent MODIS land cover and improved MODIS LAI products. Compared to the current CLM 4.0 parameters, the new parameters produced a decreased coverage by bare soil and trees, but an increased coverage by shrub, grass, and cropland. The new parameters result in a decrease in global seasonal LAI, with the biggest decrease in boreal forests; however, the new parameters also show a large increase in LAI in tropical forest. Differences between the new and the current parameters are mainly caused by changes in the sources of remotely sensed data and the representation of land cover in the source data. The new high-resolution land surface parameters have been used in a coupled land-atmosphere model (WRF-CLM) applied to the western US to demonstrate their use in high-resolution modeling. Future work will include global offline CLMsimulations to examine the impacts of source data resolution and subsequent land parameter changes on simulated land surface processes.

Description: The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1): an extended and updated framework for modeling biogenic emissions Geoscientific Model Development Discussions, 5, 1503-1560, 2012 Author(s): A. B. Guenther, X. Jiang, C. L. Heald, T. Sakulyanontvittaya, T. Duhl, L. K. Emmons, and X. Wang The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1) is a modeling framework for estimating fluxes of 147 biogenic compounds between terrestrial ecosystems and the atmosphere using simple mechanistic algorithms to account for the major known processes controlling biogenic emissions. It is available as an offline code and has also been coupled into land surface models and atmospheric chemistry models. MEGAN2.1 is an update from the previous versions including MEGAN2.0 for isoprene emissions and MEGAN2.04, which estimates emissions of 138 compounds. Isoprene comprises about half of the estimated total global biogenic volatile organic compound (BVOC) emission of 1 Pg (1000 Tg or 10 15 g). Another 10 compounds including methanol, ethanol, acetaldehyde, acetone, α-pinene, β-pinene, t −β-ocimene, limonene, ethene, and propene together contribute another 30% of the estimated emission. An additional 20 compounds (mostly terpenoids) are associated with another 17% of the total emission with the remaining 3% distributed among 125 compounds. Emissions of 41 monoterpenes and 32 sesquiterpenes together comprise about 15% and 3%, respectively, of the total global BVOC emission. Tropical trees cover about 18% of the global land surface and are estimated to be responsible for 60% of terpenoid emissions and 48% of other VOC emissions. Other trees cover about the same area but are estimated to contribute only about 10% of total emissions. The magnitude of the emissions estimated with MEGAN2.1 are within the range of estimates reported using other approaches and much of the differences between reported values can be attributed to landcover and meteorological driving variables. The offline version of MEGAN2.1 source code and driving variables is available from http://acd.ucar.edu/~guenther/MEGAN/MEGAN.htm and the version integrated into the Community Land Model version 4 (CLM4) can be downloaded from http://www.cesm.ucar.edu/ .

Description: Joint impact of rainfall and tidal level on flood risk in a coastal city with a complex river network: a case study for Fuzhou city, China Hydrology and Earth System Sciences Discussions, 9, 7475-7505, 2012 Author(s): J. J. Lian, K. Xu, and C. Ma Coastal cities are particularly vulnerable to flood under the combined effect of multivariable variables, such as heavy rainfall, high sea level and large waves. For better assessment and management of flood risk the combined effect and joint probability should be considered. This paper aims to study the joint impact of rainfall and tidal level on flood risk by estimating the combined risk degree of flood and the joint flood probability. The area of case study is a typical coastal city in China, which has a complex river system. The flood in this city is mainly caused by inundation of river system. In this paper, the combined risk degree of flood is assessed by analyzing the behavior of the complex river network of the city under the combined effect of rainfall and tidal level with diverse return periods. The hydraulic model of the complex drainage network is established using HEC-RAS and verified by comparing the simulation results with the observed data during Typhoon "Longwang". The joint distribution and combined risk probability of rainfall and tidal level are estimated using the optimal copula function. The work carried out in this paper would facilitate assessment of flood risk significantly, which can be referred for the similar cities.

Description: An ensemble approach to assess hydrological models' contribution to uncertainties in the analysis of climate change impact on water resources Hydrology and Earth System Sciences Discussions, 9, 7441-7474, 2012 Author(s): J. A. Velázquez, J. Schmid, S. Ricard, M. J. Muerth, B. Gauvin St-Denis, M. Minville, D. Chaumont, D. Caya, R. Ludwig, and R. Turcotte Over the recent years, several research efforts investigated the impact of climate change on water resources for different regions of the world. The projection of future river flows is affected by different sources of uncertainty in the hydro-climatic modelling chain. One of the aims of the QBic 3 project (Québec-Bavarian International Collaboration on Climate Change) is to assess the contribution to uncertainty of hydrological models by using an ensemble of hydrological models presenting a diversity of structural complexity (i.e. lumped, semi distributed and distributed models). The study investigates two humid, mid-latitude catchments with natural flow conditions; one located in Southern Québec (Canada) and one in Southern Bavaria (Germany). Daily flow is simulated with four different hydrological models, forced by outputs from regional climate models driven by a given number of GCMs' members over a reference (1971–2000) and a future (2041–2070) periods. The results show that the choice of the hydrological model does strongly affect the climate change response of selected hydrological indicators, especially those related to low flows. Indicators related to high flows seem less sensitive on the choice of the hydrological model. Therefore, the computationally less demanding models (usually simple, lumped and conceptual) give a significant level of trust for high and overall mean flows.

Description: Water discharge estimates from large radar altimetry datasets in the Amazon basin Hydrology and Earth System Sciences Discussions, 9, 7591-7611, 2012 Author(s): A. C. V. Getirana and C. Peters-Lidard In this study, we evaluate the use of a large radar altimetry dataset as a complementary gauging network capable of providing water discharge in ungauged regions within the Amazon basin. A rating-curve-based methodology is adopted to derive water discharge from altimetric data provided by Envisat at 444 virtual stations (VS). The stage-discharge relations at VS are built based on radar altimetry and outputs from a global flow routing scheme. In order to quantify the impact of modeling uncertainties on rating-curve based discharges, another experiment is performed using simulated discharges derived from a simplified data assimilation procedure. Discharge estimates at 90 VS are evaluated against observations during the curve fitting calibration (2002–2005) and evaluation (2006–2008) periods, resulting in mean relative RMS errors as high as 52% and 12% for experiments without and with assimilation, respectively. Without data assimilation, uncertainty of discharge estimates can be mostly attributed to forcing errors at smaller scales, generating a positive correlation between performance and drainage area. Mean relative errors (RE) of altimetry-based discharges varied from 15% to 92% for large and small drainage areas, respectively. Rating curves produced a mean RE of 54% versus 68% from model outputs. Assimilating discharge data decreases the mean RE from 68% to 12%. These results demonstrate the feasibility of applying the proposed methodology to the regional or global scales. Also, it is shown the potential of satellite altimetry for predicting water discharge in poorly-gauged and ungauged river basins.

Description: Similarity between runoff coefficient and perennial stream density in the Budyko framework Hydrology and Earth System Sciences Discussions, 9, 7571-7589, 2012 Author(s): D. Wang and L. Wu Streams are categorized into perennial and temporal streams based on flow durations. Perennial stream is the basic network, and temporal stream (ephemeral or intermittent) is the expanded network. Connection between perennial stream and runoff generation at the mean annual scale exists since one of the hydrologic functions of perennial stream is to deliver runoff. The partitioning of precipitation into runoff and evaporation at the mean annual scale, on the first order, is represented by the Budyko hypothesis which quantifies the ratio of evaporation to precipitation ( E / P ) as a function of climate aridity index ( E p / P , ratio of potential evaporation to precipitation). In this paper, it is hypothesized that similarity exists between perennial stream density ( D p ) and runoff coefficient ( Q / P ) as a function of climate aridity index, i.e.,   D p D p * ( E p P ) and   Q P   ( E p P ) where D p * is a scaling factor and Q is mean annual runoff. To test the hypothesis, perennial stream densities for 185 watersheds in the United States are computed based on the high resolution national hydrography dataset (NHD). The similarity between perennial stream density and runoff coefficient is promising based on the case study watersheds. As a potential application for macroscale hydrological modeling, perennial stream density in ungauged basin can be predicted based on climate aridity index using the complementary Budyko curve.

Description: Reframing hydrology education to solve coupled human and environmental problems Hydrology and Earth System Sciences Discussions, 9, 7739-7759, 2012 Author(s): E. G. King, F. C. O'Donnell, and K. K. Caylor The impact of human activity on the biophysical world raises myriad challenges for sustaining earth system processes, ecosystem services, and human societies. To engage in meaningful problem-solving in the hydrosphere, this necessitates an approach that recognizes the coupled nature of human and biophysical systems. We argue that in order to produce the next generation of problem-solvers, hydrology education should ensure that students develop an appreciation and working familiarity in the context of coupled human-environmental systems. We illustrate how undergraduate-level hydrology assignments can extend beyond rote computations or basic throughput scenarios to include consideration of the dynamic interactions with social and other biophysical dimensions of complex adaptive systems. Such an educational approach not only builds appropriate breadth of dynamic understanding, but can also empower students toward assuming influential and effective roles in solving sustainability challenges.

Description: A mechanistic description of the formation and evolution of vegetation patterns Hydrology and Earth System Sciences Discussions, 9, 8737-8798, 2012 Author(s): R. Foti and J. A. Ramírez Vegetation patterns are a common and well-defined characteristic of many landscapes. In this paper we explore some of the physical mechanisms responsible for the establishment of self-organized, non-random vegetation patterns that arise at the hillslope scale in many areas of the world, especially in arid and semi-arid regions. In doing so, we provide a fundamental mechanistic understanding of the dynamics of vegetation pattern formation and development. Reciprocal effects of vegetation on the hillslope thermodynamics, runoff production and run-on infiltration, root density, surface albedo and soil moisture content are analyzed. In particular, we: (1) present a physically based mechanistic description of processes leading to vegetation pattern formation; (2) quantify the relative impact of each process on pattern formation; and (3) describe the relationships between vegetation patterns and the climatic, hydraulic and topographic characteristics of the system. We validate the model by comparing simulations with observed natural patterns in the areas of Niger near Niamey and Somalia near Garoowe. Our analyses suggest that the phenomenon of pattern formation is primarily driven by run-on infiltration and mechanisms of facilitation/inhibition among adjacent vegetation groups mediated by vegetation effects on soil properties and controls on soil moisture and albedo. Nonetheless, even in presence of those mechanisms, patterns arise only when the climatic conditions, particularly annual precipitation and net radiation, are favorable.