ALBERT

Description: Imperfect scaling in distributions of radar-derived rainfall fields Hydrology and Earth System Sciences Discussions, 10, 11385-11422, 2013 Author(s): M. J. van den Berg, L. Delobbe, and N. E. C. Verhoest Fine scale rainfall observations for modeling exercises are often not available, but rather coarser data derived from a variety of sources are used. Effectively using these data sources in models often requires the probability distribution of the data at the applicable scale. Although numerous models for scaling distributions exist, these are often based on theoretical developments, rather than on data. In this study, we develop a model based on the α-stable distribution of rainfall fields, and tested on 5 min radar data from a Belgian weather radar. We use these data to estimate functions that describe parameters of the distribution over various scales. Moreover, we study how the mean of the distribution and the intermittency change with scale, and validate and design functions to describe the shape parameter of the distribution. This information was combined into an effective model of the distribution. Finally, the model was fitted to data from numerous storms, and the resulting parameters were compared to investigate the change in scaling behavior through time.

Description: Spatially resolved information on karst conduit flow from in-cave dye-tracing Hydrology and Earth System Sciences Discussions, 10, 11311-11335, 2013 Author(s): U. Lauber, W. Ufrecht, and N. Goldscheider Artificial tracers are powerful tools to investigate karst systems. Tracers are commonly injected into sinking streams or dolines, while springs serve as monitoring sites. The obtained flow and transport parameters represent mixed information from the vadose, epiphreatic and phreatic zones, i.e., the aquifer remains a black box. Accessible active caves constitute valuable but underexploited natural laboratories to gain detailed insights into the hydrologic functioning of the aquifer. Two multi-tracer tests in the catchment of a major karst spring (Blautopf, Germany) with injections and monitoring in two associated water caves aimed at obtaining spatially and temporally resolved information on groundwater flow in different compartments of the system. Two tracers were injected in the caves to characterize the hydraulic connections between them and with the spring. Two injections at the land surface, far from the spring, aimed at resolving the aquifer's internal drainage structure. Tracer breakthrough curves were monitored by field fluorimeters in caves and at the spring. Results demonstrate the dendritic drainage structure of the aquifer. It was possible to obtain relevant flow and transport parameters for different sections of this system. The highest mean flow velocities (275 m h −1 ) were observed in the near-spring epiphreatic section (open-channel flow), while velocities in the phreatic zone (pressurized flow) were one order of magnitude lower. Determined conduit water volumes confirm results of water balances and hydrograph analyses. In conclusion, experiments and monitoring in caves can deliver spatially resolved information on karst aquifer heterogeneity and dynamics that cannot be obtained by traditional investigative methods.

Description: On the lack of robustness of hydrologic models regarding water balance simulation – a diagnostic approach on 20 mountainous catchments using three models of increasing complexity Hydrology and Earth System Sciences Discussions, 10, 11337-11383, 2013 Author(s): L. Coron, V. Andréassian, C. Perrin, M. Bourqui, and F. Hendrickx This paper investigates the robustness of rainfall–runoff models when their parameters are transferred in time. More specifically, we studied their ability to simulate water balance on periods with different hydroclimatic characteristics. The testing procedure consisted in a series of parameter transfers between 10-yr periods and the systematic analysis of mean-volume errors. This procedure was applied to three conceptual models of different structural complexity over 20 mountainous catchments in southern France. The results showed that robustness problems are common. Errors on 10-yr-mean flows were significant for all three models and calibration periods, even when the entire record was used for calibration. Various graphical and numerical tools were used to show strong similarities between the shapes of mean flow biases calculated on a 10-yr-long sliding window when various parameter sets are used. Unexpected behavioural similarities were observed between the three models tested, considering their large differences in structural complexity. While the actual causes for robustness problems in these models remain unclear, this work stresses the limited transferability in time of the water balance adjustments made through parameter optimization. Although absolute differences between simulations obtained with different calibrated parameter sets were sometimes substantial, relative differences in simulated mean flows between time periods remained similar regardless of the calibrated parameter sets.

Description: Antecedent flow conditions and nitrate concentrations in the Mississippi River Basin Hydrology and Earth System Sciences Discussions, 10, 11451-11484, 2013 Author(s): J. C. Murphy, R. M. Hirsch, and L. A. Sprague The influence of antecedent flow conditions on nitrate concentrations was explored at eight sites in the Mississippi River Basin, USA. Antecedent moisture conditions have been shown to influence nutrient export from small, relatively homogenous basins, but this influence has not been observed at a regional or continental scale. Antecedent flow conditions were quantified as the ratio between the mean daily flow of the previous year and the mean daily flow from the period of record ( Q ratio), and the Q ratio was statistically related to nitrate anomalies (the unexplained variability in nitrate concentration after filtering out season, long-term trend, and contemporaneous flow effects) at each site. Nitrate anomaly and Q ratio were negatively related at three of the four major tributary sites and upstream in the Mississippi River, indicating that when the previous year was drier than average, at these sites, nitrate concentrations were higher than expected. The strength of these relationships increased when data were subdivided by contemporaneous flow conditions. Five of the eight sites had significant negative relationships ( p ≤ 0.05) at high or moderately high contemporaneous flows, suggesting nitrate that accumulates in these basins during a drought is flushed during subsequent storm events. At half of the sites, when flow during the previous year was 50% drier than average, nitrate concentration can be from 9 and 27% higher than nitrate concentrations that follow a year with average daily flow. Conversely, nitrate concentration can be from 8 and 21% lower than expected when the previous year was 50% wetter than average. These relationships between nitrate concentration and Q ratio serve as the basis for future studies that can better define specific hydrologic processes occurring during and after a drought, which influence nitrate concentration, such as the duration or magnitude of low flows, and the timing of low and high flows.

Description: Contribution of snow and glacier melt to discharge for highly glacierised catchments in Norway Hydrology and Earth System Sciences Discussions, 10, 11485-11517, 2013 Author(s): M. Engelhardt, T. V. Schuler, and L. M. Andreassen Glacierised catchments significantly alter the streamflow regime due to snow and glacier meltwater contribution to discharge. In this study, we modelled the mass balance and discharge rates for three highly glacierised catchments (〉50% glacier cover) in western Norway over the period 1961–2012. The spatial pattern of the catchments follows a gradient in climate continentality from west to east. The model uses gridded temperature and precipitation values from seNorge ( http://senorge.no ) as input which are available at a daily resolution. It accounts for accumulation of snow, transformation of snow to firn and ice, evaporation and melt. The model was calibrated for each catchment based on measurements of seasonal glacier mass-balances and daily discharge rates. For validation, daily melt rates were compared with measurements from sonic rangers located in the ablation zones of two of the glaciers and an uncertainty analysis was performed for the third catchment. The discharge contributions from snowmelt, glacier melt and rain were analysed with respect to spatial variations and temporal evolution. The model simulations reveal an increase of the relative contribution from glacier melt for the three catchments from less than 10% in the early 1990s to 15–30% in the late 2000s. The decline in precipitation by 10–20% in the same period was therefore overcompensated resulting in an increase of the annual discharge by 5–20%. Annual discharge sums and annual glacier melt are strongest correlated with annual and winter precipitation at the most maritime glacier and, with increased climate continentality, variations in both glacier melt contribution and annual discharge are becoming stronger correlated with variations in summer temperatures.

Description: Bias correction can modify climate model-simulated precipitation changes without adverse affect on the ensemble mean Hydrology and Earth System Sciences Discussions, 10, 11585-11611, 2013 Author(s): E. P. Maurer and D. W. Pierce When applied to remove climate model biases in precipitation, quantile mapping can in some settings modify the simulated trends. This has important implications when the precipitation will be used to drive an impacts model that is sensitive to changes in precipitation. We use daily precipitation output from 12 general circulation models (GCMs) over the conterminous United States interpolated to a common 1° grid, and gridded observations aggregated to the same scale, to compare precipitation differences before and after quantile mapping bias correction. The change in seasonal mean (winter, DJF, and summer, JJA) precipitation between different 30-yr historical periods is compared to examine (1) the consensus among GCMs as to whether the bias correction tends to amplify or diminish their simulated precipitation trends, and (2) whether the modification of the change in precipitation tends to improve or degrade the correspondence to observed changes in precipitation for the same periods. In some cases, for a particular GCM, the trend modification can be as large as the original simulated change, though the areas where this occurs varies among GCMs so the ensemble median shows smaller trend modification. In specific locations and seasons the trend modification by quantile mapping improves correspondence with observed trends, and in others it degrades it. In the majority of the domain the ensemble median is for little effect on the correspondence of simulated precipitation trends with observed. This highlights the need to use an ensemble of GCMs rather than relying on a small number of models to estimate impacts.

Description: Quantification of model uncertainty in aerosol optical thickness retrieval from Ozone Monitoring Instrument (OMI) measurements Atmospheric Measurement Techniques Discussions, 6, 8509-8541, 2013 Author(s): A. Määttä, M. Laine, J. Tamminen, and J. P. Veefkind We study uncertainty quantification in remote sensing of aerosols in the atmosphere with top of the atmosphere reflectance measurements from the nadir-viewing Ozone Monitoring Instrument (OMI). Focus is on the uncertainty in aerosol model selection of pre-calculated aerosol models and on the statistical modelling of the model inadequacies. The aim is to apply statistical methodologies that improve the uncertainty estimates of the aerosol optical thickness (AOT) retrieval by propagating model selection and model error related uncertainties more realistically. We utilise Bayesian model selection and model averaging methods for the model selection problem and use Gaussian processes to model the smooth systematic discrepancies from the modelled to observed reflectance. The systematic model error is learned from an ensemble of operational retrievals. The operational OMI multi-wavelength aerosol retrieval algorithm OMAERO is used for cloud free, over land pixels of the OMI instrument with the additional Bayesian model selection and model discrepancy techniques. The method is demonstrated with four examples with different aerosol properties: weakly absorbing aerosols, forest fires over Greece and Russia, and Sahara dessert dust. The presented statistical methodology is general; it is not restricted to this particular satellite retrieval application.

Description: The effect of training image and secondary data integration with multiple-point geostatistics in groundwater modeling Hydrology and Earth System Sciences Discussions, 10, 11829-11860, 2013 Author(s): X. He, T. O. Sonnenborg, F. Jørgensen, and K. H. Jensen Multiple-point geostatistic simulation (MPS) has recently become popular in stochastic hydrogeology, primarily because of its capability to derive multivariate distributions from the training image (TI). However, its application in three dimensional simulations has been constrained by the difficulty of constructing 3-D TI. The object-based TiGenerator may be a useful tool in this regard; yet the sensitivity of model predictions to the training image has not been documented. Another issue in MPS is the integration of multiple geophysical data. The best way to retrieve and incorporate information from high resolution geophysical data is still under discussion. This work shows that TI from TiGenerator delivers acceptable results when used for groundwater modeling, although the TI directly converted from high resolution geophysical data leads to better simulation. The model results also indicate that soft conditioning in MPS is a convenient and efficient way of integrating secondary data such as 3-D airborne electromagnetic data, but over conditioning has to be avoided.

Description: Multi-decadal river flows variations in France Hydrology and Earth System Sciences Discussions, 10, 11861-11900, 2013 Author(s): J. Boé and F. Habets In this article, multi-decadal variations in French hydroclimate are investigated, with a specific focus on river flows. Based on long observed series, it is shown that river flows in France generally exhibit large multi-decadal variations on the historical period, especially in spring. Differences of means between two 21 yr periods of the 20th century as large as 40% are indeed found for many gauging stations. Multi-decadal spring river flows variations are associated with variations in spring precipitation and temperature. These multi-decadal variations in precipitation are themselves found to be driven by large-scale atmospheric circulation, more precisely by a multi-decadal oscillation in a sea level pressure dipole between western Europe and the East Atlantic. It is suggested that the Atlantic Multidecadal Variability, the main mode of decadal variability in the North Atlantic/Europe sector, controls those variations in large-scale circulation and is therefore the main ultimate driver of multi-decadal variations in spring river flows. Multi-decadal variations in river flows in other seasons, and in particular summer, are also noted. As they are not associated with significant surface climate anomalies (i.e. temperature, precipitation) in summer, other mechanisms are investigated based on hydrological simulations. The impact of climate variations in spring on summer soil moisture, and the impact of soil moisture in summer on the runoff to precipitation ratio, could potentially play a role in multi-decadal summer river flows variations. The large amplitude of the multi-decadal variations in French river flows suggests that internal variability may play a very important role in the evolution of river flows during the next decades, potentially temporarily limiting, reversing or seriously aggravating the long-term impacts of anthropogenic climate change.

Description: Teleconnection analysis of runoff and soil moisture over the Pearl River basin in South China Hydrology and Earth System Sciences Discussions, 10, 11943-11982, 2013 Author(s): J. Niu, J. Chen, and B. Sivakumar This study explores the teleconnection of two climatic patterns, namely the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), with hydrological processes over the Pearl River basin in South China. The Variable Infiltration Capacity (VIC) model is used to simulate the daily hydrological processes over the basin for the study period 1952–2000, and then, using the simulation results, the time series of the monthly runoff and soil moisture anomalies for its ten sub-basins are aggregated. Wavelet analysis is performed to explore the variability properties of these time series at 49 timescales ranging from 2 months to 9 yr. Use of wavelet coherence and rank correlation method reveals that the dominant variabilities of the time series of runoff and soil moisture are basically correlated with IOD. The influences of ENSO on the terrestrial hydrological processes are mainly found in the eastern sub-basins. The teleconnections between climatic patterns and hydrological variability also serve as a reference basis for inferences on the occurrence of extreme hydrological events (e.g. floods and droughts).

Description: The Greenhouse Gas Climate Change Initiative (GHG-CCI): comparative validation of GHG-CCI SCIAMACHY/ENVISAT and TANSO-FTS/GOSAT CO 2 and CH 4 retrieval algorithm products with measurements from the TCCON network Atmospheric Measurement Techniques Discussions, 6, 8679-8741, 2013 Author(s): B. Dils, M. Buchwitz, M. Reuter, O. Schneising, H. Boesch, R. Parker, S. Guerlet, I. Aben, T. Blumenstock, J. P. Burrows, A. Butz, N. M. Deutscher, C. Frankenberg, F. Hase, O. P. Hasekamp, J. Heymann, M. De Mazière, J. Notholt, R. Sussmann, T. Warneke, D. Griffith, V. Sherlock, and D. Wunch Column-averaged dry-air mole fractions of carbon dioxide and methane have been retrieved from spectra acquired by the TANSO-FTS and SCIAMACHY instruments on board GOSAT and ENVISAT using a range of European retrieval algorithms. These retrievals have been compared with data from ground-based high-resolution Fourier Transform Spectrometers (FTS) from the Total Carbon Column Observing Network (TCCON). The participating algorithms are the Weighting Function Modified Differential Optical Absorption Spectroscopy (DOAS) algorithm (WFMD, University of Bremen), the Bremen Optimal Estimation DOAS algorithm (BESD, University of Bremen), the Iterative Maximum A Posteriori DOAS (IMAP, Jet Propulsion Laboratory (JPL) and Netherlands Institute for Space Research algorithm (SRON)), the proxy and full-physics versions of SRON's RemoTeC algorithm (SRPR and SRFP respectively) and the proxy and full-physics versions of the University of Leicester's adaptation of the OCO (Orbiting Carbon Observatory) algorithm (OCPR and OCFP respectively). The goal of this algorithm inter-comparison was to identify strengths and weaknesses of the various so-called Round Robin data sets generated with the various algorithms so as to determine which of the competing algorithms would proceed to the next round of the European Space Agency's (ESA) Greenhouse Gas Climate Change Initiative (GHG-CCI) project, which is the generation of the so-called Climate Research Data Package (CRDP), which is the first version of the Essential Climate Variable (ECV) "Greenhouse Gases" (GHG). For CO 2 , all algorithms reach the precision requirements for inverse modelling ( 〈 8 ppb), with only WFMD having a lower precision (4.7 ppm) than the other algorithm products (2.4–2.5 ppm). When looking at the seasonal relative accuracy (SRA, variability of the bias in space and time), none of the algorithms have reached the demanding 〈 0.5 ppm threshold. For CH 4 , the precision for both SCIAMACHY products (50.2 ppb for IMAP and 76.4 ppb for WFMD) fail to meet the 〈 34 ppb threshold, but note that this work focusses on the period after the 2005 SCIAMACHY detector degradation. The GOSAT X CH 4 precision ranges between 18.1 and 14.0 ppb. Looking at the SRA, all GOSAT algorithm products reach the 〈 10 ppm threshold (values ranging between 5.4 and 6.2 ppb). For SCIAMACHY, IMAP and WFMD have a SRA of 17.2 ppb and 10.5 ppb respectively.

Description: Hydrological functions of sinkholes and characteristics of point recharge in groundwater basins Hydrology and Earth System Sciences Discussions, 10, 11423-11449, 2013 Author(s): N. Somaratne, K. Smettem, J. Lawson, K. Nguyen, and J. Frizenschaf Karstic limestone aquifers are hydrologically and hydrochemically extremely heterogeneous and point source recharge via sinkholes and fissures is a common feature. We studied three groundwater systems in karstic settings dominated by point source recharge in order to assess the relative contributions to total recharge from point sources using chloride and δ 18 O relations. Preferential groundwater flows were observed through an inter-connected network of highly conductive zones with groundwater mixing along flow paths. Measurements of salinity and chloride indicated that fresh water pockets exist at point recharge locations. A measurable fresh water plume develops only when a large quantity of surface water enters the aquifer as a point recharge source. The difference in chloride concentrations in diffuse and point recharge zones decreases as aquifer saturated thickness increases and the plumes become diluted through mixing. The chloride concentration in point recharge fluxes crossing the watertable plane can remain at or near surface runoff chloride concentrations, rather than in equilibrium with groundwater chloride. In such circumstances the conventional chloride mass balance method that assumes equilibrium of recharge water chloride with groundwater requires modification to include both point and diffuse recharge mechanisms.

Description: Climate information based streamflow and rainfall forecasts for Huai River Basin using Hierarchical Bayesian Modeling Hydrology and Earth System Sciences Discussions, 10, 11559-11584, 2013 Author(s): X. Chen, Z. Hao, N. Devineni, and U. Lall A Hierarchal Bayesian model for forecasting regional summer rainfall and streamflow season-ahead using exogenous climate variables for East Central China is presented. The model provides estimates of the posterior forecasted probability distribution for 12 rainfall and 2 streamflow stations considering parameter uncertainty, and cross-site correlation. The model has a multilevel structure with regression coefficients modeled from a common multivariate normal distribution results in partial-pooling of information across multiple stations and better representation of parameter and posterior distribution uncertainty. Covariance structure of the residuals across stations is explicitly modeled. Model performance is tested under leave-10-out cross-validation. Frequentist and Bayesian performance metrics used include Receiver Operating Characteristic, Reduction of Error, Coefficient of Efficiency, Rank Probability Skill Scores, and coverage by posterior credible intervals. The ability of the model to reliably forecast regional summer rainfall and streamflow season-ahead offers potential for developing adaptive water risk management strategies.

Description: The effect of phase partitioning of semivolatile compounds on the measured CCN activity of aerosol particles Atmospheric Measurement Techniques Discussions, 6, 8413-8433, 2013 Author(s): S. Romakkaniemi, A. Jaatinen, A. Laaksonen, A. Nenes, and T. Raatikainen The effect of inorganic semivolatile aerosol compounds on the CCN activity of aerosol particles was studied by using a computational model for a DMT-CCN counter, a cloud parcel model for condensation kinetics and experiments to quantify the modelled results. Concentrations of water vapour and semivolatiles as well as aerosol trajectories in the CCN column were calculated by a computational fluid dynamics model. These trajectories and vapour concentrations were then used as an input for the cloud parcel model to simulate mass transfer kinetics of water and semivolatiles between aerosol particles and the gas phase. Two different questions were studied: (1) how big fraction of semivolatiles is evaporated from particles before activation in the CCN counter? (2) How much the CCN activity can be increased due to condensation of semivolatiles prior to the maximum water supersaturation in the case of high semivolatile concentration in the gas phase? The results show that, to increase the CCN activity of aerosol particles, a very high gas phase concentration (as compared to typical ambient conditions) is needed. We used nitric acid as a test compound. A concentration of several ppb or higher is needed for measurable effect. In the case of particle evaporation, we used ammonium nitrate as a test compound and found that it partially evaporates before maximum supersaturation is reached in the CCN counter, thus causing an underestimation of CCN activity. The effect of evaporation is clearly visible in all supersaturations, leading to an underestimation of the critical dry diameter by 10 to 15 nanometres in the case of ammonium nitrate particles in different supersaturations. This result was also confirmed by measurements in supersaturations between 0.1 and 0.7%.

Description: A spatial bootstrap technique for parameter estimation of rainfall annual maxima distribution Hydrology and Earth System Sciences Discussions, 10, 11755-11794, 2013 Author(s): F. Uboldi, A. N. Sulis, C. Lussana, M. Cislaghi, and M. Russo Estimation of extreme event distributions and depth-duration-frequency (DDF) curves is achieved at any target site by repeated sampling among all available raingauge data in the surrounding area. The estimate is computed over a gridded domain in Northern Italy, using precipitation time series from 1929 to 2011, including data from historical analog stations and from the present-day automatic observational network. The presented local regionalisation naturally overcomes traditional station-point methods, with their demand of long historical series and their sensitivity to very rare events occurring at very few stations, possibly causing unrealistic spatial gradients in DDF relations. At the same time, the presented approach allows for spatial dependence, necessary in a geographical domain such as Lombardy, complex for both its topography and its climatology. The bootstrap technique enables evaluating uncertainty maps for all estimated parameters and for rainfall depths at assigned return periods.

Description: Overview of the first HyMeX Special Observation Period over Italy: observations and model results Hydrology and Earth System Sciences Discussions, 10, 11643-11710, 2013 Author(s): R. Ferretti, E. Pichelli, S. Gentile, I. Maiello, D. Cimini, S. Davolio, M. M. Miglietta, G. Panegrossi, L. Baldini, F. Pasi, F. S. Marzano, A. Zinzi, S. Mariani, M. Casaioli, G. Bartolini, N. Loglisci, A. Montani, C. Marsigli, A. Manzato, A. Pucillo, M. E. Ferrario, V. Colaiuda, and R. Rotunno During the first Hymex campaign (5 September–6 November 2012) referred to as Special Observation Period (SOP-1), dedicated to heavy precipitation events and flash floods in Western Mediterranean, three Italian hydro-meteorological monitoring sites were activated: Liguria-Tuscany, North-Eastern Italy and Central Italy. The extraordinary deployment of advanced instrumentation, including instrumented aircrafts, and the use of several different operational weather forecast models has allowed an unprecedented monitoring and analysis of high impact weather events around the Italian hydro-meteorological sites. This activity has seen the strict collaboration between the Italian scientific and operational communities. In this paper, an overview of the Italian organization during the SOP-1 is provided, and selected Intensive Observation Periods (IOPs) are described. A significant event for each Italian target area is chosen for this analysis: IOP2 (12–13 September 2012) in North-Eastern Italy, IOP13 (15–16 October 2012) in Central Italy and IOP19 (3–5 November 2012) in Liguria and Tuscany. For each IOP the meteorological characteristics, together with special observations and weather forecasts, are analyzed with the aim of highlighting strengths and weaknesses of the forecast modeling systems. Moreover, using one of the three events, the usefulness of different operational chains is highlighted.

Description: Correcting basin-scale snowfall in a mountainous basin using a distributed snowmelt model and remote sensing data Hydrology and Earth System Sciences Discussions, 10, 11711-11753, 2013 Author(s): M. Shrestha, L. Wang, T. Koike, H. Tsutsui, Y. Xue, and Y. Hirabayashi Adequate estimation of the spatial distribution of snowfall is critical in hydrologic modeling. However, this is a well-known problem in estimating basin-scale snowfall, especially in mountainous basins with data scarcity. This study focuses on correction and estimation of this spatial distribution, which considers topographic effects within the basin. A method is proposed that optimizes an altitude-based snowfall correction factor ( C fsnow ). This is done through multi-objective calibration of a spatially distributed, multilayer energy and water balance-based snowmelt model (WEB-DHM-S) with observed discharge and remotely sensed snow cover data from the Moderate Resolution Imaging Spectroradiometer (MODIS). The Shuffled Complex Evolution – University of Arizona automatic search algorithm is used to obtain the optimal value of C fsnow for minimum cumulative error in discharge and snow cover simulations. Discharge error is quantified by Nash–Sutcliffe efficiency and relative volume deviation, and snow cover error was estimated by pixel-by-pixel analysis. The study region is the heavily snow-fed Yagisawa Basin of the Upper Tone River in northeast Japan. First, the system was applied to one snow season (2002–2003), obtaining an optimized C fsnow of 0.0007 m −1 . For validation purposes, the optimized C fsnow was implemented to correct snowfall in 2004, 2002 and 2001. Overall, the system was effective, implying improvements in correlation of simulated vs. observed discharge and snow cover. The 4 yr mean of basin-average snowfall for the corrected spatial snowfall distribution was 1160 mm (780 mm before correction). Execution of sensitivity runs against other model input and parameters indicated that C fsnow could be affected by uncertainty in shortwave radiation and setting of the threshold air temperature parameter. Our approach is suitable to correct snowfall and estimate its distribution in poorly-gauged basins, where elevation dependence of snowfall amount is strong.

Description: Attribution of hydrologic forecast uncertainty within scalable forecast windows Hydrology and Earth System Sciences Discussions, 10, 11795-11828, 2013 Author(s): L. Yang, F. Tian, Y. Sun, X. Yuan, and H. Hu Hindcasts based on the Extended Streamflow Prediction (ESP) approach are carried out in a typical rainfall-dominated basin in China, aiming to examine the roles of initial condition (IC), future atmospheric forcing (FC) and hydrologic model uncertainty (MU) in the streamflow forecast skill. The combined effects of IC and FC are explored within the framework of a forecast window. By implementing virtual numerical simulations without the consideration of MU, it is found that the dominance of IC could last up to 90 days in dry season, while its impact gives way to FC for lead times exceeding 30 days in the wet season. The combined effects of IC and FC on the forecast skill are further investigated by proposing a dimensionless parameter ( β ) that represents the ratio of the total amount of initial water storage and the incoming rainfall. The forecast skill increases exponentially with β , and varies greatly in different forecast windows. Moreover, the influence of MU on forecast skill is examined by focusing on the uncertainty of model parameters. Two different hydrologic model calibration strategies are carried out. The results indicate that the uncertainty of model parameters exhibits a more significant influence on the forecast skill in the dry season than in the wet season. The ESP approach is more skillful in monthly streamflow forecast during the transition period from wet to dry than otherwise. For the transition period from dry to wet, the low skill of the forecasts could be attributed to the combined effects of IC and FC, but less to the biases in the hydrologic model parameters. For the forecasting in dry season, the usefulness of the ESP approach is heavily dependent on the strategy of the model calibration.

Description: A conceptual model of check dam hydraulics for gully control Hydrology and Earth System Sciences Discussions, 10, 11901-11941, 2013 Author(s): C. Castillo, R. Pérez, and J. A. Gómez There is little information in scientific literature regarding the modifications induced by check dam systems in flow regimes in restored gully reaches, despite it being a crucial issue for the design of conservation measures. Here, we develop a conceptual model to classify flow regimes in straight rectangular channels for initial and dam-filling conditions as well as a method of estimating efficiency in order to provide guidelines for optimal design. The model integrates several previous mathematical approaches for assessing the main processes involved (hydraulic jump HJ, impact flow, gradually varied flows). Its performance was compared with the simulations obtained from IBER, a bi-dimensional hydrodynamic model. The impact of check dam spacing (defined by the geometric factor of influence c ) on efficiency was explored. Eleven main classifications of flow regimes were identified depending on the element and level of influence. The model produced similar results when compared with IBER, but led to higher estimations of HJ and impact lengths. Total influence guaranteed maximum efficiency and HJ control defining the location of the optimal c . Geometric total influence ( c = 1) was a valid criterion for the different stages of the structures in a wide range of situations provided that hydraulic roughness conditions remained high within the gully, e.g. through revegetation. Our total influence criterion involved shorter spacing than that habitually recommended in technical manuals for restoration, but was in line with those values found in spontaneous and stable step-pools systems, which might serve as a reference for man-made interventions.

Description: Senstitivity of water balance components to environmental changes in a mountainous watershed: uncertainty assessment based on models comparison Hydrology and Earth System Sciences Discussions, 10, 11983-12026, 2013 Author(s): E. Morán-Tejeda, J. Zabalza, K. Rahman, A. Gago-Silva, J. I. López-Moreno, S. Vicente-Serrano, A. Lehmann, C. L. Tague, and M. Beniston This paper evaluates the response of stream flow and other components of the water balance to changes in climate and land-use in a Pyrenean watershed. It further provides a measure of uncertainty in water resources forecasts by comparing the performance of two hydrological models: Soil and Water Assessment Tool (SWAT) and Regional Hydro-Ecological Simulation System (RHESSys). Regional Climate Model outputs for the 2021–2050 time-frame, and hypothetical (but plausible) land-use scenarios considering re-vegetation and wildfire processes were used as inputs to the models. Results indicate an overall decrease in river flows when the scenarios are considered, except for the post-fire vegetation scenario, in which stream flows are simulated to increase. However the magnitude of these projections varies between the two models used, as SWAT tends to produce larger hydrological changes under climate change scenarios, and RHESSys shows more sensitivity to changes in land-cover. The final prediction will therefore depend largely on the combination of the land-use and climate scenarios, and on the model utilized.

Description: Spectral Aerosol Extinction Monitoring System (SÆMS): setup, observational products, and comparisons Atmospheric Measurement Techniques Discussions, 6, 8647-8677, 2013 Author(s): A. Skupin, A. Ansmann, R. Engelmann, and H. Baars A Spectral Aerosol Extinction Monitoring System (SÆMS) is presented that allows us to continuously measure the spectral extinction coefficient of atmospheric aerosol particles along an about 2.7 km long optical path at 30–50 m height above ground at Leipzig (51.3° N, 12.4° E), Germany. The fully automated instrument measures the ambient aerosol extinction coefficients from 300–1000 nm. The main goal of SÆMS observations are long-term studies of the relationship between particle extinction and relative humidity from below 40 % to almost 100 %. The setup is presented and observations (a case study and statistical results for 2009) are discussed in terms of time series of 550 nm particle optical depth, Ångström exponent, and particle size distribution retrieved from the spectrally resolved extinction. The SÆMS measurements are compared with simultaneously performed EARLINET lidar, AERONET photometer, and in situ aerosol observations of particle size distribution and related extinction coefficients at the roof of our institute. Consistency between the different measurements is found which corroborates the quality of the SÆMS observations.

Description: Validation of SCIAMACHY O 2 A band cloud heights using Cloudnet radar/lidar measurements Atmospheric Measurement Techniques Discussions, 6, 8603-8645, 2013 Author(s): P. Wang and P. Stammes For the first time two SCIAMACHY O 2 A band cloud height products are validated using ground-based radar/lidar measurements between January 2003 and December 2011. The products are the ESA Level 2 (L2) version 5.02 cloud top height and the FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band) version 6 cloud height. The radar/lidar profiles are obtained at the Cloudnet sites of Cabauw and Lindenberg, and are averaged for one hour centered at the SCIAMACHY overpass time to achieve an optimal temporal and spatial match. In total we have about 220 cases of single layer clouds and 200 cases of multi-layer clouds. The FRESCO cloud height and ESA L2 cloud top height are compared with the Cloudnet cloud top height and Cloudnet cloud middle height. We find that the ESA L2 cloud top height has a better agreement with the Cloudnet cloud top height than the Cloudnet cloud middle height. The ESA L2 cloud top height is on average 0.44 km higher than the Cloudnet cloud top height, with a standard deviation of 3.07 km. The FRESCO cloud height is closer to the Cloudnet cloud middle height than the Cloudnet cloud top height. The mean difference between the FRESCO cloud height and the Cloudnet cloud middle height is −0.14 km with a standard deviation of 1.88 km. The SCIAMACHY cloud height products are further compared to the Cloudnet cloud top height and the Cloudnet cloud middle height in 1 km bins. For single layer clouds, the difference between the ESA L2 cloud top height and the Cloudnet cloud top height is less than 1 km for each cloud bin at 3–7 km, which is 24 % percent of the data. The difference between the FRESCO cloud height and the Cloudnet cloud middle height is less than 1 km for each cloud bin at 0–6 km, which is 85 % percent of the data. The results are similar for multi-layer clouds, but the percentage of cases having a bias within 1 km is smaller than for single layer clouds. Since globally about 60 % of all clouds are low clouds and 42 % are single-layer low clouds, we expect that globally for a large percentage of cases the FRESCO cloud height would be close to the cloud middle height.

Description: Characteristics of cloud liquid water path from SEVIRI on the Meteosat Second Generation 2 satellite for several cloud types Atmospheric Measurement Techniques Discussions, 6, 8743-8782, 2013 Author(s): A. Kniffka, M. Stengel, M. Lockhoff, R. Bennartz, and R. Hollmann In this study the temporal and spatial characteristics of liquid water path (LWP) of low, middle level and high clouds are analysed using space-based observations of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard the Meteosat Second Generation 2 (MSG2) satellite. Both geophysical quantities are part of the dataset CLAAS (CLoud property dAtAset using SEVIRI) and are generated by EUMETSAT's Satellite Application Facility on Climate Monitoring (CM SAF). In this article we focus on the statistical properties of LWP retrieved at daylight associated with the individual cloud type. Our results reveal that each cloud type possesses a characteristic LWP distribution. These frequency distributions are constant with time in the entire SEVIRI field of view, but vary for smaller regions like Central Europe. The average LWP is higher over land than over sea, in case of low clouds 15–27% for 2009 and the variance of the frequency distributions is enhanced. Also, the average diurnal cycle of LWP is related to cloud type where most pronounced diurnal variations were detected for middle level clouds. With SEVIRI it is possible to distinguish between intrinsic LWP variability and variations driven by cloud amount. The relative amplitude of the intrinsic diurnal cycle can exceed the cloud amount driven amplitude.

Description: Hydrologic impact of climate change on Murray Hotham catchment of Western Australia: a projection of rainfall-runoff for future water resources planning Hydrology and Earth System Sciences Discussions, 10, 12027-12076, 2013 Author(s): S. A. Islam, M. A. Bari, and A. H. M. F. Anwar Reduction of rainfall and runoff in recent years across South West Western Australia (SWWA) has drawn attention about climate change impact on water resources and its availability in this region. In this paper, hydrologic impact of climate change on Murray Hotham catchment in SWWA is investigated using multi-model ensemble approach. The Land Use Change Incorporated Catchment (LUCICAT) model was used for hydrologic modelling. Model calibration was performed using (5 km) grid rainfall data from Australian Water Availability Project (AWAP). Downscaled and bias corrected rainfall data from 11 General Circulation Models (GCMs) for Intergovernmental Panel on Climate Change (IPCC) emission scenarios A2 and B1 was used in LUCICAT model to derive rainfall and runoff scenarios for 2046–2065 (mid this century) and 2081–2100 (late this century). The results of climate scenarios were compared with observed past (1961–1980) climate. The mean annual rainfall averaged over the catchment during recent time (1981–2000) was reduced by 2.3% with respect to observed past (1961–1980) and resulting runoff reduction was found 14%. Compared to the past, the mean annual rainfall reductions, averaged over 11 ensembles and over the period for the catchment for A2 scenario are 13.6 and 23.6% for mid and late this century respectively while the corresponding runoff reductions are 36 and 74%. For B1 scenario, the rainfall reductions were 11.9 and 11.6% for mid and late this century and corresponding runoff reductions were 31 and 38%. Spatial distribution of rainfall and runoff changes showed that the rate of changes were higher in high rainfall part compared to the low rainfall part. Temporal distribution of rainfall and runoff indicate that high rainfall in the catchment reduced significantly and further reductions are projected resulting significant runoff reductions. A catchment scenario map has been developed through plotting decadal runoff reduction against corresponding rainfall reduction at four gauging stations for observed and projected period. This could be useful for planning future water resources in the catchment. Projection of rainfall and runoff made based on the GCMs varied significantly for the time periods and emission scenarios. Hence, considerable uncertainty involved in this study though ensemble mean was used to explain the findings.

Description: Remote sensing of atmospheric trace gas columns: an efficient approach for regularization and calculation of total column averaging kernels Atmospheric Measurement Techniques Discussions, 6, 4999-5031, 2013 Author(s): T. Borsdorff, O. P. Hasekamp, A. Wassmann, and J. Landgraf A concept is proposed to retrieve the vertical column densities of atmospheric trace gases from remote sensing measurements. It combines the numerical simplicity of a least-squares profile scaling retrieval with the numerically robust calculation of the total column averaging kernel using an analytic expression. The approach enables calculation of the total column averaging kernel on arbitrary vertical grids. Formally, the proposed method is equivalent to Tikhonov regularization of the first kind with an infinite regularization strength. Due to its efficiency it is particularly suited for implementation in operational data processing with high demands on processing time. To demonstrate the method, we apply it to CO column retrieval from simulated measurements in the 2.3 μm spectral region and to O 3 column retrieval from the UV, which represents ideal measurements of a series of space-borne spectrometers like SCIAMACHY, TROPOMI, GOME, and GOME-2. For both spectral ranges, we consider clear-sky and cloudy scenes where clouds are modelled as an elevated Lambertian surface. Here, the smoothing error for the clear-sky and cloudy atmosphere is significant and reaches several percent, depending on the reference profile which is used for scaling. This underlines the importance of the column averaging kernel for a proper interpretation of retrieved column densities. Furthermore, we show that the total column smoothing error is affected by a discretization error when total column averaging kernels are not represented on a fine enough vertical grid. For both retrievals this effect becomes negligible by using a vertical grid with 20–40 equally thick layers between 0 and 50 km.

Description: Mixing layer height retrievals by multichannel microwave radiometer observations Atmospheric Measurement Techniques Discussions, 6, 4971-4998, 2013 Author(s): D. Cimini, F. De Angelis, J.-C. Dupont, S. Pal, and M. Haeffelin The mixing layer height (MLH) is a key parameter for boundary layer studies, including meteorology, air quality, and climate. MLH estimates are inferred from in situ radiosonde measurements or remote sensing observations from instruments like lidar, wind profiling radar, or sodar. Methods used to estimate MLH from radiosonde profiles are also used with atmospheric temperature and humidity profiles retrieved by microwave radiometers (MWR). This paper proposes an alternative approach to estimate MLH from MWR data, based on direct observations (brightness temperatures, Tb) instead of retrieved profiles. To our knowledge, MLH estimates directly from Tb observations has never been attempted before. The method consists of a multivariate linear regression trained with an a priori set of collocated MWR Tb observations (multi-frequency and multi-angle) and MLH estimates from a state-of-the-art lidar system. Results show that the method is able to follow both the diurnal cycle and the day-to-day variability as suggested by the lidar measurements, and also it can detect low MLH values that are below the full overlap limit (~ 200 m) of the lidar system used. Statistics of the comparison between MWR- and reference lidar-based MLH retrievals show mean difference within 10 m, RMS within 340 m, and correlation coefficient higher than 0.77. Monthly mean analysis for day-time MLH from MWR, lidar, and radiosonde shows consistent seasonal variability, peaking at ~ 1200–1400 m in June and decreasing down to ~ 600 m in October. Conversely, night-time monthly mean MLH from all methods are within 300–500 m without any significant seasonal variability. The proposed method provides results that are more consistent with radiosonde estimates than MLH estimates from MWR retrieved profiles. MLH monthly mean values agree well within 1 std with bulk Richardson number method applied at radiosonde profiles at 11:00 and 23:00 UTC. The method described herewith operates continuously and it is expected to work with analogous performances for the entire diurnal cycle, except during considerable precipitation, demonstrating new potential for atmospheric observation by ground-based microwave radiometry.

Description: Stratospheric aerosol particle size information in Odin-OSIRIS limb scatter spectra Atmospheric Measurement Techniques Discussions, 6, 5065-5099, 2013 Author(s): L. A. Rieger, A. E. Bourassa, and D. A. Degenstein The Optical Spectrograph and InfraRed Imaging System (OSIRIS) on-board the Odin satellite has now taken over a decade of limb scatter measurements that have been used to retrieve the Version 5 stratospheric aerosol extinction product. This product is retrieved using a representative particle size distribution to calculate scattering cross sections and scattering phase functions for the forward model calculations. In this work the information content of OSIRIS measurements with respect to stratospheric aerosol is systematically examined for the purpose of retrieving particle size information along with the extinction coefficient. The benefit of using measurements at different wavelengths and scattering angles in the retrieval is studied and it is found that incorporation of the 1530 nm radiance measurement is key for a robust retrieval of particle size information. It is also found that using OSIRIS measurements at different solar geometries simultaneously provides little additional benefit. Based on these results, an improved aerosol retrieval algorithm is developed that couples the retrieval of aerosol extinction and mode radius of a log-normal particle size distribution. Comparison of these results with coincident measurements from SAGE III show agreement in retrieved extinction to within approximately 10% over the bulk of the aerosol layer, which is comparable to Version 5. The retrieved particle size, when converted to Ångström coefficient, shows good qualitative agreement with SAGE II measurements made at somewhat shorter wavelengths.

Description: Upscaling of evapotranspiration fluxes from instantaneous to daytime scales for thermal remote sensing applications Hydrology and Earth System Sciences Discussions, 10, 7325-7350, 2013 Author(s): C. Cammalleri, M. C. Anderson, and W. P. Kustas Four upscaling methods for estimating daytime evapotranspiration (ET) from single time-of-day snapshots, as commonly retrieved using remote sensing, were compared. These methods are based on the assumption of self-preservation of the ratio between ET and a given reference variable over the daytime hours. The analysis was performed using eddy covariance data collected at 12 AmeriFlux towers, sampling a fairly wide range in climatic and land cover conditions. The choice of energy budget closure method significantly impacted performance using different scaling methodologies. Therefore, a statistical evaluation approach was adopted to better account for the inherent uncertainty in ET fluxes using eddy covariance technique. Overall, this approach suggests that at-surface solar radiation is the most robust reference variable amongst those tested, due to high accuracy of upscaled fluxes and absence of systematic biases. Top-of-atmosphere irradiance was also tested and proved to be reliable under near clear-sky conditions, but tended to overestimate the observed daytime ET during cloudy days. Use of reference ET as a scaling flux did not perform as well as the solar radiation method, but similarly had errors with little seasonal dependency. Finally, the commonly-used evaporative fraction method yielded satisfactory results only in summer months, July and August, and tended to underestimate the observations in the fall/winter seasons from November to January at the flux sites studied.

Description: SAGE version 7.0 algorithm: application to SAGE II Atmospheric Measurement Techniques Discussions, 6, 5101-5171, 2013 Author(s): R. P. Damadeo, J. M. Zawodny, L. W. Thomason, and N. Iyer This paper details the SAGE version 7.0 algorithm and how it is applied to SAGE II. Changes made between the previous (v6.2) and current (v7.0) versions are described and their impacts on the data products explained for both coincident event comparisons and time-series analysis. Users of the data will notice a general improvement in all of the SAGE II data products, which are now in better agreement with more modern data sets (e.g. SAGE III) and more robust for use with trend studies.

Description: Global isoscapes for δ 18 O and δ 2 H in precipitation: improved prediction using regionalized climatic regression models Hydrology and Earth System Sciences Discussions, 10, 7351-7393, 2013 Author(s): S. Terzer, L. I. Wassenaar, L. J. Araguás-Araguás, and P. K. Aggarwal A Regionalized Climatic Water Isotope Prediction (RCWIP) approach, based on the Global Network for Isotopes in Precipitation (GNIP), was demonstrated for the purposes of predicting point- and large-scale spatiotemporal patterns of the stable isotope compositions of water (δ 2 H, δ 18 O) in precipitation around the world. Unlike earlier global domain and fixed regressor models, RCWIP pre-defined thirty-six climatic cluster domains, and tested all model combinations from an array of climatic and spatial regressor variables to obtain the best predictive approach to each cluster domain, as indicated by RMSE and variogram analysis. Fuzzy membership fractions were thereafter used as the weights to seamlessly amalgamate results of the optimized climatic zone prediction models into a single predictive mapping product, such as global or regional amount-weighted mean annual, mean monthly or growing-season δ 18 O/δ 2 H in precipitation. Comparative tests revealed the RCWIP approach outperformed classical global-fixed regression-interpolation based models more than 67% of the time, and significantly improved upon predictive accuracy and precision. All RCWIP isotope mapping products are available as gridded GeoTIFF files from the IAEA website ( www.iaea.org/water ) and are for use in hydrology, climatology, food authenticity, ecology, and forensics.

Description: Water displacement by sewer infrastructure in the Grote Nete catchment, Belgium, and its hydrological regime effects Hydrology and Earth System Sciences Discussions, 10, 7425-7467, 2013 Author(s): D. Vrebos, T. Vansteenkiste, J. Staes, P. Willems, and P. Meire Urbanization and especially impervious areas, in combination with wastewater treatment infrastructure, can exert several pressures on the hydrological cycle. These pressures were studied for the Grote Nete catchment in Belgium (8.18% impervious area and 3.89% effective impervious area), based on a combination of empirical and model-based approaches. The effective impervious area, combined with the extent of the wastewater collection regions which do not coincide with the natural catchment boundaries, was used as an indicator for the urbanization pressure. Our study revealed changes in the total upstream areas of the subcatchments between −16% and +3%, and in upstream impervious areas between −99% and +64%. These changes lead to important inter-catchment water transfers. Based on simulations with a physically-based and spatially-distributed hydrological catchment model, profound impacts of effective impervious area on infiltration and runoff were found. The model results show that the changes in impervious areas and related water displacements in and between catchments due to the installation of the wastewater treatment infrastructure severely impacted low flows, peak flows and seasonal trends. They moreover show that it is difficult, but of utmost importance, to incorporate these pressures and artificial processes in an accurate way during the development of hydrological models for urbanized catchments.

Description: Virtual water trade and development in Africa Hydrology and Earth System Sciences Discussions, 10, 7291-7324, 2013 Author(s): M. Konar and K. Caylor A debate has long existed on the relationships between human population, natural resources, and development. Recent research has expanded this debate to include the impacts of trade; specifically, virtual water trade, or the water footprint of traded commodities. We conduct an empirical analysis of the relationships between virtual water trade, population, and development in Africa. We find that increases in virtual water imports do not lead to increases in population growth nor do they diminish human welfare. We establish a new index of virtual water trade openness and show that levels of undernourishment tend to fall with increased values of virtual water trade openness. Countries with small dam storage capacity obtain a higher fraction of their agricultural water requirements from external sources, which may indicate implicit "infrastructure sharing" across nations. Globally, increased crop exports tends to correlate with increased crop water use efficiency, though this relationship does not hold for Africa. However, internal African trade is much more efficient in terms of embodied water resources than any other region in the world. Thus, internal African trade patterns may be compensating for poor internal production systems.

Description: Technical Note: Aeolian dust proxies produce visible luminescence upon intense laser-illumination that results from incandescence of internally mixed carbon Atmospheric Measurement Techniques Discussions, 6, 5173-5194, 2013 Author(s): L. Ma, T. Cao, and J. E. Thompson Mineral dust mimics dispersed in air produced visible luminescence between 550–800 nm when illuminated with a high peak power (MW range) Nd:YAG laser beam at 532 or 1064 nm. The luminescence persists for a few microseconds after the laser pulse and the measured emission spectrum is roughly consistent with a blackbody emitter at ≈4300 K. Both observations are consistent with assigning laser-induced incandescence (LII) as the source of the luminescence. However, light emission intensity from the mineral dust proxies is 240–4600 less intense than incandescence from fresh kerosene soot on a per-mass basis at laser pulse energies 90% on average. Heating to 350 °C reduced emission by 45–72%. Since black carbon soot and char (BC) oxidizes at elevated temperatures and BC is known to be present in soils, we conclude emission of light from the mineral dust aerosol proxies is likely a result of black carbon or char internally mixed within the soil dust sample. The reduction in LII response for samples heated to temperatures of 250–350 °C may result from partial oxidation of BC, but alternatively, could implicate a role for carbon present within organic molecules. The study suggests laser-induced incandescence measurements may allow quantitation of black carbon in soils and that soil dust is not truly an interferent in BC analysis by LII, but rather, a BC containing material.

Description: An interdisciplinary swat ecohydrological model to define catchment-scale hydrologic partitioning Hydrology and Earth System Sciences Discussions, 10, 7235-7290, 2013 Author(s): C. L. Shope, G. R. Maharjan, J. Tenhunen, B. Seo, K. Kim, J. Riley, S. Arnhold, T. Koellner, Y. S. Ok, S. Peiffer, B. Kim, J.-H. Park, and B. Huwe Land use and climate change have long been implicated in modifying ecosystem services, such as water quality and water yield, biodiversity, and agricultural production. To account for future effects on ecosystem services, the integration of physical, biological, economic, and social data over several scales must be implemented to assess the effects on natural resource availability and use. Our objective is to assess the capability of the SWAT model to capture short-duration monsoonal rainfall-runoff processes in complex mountainous terrain under rapid, event-driven processes in a monsoonal environment. To accomplish this, we developed a unique quality-control gap-filling algorithm for interpolation of high frequency meteorological data. We used a novel multi-location, multi-optimization calibration technique to improve estimations of catchment-wide hydrologic partitioning. We calibrated the interdisciplinary model to a combination of statistical, hydrologic, and plant growth metrics. In addition, we used multiple locations of different drainage area, aspect, elevation, and geologic substrata distributed throughout the catchment. Results indicate scale-dependent sensitivity of hydrologic partitioning and substantial influence of engineered features. While our model accurately reproduced observed discharge variability, the addition of hydrologic and plant growth objective functions identified the importance of culverts in catchment-wide flow distribution. The results of this study provide a valuable resource to describe landscape controls and their implication on discharge, sediment transport, and nutrient loading. This study also shows the challenges of applying the SWAT model to complex terrain and extreme environments. By incorporating anthropogenic features into modeling scenarios, we can greatly enhance our understanding of the hydroecological impacts on ecosystem services.

Description: Technical note: Method of Morris effectively reduces the computational demands of global sensitivity analysis for distributed watershed models Hydrology and Earth System Sciences Discussions, 10, 4275-4299, 2013 Author(s): J. D. Herman, J. B. Kollat, P. M. Reed, and T. Wagener The increase in spatially distributed hydrologic modeling warrants a corresponding increase in diagnostic methods capable of analyzing complex models with large numbers of parameters. Sobol ' sensitivity analysis has proven to be a valuable tool for diagnostic analyses of hydrologic models. However, for many spatially distributed models, the Sobol ' method requires a prohibitive number of model evaluations to reliably decompose output variance across the full set of parameters. We investigate the potential of the method of Morris, a screening-based sensitivity approach, to provide results sufficiently similar to those of the Sobol ' method at a greatly reduced computational expense. The methods are benchmarked on the Hydrology Laboratory Research Distributed Hydrologic Model (HL-RDHM) model over a six-month period in the Blue River Watershed, Oklahoma, USA. The Sobol ' method required over six million model evaluations to ensure reliable sensitivity indices, corresponding to more than 30 000 computing hours and roughly 180 gigabytes of storage space. We find that the method of Morris is able to correctly identify sensitive and insensitive parameters with 300 times fewer model evaluations, requiring only 100 computing hours and 1 gigabyte of storage space. Method of Morris proves to be a promising diagnostic approach for global sensitivity analysis of highly parameterized, spatially distributed hydrologic models.

Description: Socio-hydrology: conceptualising human-flood interactions Hydrology and Earth System Sciences Discussions, 10, 4515-4536, 2013 Author(s): G. Di Baldassarre, A. Viglione, G. Carr, L. Kuil, J. L. Salinas, and G. Blöschl Over history, humankind has tended to settle near streams because of the role of rivers as transportation corridors and the fertility of riparian areas. However, human settlements in floodplains have been threatened by the risk of flooding. Possible responses have been to resettle away and/or modify the river system by building flood control structures. This has led to a complex web of interactions and feedback mechanisms between hydrological and social processes in settled floodplains. This paper is an attempt to conceptualise these interplays for hypothetical human-flood systems. We develop a simple, dynamic model to represent the interactions and feedback loops between hydrological and social processes. The model is then used to explore the dynamics of the human-flood system and the effect of changing individual characteristics, including external forcing such as technological development. The results show that the conceptual model is able to reproduce reciprocal effects between floods and people as well as the emergence of typical patterns. For instance, when levees are built or raised to protect floodplain areas, their presence not only reduces the frequency of flooding, but also exacerbates high water levels. Then, because of this exacerbation, higher flood protection levels are required by the society. As a result, more and more flooding events are avoided, but rare and catastrophic events take place.

Description: Integration of remote sensing, RUSLE and GIS to model potential soil loss and sediment yield (SY) Hydrology and Earth System Sciences Discussions, 10, 4567-4596, 2013 Author(s): H. Kamaludin, T. Lihan, Z. Ali Rahman, M. A. Mustapha, W. M. R. Idris, and S. A. Rahim Land use activities within a basin serve as one of the contributing factors which cause deterioration of river water quality through its potential effect on erosion. Sediment yield in the form of suspended solid in the river water body which is transported to the coastal area occurs as a sign of lowering of the water quality. Hence, the aim of this study was to determine potential soil loss using the Revised Universal Soil Loss Equation (RUSLE) model and the sediment yield, in the Geographical Information Systems (GIS) environment within selected sub-catchments of Pahang River Basin. RUSLE was used to estimate potential soil losses and sediment yield by utilizing information on rainfall erosivity ( R ) using interpolation of rainfall data, soil erodibility ( K ) using field measurement and soil map, vegetation cover ( C ) using satellite images, topography (LS) using DEM and conservation practices ( P ) using satellite images. The results indicated that the rate of potential soil loss in these sub-catchments ranged from very low to extremely high. The area covered by very low to low potential soil loss was about 99%, whereas moderate to extremely high soil loss potential covered only about 1% of the study area. Sediment yield represented only 1% of the potential soil loss. The sediment yield (SY) value in Pahang River turned out to be higher closer to the river mouth because of the topographic character, climate, vegetation type and density, and land use within the drainage basin.

Description: Probability distributions for explaining hydrological losses in South Australian catchments Hydrology and Earth System Sciences Discussions, 10, 4597-4626, 2013 Author(s): S. H. P. W. Gamage, G. A. Hewa, and S. Beecham The wide variability of hydrological losses in catchments is due to multiple variables that affect the rainfall-runoff process. Accurate estimation of hydrological losses is required for making vital decisions in design applications that are based on design rainfall models and rainfall-runoff models. Using representative single values of losses, despite their wide variability, is common practice, especially in Australian studies. This practice leads to issues such as over or under estimation of design floods. Probability distributions can be used as a better representation of losses. In particular, using joint probability approaches (JPA), probability distributions can be incorporated into hydrological loss parameters in design models. However, lack of understanding of loss distributions limits the benefit of using JPA. The aim of this paper is to identify a probability distribution function that can successfully describe hydrological losses in South Australian (SA) catchments. This paper describes suitable parametric and non-parametric distributions that can successfully describe observed loss data. The goodness-of-fit of the fitted distributions and quantification of the errors associated with quantile estimation are also discussed a two-parameter Gamma distribution was identified as one that successfully described initial loss (IL) data of the selected catchments. Also, a non-parametric standardised distribution of losses that describes both IL and continuing loss (CL) data were identified. The results obtained for the non-parametric methods were compared with similar studies carried out in other parts of Australia and a remarkable degree of consistency was observed. The results will be helpful in improving design flood applications.

Description: Technical Note: Using wavelet analyses on water depth time series to detect glacial influence in high-mountain hydrosystems Hydrology and Earth System Sciences Discussions, 10, 4369-4395, 2013 Author(s): S. Cauvy-Fraunié, T. Condom, A. Rabatel, M. Villacis, D. Jacobsen, and O. Dangles Worldwide, the rapid shrinking of glaciers in response to ongoing climate change is currently modifying the glacial meltwater contribution to hydrosystems in glacierized catchments. Assessing the contribution of glacier run-off to stream discharge is therefore of critical importance to evaluate potential impact of glacier retreat on water quality and aquatic biota. This task has challenged both glacier hydrologists and ecologists over the last 20 yr due to both structural and functional complexity of the glacier-stream system interface. Here we propose a new methodological approach based on wavelet analyses on water depth time series to determine the glacial influence in glacierized catchments. We performed water depth measurement using water pressure loggers over ten months in 15 stream sites in two glacier-fed catchments in the Ecuadorian Andes (〉 4000 m). We determined the global wavelet spectrum of each time series and defined the Wavelet Glacier Signal (WGS) as the ratio between the global wavelet power spectrum value at a 24 h-scale and its corresponding significance value. To test the relevance of the WGS we compared it with the percentage of the glacier cover in the catchments, a metric of glacier influence often used in the literature. We then tested whether one month data could be sufficient to reliably determine the glacial influence. As expected we found that the WGS of glacier-fed streams decreased downstream with the increasing of non-glacial tributaries. We also found that the WGS and the percentage of the glacier cover in the catchment were significantly positively correlated and that one month data was sufficient to identify and compare the glacial influence between two sites, provided that the water level time series were acquired over the same period. Furthermore, we found that our method permits to detect glacial signal in supposedly non-glacial sites, thereby evidencing glacial meltwater infiltrations. While we specifically focused on the tropical Andes in this paper, our approach to determine glacier influence would be applicable to temperate and arctic glacierized catchments. The WGS therefore appears as a powerful and cost effective tool to better understand the hydrological links between glaciers and hydrosystems and assess the consequences of rapid glacier melting.

Description: Spatio-temporal heterogeneity of riparian soil morphology in a restored floodplain Hydrology and Earth System Sciences Discussions, 10, 4337-4367, 2013 Author(s): B. Fournier, C. Guenat, G. Bullinger-Weber, and E. A. D. Mitchell Floodplains have been intensively altered in industrialized countries, but are now increasingly being restored and it is therefore important to assess the effect of these restoration projects on the aquatic and terrestrial components of ecosystems. Soils are a functionally crucial component of terrestrial ecosystems but are generally overlooked in floodplain restoration assessment. We studied the spatio-temporal heterogeneity of soil morphology in a restored (riverbed widening) river reach along River Thur (Switzerland) using three criteria (soil diversity, dynamism and typicality) and their associated indicators. We hypothesized that these criteria would correctly discriminate the post-restoration changes in soil morphology within the study site, and that these changes correspond to patterns of vascular plant diversity. Soil diversity and dynamism increased five years after the restoration, but typical soils of braided rivers were still missing. Soil typicality and dynamism correlated to vegetation changes. These results suggest a limited success of the project in agreement with evaluations carried out at the same site using other, more resource demanding methods (e.g. soil fauna, fish, ecosystem functioning). Soil morphology provides structural and functional information on floodplain ecosystems and allows predicting broad changes in plant diversity. The spatio-temporal heterogeneity of soil morphology represents a cost-efficient ecological indicator that could easily be integrated into rapid assessment protocols of floodplain and river restoration projects.

Description: Space-time kriging extension of precipitation variability at 12 km spacing from tree-ring chronologies and its implications for drought analysis Hydrology and Earth System Sciences Discussions, 10, 4301-4335, 2013 Author(s): F. Biondi Understanding and preparing for future hydroclimatic variability greatly benefits from long (i.e., multi-century) records at seasonal to annual time steps that have been gridded at km-scale spatial intervals over a geographic region. Kriging is a geostatistical technique commonly used for optimal interpolation of environmental data, and space-time geostatistical models can improve kriging estimates when long temporal sequences of observations exist at relatively few points on the landscape. Here I present how a network of 22 tree-ring chronologies from single-leaf pinyon ( Pinus monophylla ) in the central Great Basin of North America was used to extend hydroclimatic records both temporally and spatially. First, the Line of Organic Correlation (LOC) method was used to reconstruct October–May total precipitation anomalies at each tree-ring site, as these ecotonal environments at the lower forest border are typically moisture limited. Individual site reconstructions were then combined using a hierarchical model of spatio-temporal kriging that produced annual anomaly maps on a 12 × 12 km grid during the period in common among all chronologies (1650–1976). Hydro-climatic episodes were numerically identified and modeled using their duration, magnitude, and peak. Spatial patterns were more variable during wet years than during dry years, and the evolution of drought episodes over space and time could be visualized and quantified. The most remarkable episode in the entire reconstruction was the early 1900s pluvial, followed by the late 1800s drought. The 1930s "Dust Bowl" drought was among the top ten hydroclimatic episodes in the past few centuries. These results directly address the needs of water and natural resource managers with respect to planning for "worst case" scenarios of drought duration and magnitude at the watershed level. For instance, it is possible to analyze which geographical areas are more likely to be impacted by severe and sustained droughts at annual or multiannual timescales and at spatial resolutions commonly used by regional climate models.

Description: Retrieval of aerosol parameters from the oxygen A band in the presence of chlorophyll fluorescence Atmospheric Measurement Techniques Discussions, 6, 3181-3213, 2013 Author(s): A. F. J. Sanders and J. F. de Haan We have investigated precision of retrieved parameters for a generic aerosol retrieval algorithm over vegetated land using the O 2 A band. Chlorophyll fluorescence is taken into account in the forward model. Fluorescence emissions are modeled as isotropic contributions to the upwelling radiance field at the surface and they are retrieved along with aerosol parameters. Precision is calculated by propagating measurement noise using the forward model's derivatives. We assume that measurement noise is dominated by shot noise; thus, results apply to grating spectrometers in particular. In a number of retrieval simulations, we describe precision for various atmospheric states, observation geometries and spectral resolutions of the instrument. Our results show that aerosol optical thickness, aerosol pressure, fluorescence emission and surface albedo can be simultaneously retrieved from the O 2 A band. We also show that most of the fluorescence signal is provided by filling-in of the O 2 A band and to a lesser extent by filling-in of Fraunhofer lines.

Description: Optimising predictor domains for spatially coherent precipitation downscaling Hydrology and Earth System Sciences Discussions, 10, 4015-4061, 2013 Author(s): S. Radanovics, J.-P. Vidal, E. Sauquet, A. Ben Daoud, and G. Bontron Statistical downscaling is widely used to overcome the scale gap between predictors from Numerical Weather Prediction (NWP) models or General Circulation Models (GCMs) and predictands like local precipitation, required for example for medium-term operational forecasts or climate change impact studies. The predictors are considered over a given spatial domain which is rarely optimised with respect to the target predictand location. In this study the geopotential predictor domains used by an analogue downscaling method are optimised for 608 target zones covering France. An extended version of the growing rectangular domain algorithm provides an ensemble of five near-optimum domains for each target zone. All five near-optimum domains are consistently equally skillful based on the Continuous Rank Probability Score. Relevance maps calculated for selected target zones first reveal high skill geopotential regions with specific shapes for locations in south-eastern France compared to the rest of the country. In all cases, the optimised domains tend to include the most relevant area on the relevance maps. The domain centers of the optimised domains are mainly distributed following the geographical location of the target location, but there are apparent differences between the windward and the lee side of mountain ridges. Moreover, domains for target zones located in south-eastern France are centered more east and south than the ones for target locations on the same longitude. The size of the optimised domains tends to be larger in the southeastern part of the country, while domains with a very small meridional extent can be found in a east-west band around 47° N. Sensitivity tests on the archive length for the analogue method show a general robustness except for zones with high interannual variability like in the Cévennes area. Moreover, results appear to be rather unsensitive to the starting point of the optimisation algorithm except for zones located in the transition area north of the zones having optimized domains with a small meridional extent. This study paves the way for defining regions with homogeneous geopotential predictor domains for precipitation downscaling over France.

Description: Opportunities and challenges for the use of scintillometer-based catchment-averaged evapotranspiration estimates as model forcing Hydrology and Earth System Sciences Discussions, 10, 3973-4013, 2013 Author(s): B. Samain and V. R. N. Pauwels To date, lumped rainfall-runoff models rely on rough estimates of catchment-averaged potential evapotranspiration (ET p ) rates as meteorological forcing. A model parameter converts this ET p input into actual evapotranspiration (ET act ) estimates. This paper examines the potential use of scintillometer-based ET act rates for rainfall-runoff modeling. It has been found that the reservoir-structure of the rainfall-runoff model functions as a low-pass filter for the ET p input. If the long-term volume of the ET p used in the model simulations is consistent with the data set used for calibration, a good match of the seasonal pattern, using temporally constant ET p data, is sufficient to obtain adequate discharge simulations. However, these results are then obtained with strongly erroneous evapotranspiration estimates. A better match of the diurnal cycle does not lead to better model results. Replacing the ET p inputs by scintillometer-based ET act estimates does not lead to better model predictions. Small underestimations of ET act under stable conditions, which occur at night and during the Winter, and which accumulate to significant amounts, are the cause of this problem. Consistent with other studies, the scintillometer-based ET act estimates can be considered reliable and realistic under unstable conditions. These values can thus be used as forcing for rainfall-runoff models.

Description: Temporal stability of soil moisture patterns measured by proximal ground-penetrating radar Hydrology and Earth System Sciences Discussions, 10, 4063-4097, 2013 Author(s): J. Minet, N. E. C. Verhoest, S. Lambot, and M. Vanclooster We analyzed the temporal stability of soil moisture patterns acquired using a proximal ground-penetrating radar (GPR) in a 2.5 ha agricultural field at five different dates over three weeks. The GPR system was mounted on a mobile platform, allowing for real-time mapping of soil moisture with a high spatial resolution (2–5 m). The spatio-temporal soil moisture patterns were in accordance with the meteorological data and with soil moisture measurements from soil core sampling. Time-stable areas showing the field-average moisture could be revealed by two methods: (1) by the computation of temporal stability indicators based on relative differences of soil moisture to the field-average and (2) by the spatial intersection of the areas showing the field-average. Locations where the mean relative difference was below 0.02 m 3 m −3 extended up to 10% of the field area whereas the intersection of areas showing the field-average within a tolerance of 0.02 m 3 m −3 covered 5% of the field area. Compared to most of the previous studies about temporal stability of soil moisture, time-stable areas and their spatial patterns could be revealed instead of single point locations, owing to the advanced GPR method for real-time mapping. It is believed that determining spatially coherent time-stable areas is more informative rather than determining time-stable points. Other acquisitions over larger time periods would be necessary to assert the robustness of the time-stable areas.

Description: McClear: a new model estimating downwelling solar radiation at ground level in clear-sky conditions Atmospheric Measurement Techniques Discussions, 6, 3367-3405, 2013 Author(s): M. Lefèvre, A. Oumbe, P. Blanc, B. Espinar, B. Gschwind, Z. Qu, L. Wald, M. Schroedter-Homscheidt, C. Hoyer-Klick, A. Arola, A. Benedetti, J. W. Kaiser, and J.-J. Morcrette A new fast clear-sky model called McClear was developed to estimate the downwelling shortwave direct and global irradiances received at ground level under clear skies. McClear implements a fully physical modelling replacing empirical relations or simpler models used before. It exploits the recent results on aerosol properties, and total column content in water vapor and ozone produced by the MACC project (Monitoring Atmosphere Composition and Climate). It accurately reproduces the irradiance computed by the libRadtran reference radiative transfer model with a computational speed approximately 10 5 times greater by adopting the abaci, or look-up tables, approach combined with interpolation functions. It is therefore suited for geostationary satellite retrievals or numerical weather prediction schemes with many pixels or grid points, respectively. McClear irradiances were compared to 1 min measurements made in clear-sky conditions in several stations within the Baseline Surface Radiation Network in various climates. For global, respectively direct, irradiance, the correlation coefficient ranges between 0.95 and 0.99, resp. 0.86 and 0.99. The bias is comprised between −14 and 25 W m −2 , resp. −49 and +33 W m −2 . The RMSE ranges between 20 W m −2 (3% of the mean observed irradiance) and 36 W m −2 (5%), resp. 33 W m −2 (5%) and 64 W m −2 (10%). These results are much better than those from state-of-the-art models. This work demonstrates the quality of the McClear model combined with MACC products, and indirectly the quality of the aerosol properties modeled by the MACC reanalysis.

Description: Experimental quantification of contact freezing in an electrodynamic balance Atmospheric Measurement Techniques Discussions, 6, 3407-3437, 2013 Author(s): N. Hoffmann, A. Kiselev, D. Rzesanke, D. Duft, and T. Leisner Heterogeneous nucleation of ice in a supercooled water droplet induced by an external contact with a dry aerosol particle has long been known to be more effective than freezing induced by the same nucleus immersed in the droplet. However, the experimental quantification of contact freezing is challenging. Here we report an experimental method allowing to determine the temperature dependent ice nucleation probability of size selected aerosol particles. The method uses supercooled charged water droplets suspended in a laminar flow of air containing aerosol particles as contact freezing nuclei. The rate of droplet–particle collisions is calculated numerically with account for Coulomb attraction, drag force and induced dipole interaction between charged droplet and aerosol particles. The calculation is verified by direct counting of aerosol particles collected by a levitated droplet. By repeating the experiment on individual droplets for a sufficient number of times, we are able to reproduce the statistical freezing behavior of a large ensemble of supercooled droplets and measure the average rate of freezing events. The freezing rate is equal to the product of the droplet–particle collision rate and the probability of freezing on a single contact, the latter being a function of temperature, size and composition of the contact ice nuclei. Based on these observations, we show that for the types of particles investigated so far, contact freezing is the dominating freezing mechanism on the time scale of our experiment.

Description: Impacts of tropical cyclones on hydrochemistry of a subtropical forest Hydrology and Earth System Sciences Discussions, 10, 4537-4566, 2013 Author(s): C. T. Chang, S. P. Hamburg, J. L. Hwong, N. H. Lin, M. L. Hsueh, M. C. Chen, and T. C. Lin Tropical cyclones (typhoons/hurricanes) have major impacts on the biogeochemistry of forest ecosystems, but the stochastic nature and the long intervals between storms means that there are limited data on their effects. We characterized the impacts of 14 typhoons over six years on hydrochemistry of a subtropical forest plantation in Taiwan, a region experiencing frequent typhoons. Typhoons contributed 1/3 of annual rainfall on average, but ranged from 4% to 55%. The stochastic nature of annual typhoon related precipitation poses a challenge with respect to managing the impacts of these extreme events. This challenge is exacerbated by the fact that typhoon-related rainfall is not significantly correlated with wind velocity, the current focus of weather forecasts. Thus little advance warning is provided for the hydrological impacts of these storms. The typhoons we studied contributed approximately one third of the annual input and output of most nutrients (except nitrogen) during an average 9.5d yr −1 period, resulting in nutrient input/output rates an order of magnitude greater than during non-typhoon period. Nitrate output balanced input during the non-typhoon period, but during the typhoon period an average of 10 kg ha −1 yr −1 nitrate was lost. Streamwater chemistry exhibited similarly high variability during typhoon and non-typhoon periods and returned to pre-typhoon levels one to three weeks following each typhoon. The streamwater chemistry appears to be very resilient in response to typhoons, resulting in minimal loss of nutrients.

Description: Separating mixtures of aerosol types in airborne High Spectral Resolution Lidar data Atmospheric Measurement Techniques Discussions, 6, 8269-8309, 2013 Author(s): S. P. Burton, M. A. Vaughan, R. A. Ferrare, and C. A. Hostetler Knowledge of aerosol type is important for source attribution and for determining the magnitude and assessing the consequences of aerosol radiative forcing. However, atmospheric aerosol is frequently not a single pure type, but instead occurs as a mixture of types, and this mixing affects the optical and radiative properties of the aerosol. This paper extends the work of earlier researchers by using the aerosol intensive parameters measured by the NASA Langley Research Center airborne High Spectral Resolution Lidar (HSRL-1) to develop a comprehensive and unified set of rules for characterizing the external mixing of several key aerosol intensive parameters: extinction-to-backscatter ratio (i.e. lidar ratio), backscatter color ratio, and depolarization ratio. We present the mixing rules in a particularly simple form that leads easily to mixing rules for the covariance matrices that describe aerosol distributions, rather than just scalar values of measured parameters. These rules can be applied to infer mixing ratios from the lidar-observed aerosol parameters, even for cases without significant depolarization. We demonstrate our technique with measurement curtains from three HSRL-1 flights which exhibit mixing between two aerosol types, urban pollution plus dust, marine plus dust, and smoke plus marine. For these cases, we infer a time-height cross-section of mixing ratio along the flight track, and partition aerosol extinction into portions attributed to the two pure types.

Description: Lidar-based remote sensing of atmospheric boundary layer height over land and ocean Atmospheric Measurement Techniques Discussions, 6, 8311-8338, 2013 Author(s): T. Luo, R. Yuan, and Z. Wang Atmospheric boundary layer (ABL) processes are important in climate, weather and air quality. A better understanding of the structure and the behavior of the ABL is required for understanding and modeling of the chemistry and dynamics of the atmosphere on all scales. Based on the systematic variations of ABL structures over different surfaces, different lidar-based methods were developed and evaluated to determine the boundary layer height and mixing layer height over land and ocean. With Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) micropulse lidar (MPL) and radiosonde measurements, diurnal and season cycles of atmospheric boundary layer depth and ABL vertical structure over ocean (TWP_C2 cite) and land (SGP_C1) are analyzed. The new methods are also applied to satellite lidar measurements. The derived global marine boundary layer structure database shows good agreement with marine ABL stratiform cloud top height.

Description: Fast reconstruction of hyperspectral radiative transfer simulations by using small spectral subsets: application to the oxygen A band Atmospheric Measurement Techniques Discussions, 6, 8339-8370, 2013 Author(s): A. Hollstein and R. Lindstrot Hyperspectral radiative transfer simulations are a versatile tool in remote sensing but can pose a major computational burden. We describe a simple method to construct hyperspectral simulation results by using only a small spectral subsample of the simulated wavelength range, thus leading to major speedups in such simulations. This is achieved by computing principal components for a small number of representative hyperspectral spectra and then deriving a reconstruction matrix for a specific spectral subset of channels to compute the hyperspectral data. The method is applied and discussed in detail using the example of top of atmosphere radiances in the oxygen A band, leading to speedups in the range of one to two orders of magnitude when compared to radiative transfer simulations at full spectral resolution.

Description: An assessment of cloud top thermodynamic phase products obtained from A-Train passive and active sensors Atmospheric Measurement Techniques Discussions, 6, 8371-8411, 2013 Author(s): S. Zeng, J. Riedi, F. Parol, C. Cornet, and F. Thieuleux The A-Train observations provide an unprecedented opportunity for the production of high quality dataset describing cloud properties. We illustrate in this study the use of one year of coincident POLDER (Polarization and Directionality of the Earth Reflectance), MODIS (MODerate Resolution Imaging Spectroradiometer) and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) observations to establish a reference dataset for the description of cloud top thermodynamic phase at global scale. We present the results of an extensive comparison between POLDER and MODIS cloud top phase products and discuss those in view of cloud vertical structure and optical properties derived simultaneously from collocated CALIOP active measurements. These results allow to identify and quantify potential biases present in the 3 considered dataset. Among those, we discuss the impacts of observation geometry, thin cirrus in multilayered and single layered cloud systems, supercooled liquid droplets, aerosols, fractional cloud cover and snow/ice or bright surfaces on global statistics of cloud phase derived from POLDER and MODIS passive measurements. Based on these analysis we define criteria for the selection of high confidence cloud phase retrievals which in turn can serve for the establishment of a reference cloud phase product. This high confidence joint product derived from POLDER/PARASOL and MODIS/Aqua can be used in the future as a benchmark for the evaluation of other cloud climatologies, for the assessment of cloud phase representation in models and the development of better cloud phase parametrization in the general circulation models (GCMs).

Description: Hurricane impacts on a pair of coastal forested watersheds: implications of selective hurricane damage to forest structure and streamflow dynamics Hydrology and Earth System Sciences Discussions, 10, 11519-11557, 2013 Author(s): A. D. Jayakaran, T. M. Williams, H. Ssegane, D. M. Amatya, B. Song, and C. C. Trettin Hurricanes are infrequent but influential disruptors of ecosystem processes in the southeastern Atlantic and Gulf coasts. Every southeastern forested wetland has the potential to be struck by a tropical cyclone. We examined the impact of Hurricane Hugo on two paired coastal watersheds in South Carolina in terms of stream flow and vegetation dynamics, both before and after the hurricane's passage in 1989. The study objectives were to quantify the magnitude and timing of changes including a reversal in relative streamflow-difference between two paired watersheds, and to examine the selective impacts of a hurricane on the vegetative composition of the forest. We related these impacts to their potential contribution to change watershed hydrology through altered evapotranspiration processes. Using over thirty years of monthly rainfall and streamflow data we showed that there was a significant transformation in the hydrologic character of the two watersheds – a transformation that occurred soon after the hurricane's passage. We linked the change in the rainfall-runoff relationship to a catastrophic shift in forest vegetation due to selective hurricane damage. While both watersheds were located in the path of the hurricane, extant forest structure varied between the two watersheds as a function of experimental forest management techniques on the treatment watershed. We showed that the primary damage was to older pines, and to some extent larger hardwood trees. We believe that lowered vegetative water use impacted both watersheds with increased outflows on both watersheds due to loss of trees following hurricane impact. However, one watershed was able to recover to pre hurricane levels of canopy transpiration at a quicker rate due to the greater abundance of pine seedlings and saplings in that watershed.

Description: Recent evolution of China's virtual water trade: analysis of selected crops and considerations for policy Hydrology and Earth System Sciences Discussions, 10, 11613-11641, 2013 Author(s): J. Shi, J. Liu, and L. Pinter China has dramatically increased its virtual water import unconsciously for recent years. Many studies have focused on the quantity of traded virtual water but very few go into analysing geographic distribution and the properties of China's virtual water trade network. This paper provides a calculation and analysis of the crop-related virtual water trade network of China based on 27 major primary crops between 1986 and 2009. The results show that China is a net importer of virtual water from water-abundant areas of North and South America, and a net virtual water exporter to water-stressed areas of Asia, Africa, and Europe. Virtual water import is far larger than virtual water export and in both import and export a small number of trade partners control the supply chain. Grain crops are the major contributors to virtual water trade, and among grain crops soybeans, mostly imported from the US, Brazil and Argentina are the most significant. As crop yield and crop water productivity in North and South America are generally higher than those in Asia and Africa, the effect of China's crop-related virtual water trade positively contributes to optimizing crop water use efficiency at the global scale. In order to mitigate water scarcity and secure the food supply, virtual water should be actively incorporated into national water management strategies. From the national perspective, China should reduce the export and increase the import of water-intensive crops. But the sources of virtual water import need to be further diversified to reduce supply chain risks and increase resilience.

Description: Odin-OSIRIS detection of the Chelyabinsk meteor Atmospheric Measurement Techniques Discussions, 6, 8435-8443, 2013 Author(s): L. A. Rieger, A. E. Bourassa, and D. A. Degenstein On 15 February 2013 an 11 000 ton meteor entered Earth's atmosphere south east of Chelyabinsk creating a large fireball at 23 km altitude. The resulting stratospheric aerosol loading was detected by the Ozone Mapping and Profiler Suite (OMPS) in a high altitude polar belt. This work confirms the presence and lifetime of the stratospheric debris using the Optical Spectrograph and InfraRed Imaging System (OSIRIS) onboard the Odin satellite. Although OSIRIS coverage begins in mid-March, the measurements show a belt of enhanced scattering near 35 km altitude between 50° N and 70° N. Initially, enhancements show increased scattering of up to 15% over the background conditions, decaying in intensity and dropping in altitude until they are indistinguishable from background conditions by mid-May.

Description: Probabilistic approach to cloud and snow detection on AVHRR imagery Atmospheric Measurement Techniques Discussions, 6, 8445-8507, 2013 Author(s): J. P. Musial, F. Hüsler, M. Sütterlin, C. Neuhaus, and S. Wunderle The derivation of probability estimates complementary to geophysical data sets has gained special attention over the last years. The information about a confidence level of provided physical quantities is required to construct an error budget of higher level products and to correctly interpret final results of a particular analysis. Regarding the generation of products based on satellite data the common input consists of a cloud mask which allows discrimination between surface and cloud signals. Further the surface information is divided between snow and snow-free components. At any step of this discrimination process a misclassification in a cloud/snow mask propagates to higher level products and may alter their usability. Within this scope a novel Probabilistic Cloud Mask (PCM) algorithm suited for the 1×1 km Advanced Very High Resolution Radiometer (AVHRR) data is proposed which provides three types of probability estimates between: cloudy/clear-sky, cloudy/snow and clear-sky/snow conditions. As opposed to the majority of available techniques which are usually based on a decision-tree approach in the PCM algorithm all spectral, angular and ancillary information is used in a single step to retrieve the probability estimates from the pre-computed Look Up Tables (LUTs). Moreover, the issue of derivation of a single threshold value for a spectral test was overcome by the concept of multidimensional information space which is divided into small bins by an extensive set of thresholds. The discrimination between snow and ice clouds and detection of broken, thin clouds was enhanced by means of the Invariant Coordinate System (ICS) transformation. The study area covers a wide range of environmental conditions spanning from Iceland through central Europe to northern parts of Africa which exhibit diverse difficulties for cloud/snow masking algorithms. The retrieved PCM cloud classification was compared to the PPSv2012 and MOD35 collection 6 cloud masks, SYNOP weather reports, CALIPSO vertical feature mask version 3 and to MOD10A1 collection 5 snow mask. The outcomes of conducted analyses proved fine detection skills of the PCM method with comparable or better results than the reference PPS algorithm.

Description: Retrieval techniques for airborne imaging of methane concentrations using high spatial and moderate spectral resolution: application to AVIRIS Atmospheric Measurement Techniques Discussions, 6, 8543-8588, 2013 Author(s): A. K. Thorpe, C. Frankenberg, and D. A. Roberts Two quantitative retrieval techniques were evaluated to estimate methane (CH 4 ) enhancement in concentrated plumes using high spatial and moderate spectral resolution data from the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). An Iterative Maximum a Posteriori Differential Optical Absorption Spectroscopy (IMAP-DOAS) algorithm performed well for an ocean scene containing natural CH 4 emissions from the Coal Oil Point (COP) seep field near Santa Barbara, California. IMAP-DOAS retrieval precision errors are expected to equal between 0.31 to 0.61 ppm CH 4 over the lowest atmospheric layer (height up to 1.04 km), corresponding to about a 30 to 60 ppm error for a 10 m thick plume. However, IMAP-DOAS results for a terrestrial scene were adveresly influenced by the underlying landcover. A hybrid approach using Singular Value Decomposition (SVD) was particularly effective for terrestrial surfaces because it could better account for spectral variability in surface reflectance. Using this approach, a CH 4 plume was observed immediately downwind of two hydrocarbon storage tanks at the Inglewood Oil Field in Los Angeles, California, with a maximum near surface enhancement of 8.45 ppm above background. At COP, the distinct plume had a maximum enhancement of 2.85 ppm CH 4 above background and was consistent with known seep locations and local wind direction. A sensitivity analysis also indicates CH 4 sensitivity should be more than doubled for the next generation AVIRIS sensor (AVIRISng) due to improved spectral resolution and sampling. AVIRIS-like sensors offer the potential to better constrain emissions on local and regional scales, including sources of increasing concern like industrial point source emissions and fugitive CH 4 from the oil and gas industry.

Description: Effects of solar activity and geomagnetic field on noise in CALIOP profiles above the South Atlantic Anomaly Atmospheric Measurement Techniques Discussions, 6, 8589-8602, 2013 Author(s): V. Noel, H. Chepfer, C. Hoareau, M. Reverdy, and G. Cesana By documenting noise levels in 6.5 yr of nighttime measurements by the spaceborne lidar CALIOP above the South Atlantic Anomaly (SAA), we show they contain information about the evolution of upwelling high-energy radiation levels in the area. We find the amount of noisy profiles is influenced by the 11 yr cycle of solar activity, fluctuates by ±5% between 2006 and 2013, and is anticorrelated with solar activity with a 1 yr lag. The size of the SAA grows as solar activity decreases, and an overall westward shift of the SAA region is detectable. We predict SAA noise levels will increase anew after 2014, and will affect future spaceborne lidar missions most near 2020. In other areas, supposedly unaffected by incoming sunlight, nighttime noise levels are much weaker but follow the same 11 yr cycle, superimposed with a one-year cycle that affects both hemispheres similarly and could be attributed to geomagnetic activity.

Description: Extending periodic eddy covariance latent heat fluxes through tree sapflow measurements to estimate long-term total evaporation in a peat swamp forest Hydrology and Earth System Sciences Discussions, 11, 13607-13661, 2014 Author(s): A. D. Clulow, C. S. Everson, M. G. Mengistu, J. S. Price, A. Nickless, and G. P. W. Jewitt A combination of measurement and modelling was used to find a pragmatic solution to estimate the annual total evaporation (ET) from the rare and indigenous Nkazana Peat Swamp Forest (PSF) on the east coast of Southern Africa to improve the water balance estimates within the area. Total evaporation was measured during three window periods (between seven and nine days each) using an eddy covariance (EC) system on a telescopic mast above the forest canopy. Sapflow of an understory and an emergent tree was measured using a low maintenance heat pulse velocity system for an entire hydrological year (October 2009 to September 2010). An empirical model was derived, describing the relationship between the observed ET of the Nkazana PSF measured during two of the window periods ( R 2 = 0.92 and 0.90) which, overlapped with sapflow measurements, thereby providing hourly estimates of predicted ET of the Nkazana PSF for a year, totalling 1125 mm (while rainfall was 650 mm). In building the empirical model, it was found that including the understory tree sapflow provided no benefit to the model performance. In addition, the observed emergent tree sapflow relationship with observed ET between the two field campaigns was consistent and could be represented by a single empirical model ( R 2 = 0.90; RMSE = 0.08 mm). During the window periods of EC measurement, no single meteorological variable was found to describe the Nkazana PSF ET satisfactorily. However, in terms of evaporation models, the hourly FAO56 Penman–Monteith equation best described the observed ET from EC during the August 2009 ( R 2 = 0.75), November 2009 ( R 2 = 0.85) and March 2010 ( R 2 = 0.76) field campaigns, compared to the Priestley–Taylor model ( R 2 = 0.54, 0.74 and 0.62 during the respective field campaigns). From the empirical model of ET and the FAO56 Penman–Monteith equation, a monthly crop factor ( K c ) was derived for the Nkazana PSF providing a method of estimating long-term swamp forest ET from meteorological data. The monthly crop factor indicated two distinct periods. From February to May, it was between 1.2 and 1.4 compared with June to January, when the crop factor was 0.8 to 1.0. The derived monthly K c values were verified as accurate (to one significant digit) using historical data measured at the same site, also using EC, from a~previous study. The measurements provided insights into the microclimate within a subtropical peat swamp forest and the contrasting sapflow of emergent and understory trees. They showed that expensive, high maintenance equipment can be used during manageable window periods in conjunction with low maintenance systems, dedicated to individual trees, to derive a model to estimate long-term ET over remote heterogeneous forests. In addition, the contrast in ET and rainfall emphasises the reliance of the Nkazana PSF on groundwater.

Description: Dye tracing for investigating flow and transport properties of hydrocarbon-polluted Rabots glaciär, Kebnekaise, Sweden Hydrology and Earth System Sciences Discussions, 11, 13711-13744, 2014 Author(s): C. C. Clason, C. Coch, J. Jarsjö, K. Brugger, P. Jansson, and G. Rosqvist Over 11 000 L of hydrocarbon pollution was deposited on the surface of Rabots glaciär on the Kebnekaise Massif, northern Sweden, following the crash of a Royal Norwegian Air Force aircraft in March 2012. An environmental monitoring programme was subsequently commissioned, including water, snow and ice sampling. The scientific programme further included a series of dye tracing experiments during the 2013 melt season, conducted to investigate flow pathways for pollutants through the glacier hydrological system, and to gain new insight to the internal hydrological system of Rabots glaciär. Results of dye tracing reveal a degree of homogeneity in the topology of the drainage system throughout July and August, with an increase in efficiency as the season progresses, as reflected by decreasing temporary storage and dispersivity. Early onset of melting likely led to formation of an efficient, discrete drainage system early in the melt season, subject to decreasing sinuosity and braiding as the season progressed. Analysis of turbidity-discharge hysteresis further supports the formation of discrete, efficient drainage, with clockwise diurnal hysteresis suggesting easy mobilisation of readily-available sediments in channels. Dye injection immediately downstream of the pollution source zone revealed prolonged storage of dye followed by fast, efficient release. Twinned with a low dye recovery, and supported by sporadic detection of hydrocarbons in the proglacial river, we suggest that meltwater, and thus pollutants in solution, may be released periodically from this zone of the glacier hydrological system. The here identified dynamics of dye storage, dispersion and breakthrough indicate that the ultimate fate and permanence of pollutants in the glacier system is likely to be governed by storage of pollutants in the firn layer and ice mass, or within the internal hydrological system, where it may refreeze. This shows that future studies on the fate of hydrocarbons in pristine, glaciated mountain environments should address the extent to which pollutants in solution act like water molecules or whether they are more susceptible to, for example, refreezing into the surrounding ice, becoming stuck in micro-fractures and pore spaces, or sorption onto subglacial sediments.

Description: Sampling frequency trade-offs in the assessment of mean transit times of tropical montane catchment waters under semi-steady-state conditions Hydrology and Earth System Sciences Discussions, 11, 12443-12488, 2014 Author(s): E. Timbe, D. Windhorst, R. Celleri, L. Timbe, P. Crespo, H.-G. Frede, J. Feyen, and L. Breuer Stream and soil waters were collected on a weekly basis in a tropical montane cloud forest catchment for two years and analyzed for stable water isotopes in order to infer transit time distribution functions and to define the mean transit times. Depending on the water type (stream or soil water), lumped distribution functions such as Exponential-Piston flow, Linear-Piston flow and Gamma models using temporal isotopic variations of precipitation event samples as input, were fitted. Samples were aggregated to daily, weekly, biweekly, monthly and bimonthly time scales in order to check the sensitivity of temporal sampling on model predictions. The study reveals that the effect of decreasing sampling frequency depends on the water type. For soil waters with transit times in the order of weeks to months, there was a clear trend of over prediction. In contrast, the trend of prediction for stream waters, with a dampened isotopic signal and mean transit times in the order of 2 to 4 years, was less clear and depending on the type of model used. The trade-off to coarse data resolutions could potentially lead to misleading conclusions on how water actually moves through the catchment, while at the same time predictions can reach better fitting efficiencies, lesser uncertainties, errors and biases. For both water types an optimal sampling frequency seems to be one or at most two weeks. The results of our analyses provide information for the planning (in particular in terms of cost-benefit and time requirements) of future fieldwork in similar Andean or other catchments.

Description: Quantitative high-resolution observations of soil water dynamics in a complicated architecture with time-lapse Ground-Penetrating Radar Hydrology and Earth System Sciences Discussions, 11, 12365-12404, 2014 Author(s): P. Klenk, S. Jaumann, and K. Roth High-resolution time-lapse Ground-Penetrating Radar (GPR) observations of advancing and retreating water tables can yield a wealth of information about near-surface water content dynamics. In this study, we present and analyze a series of imbibition, drainage and infiltration experiments which have been carried out at our artificial ASSESS test site and observed with surface based GPR. The test site features a complicated but known subsurface architecture constructed with three different kinds of sand. It allows studying soil water dynamics with GPR under a wide range of different conditions. Here, we assess in particular (i) the accurate determination of soil water dynamics averaged over the whole vertical extent by evaluating the bottom reflection and (ii) the feasibility of monitoring the dynamic shape of the capillary fringe reflection. The phenomenology of the GPR response of a dynamically changing capillary fringe is developed from a soil physical point of view. We then explain experimentally observed phenomena based on numerical simulations of both the water content dynamics and the expected GPR response.

Description: Simulating long-term past changes in the balance between water demand and availability and assessing their main drivers at the river basin management scale Hydrology and Earth System Sciences Discussions, 11, 12315-12364, 2014 Author(s): J. Fabre, D. Ruelland, A. Dezetter, and B. Grouillet The aim of this study was to assess the balance between water demand and availability and its spatial and temporal variability from 1971 to 2009 in the Herault (2500 km 2 , France) and the Ebro (85 000 km 2 , Spain) catchments. Natural streamflow was evaluated using a conceptual hydrological model. The regulation of river flow was accounted for through a widely applicable demand-driven reservoir management model applied to the largest dam in the Herault basin and to 11 major dams in the Ebro basin. Urban water demand was estimated from population and monthly unit water consumption data. Water demand for irrigation was computed from irrigated area, crop and soil data, and climatic forcing. Finally, a series of indicators comparing water supply and water demand at strategic resource and demand nodes were computed at a 10 day time step. Variations in water stress in each catchment over the past 40 years were successfully modeled, taking into account climatic and anthropogenic pressures and changes in water management strategies over time. Observed changes in discharge were explained by separating human and hydro-climatic pressures on water resources: respectively 20 and 3% of the decrease in the Ebro and the Herault discharges were linked to human-induced changes. Although key areas of the Herault basin were shown to be highly sensitive to hydro-climatic variability, the balance between water uses and availability in the Ebro basin appears to be more critical, owing to high agricultural pressure on water resources. The proposed modeling framework is currently being used to assess water stress under climatic and socio-economic prospective scenarios. Further research will investigate the effectiveness of adaptation policies aimed at maintaining the balance between water use and availability.

Description: Time-series analysis of the long-term hydrologic impacts of afforestation in the Águeda watershed of North-Central Portugal Hydrology and Earth System Sciences Discussions, 11, 12223-12256, 2014 Author(s): D. Hawtree, J. P. Nunes, J. J. Keizer, R. Jacinto, J. Santos, M. E. Rial-Rivas, A.-K. Boulet, F. Tavares-Wahren, and K.-H. Feger The north-central region of Portugal has undergone significant afforestation of the species Pinus pinaster and Eucalyptus globulus since the early 1900s; however, the long-term hydrologic impacts of this land cover change are not fully understood. To contribute to a better understanding of the potential hydrologic impacts of this land cover change, this study examines the temporal trends in 7 years of data from the Águeda watershed (part of the Vouga Basin) over the period of 1936 to 2010. Meteorological and hydrological records were analysed using a combined Thiel–Sen/Mann–Kendall trend testing approach, to assess the magnitude and significance of patterns in the observed data. These trend tests indicated that there had been no significant reduction in streamflow yield over either the entire test period, or during sub-record periods, despite the large-scale afforestation which had taken place. This lack of change is attributed to both the characteristics of the watershed and the nature of the land cover change. By contrast, a number of significant trends were found for baseflow index, which showed positive trends in the early data record (primarily during Pinus pinaster afforestation), followed by a reversal to negative trends later in the data record (primarily during Eucalyptus globulus afforestation). These changes are attributed to vegetation impacts on streamflow generating processes, both due to the species differences and to alterations in soil properties (i.e. promoting water repellency of the topsoil). These results highlight the importance of considering both vegetation types/dynamics and watershed characteristic when assessing hydrologic impacts, in particular with respect to soil properties.

Description: Reply to D. L. Peters' comment on "Streamflow input to Lake Athabasca, Canada" by Rasouli et al. (2013) Hydrology and Earth System Sciences Discussions, 11, 12257-12270, 2014 Author(s): K. Rasouli, M. A. Hernández-Henríquez, and S. J. Déry This paper provides a reply to a comment from Peters (2014) on our recent effort focused on evaluating changes in streamflow input to Lake Athabasca, Canada. Lake Athabasca experienced a 21.2% decline in streamflow input between 1960 and 2010 that has led to a marked decline in its water levels in recent decades. A reassessment of trends in naturalized Lake Athabasca water levels shows insignificant changes from our previous findings reported in Rasouli et al. (2013), and hence our previous conclusions remain unchanged. The reply closes with recommendations for future research to minimize uncertainties in historical assessments of trends in Lake Athabasca water levels and to better project its future water levels driven by climate change and anthropogenic activities in the Athabasca Lake Basin.

Description: Is sinuosity a function of slope and bankfull discharge? – A case study of the meandering rivers in the Pannonian Basin Hydrology and Earth System Sciences Discussions, 11, 12271-12290, 2014 Author(s): J. Petrovszki, G. Timár, and G. Molnár Pre-regulation channel sinuosities of the meandering rivers of the Pannonian Basin are analysed in order to define a mathematical model to estimate the influence of the bankfull discharge and the channel slope on them. As a primary database, data triplets of slope, discharge and sinuosity values were extracted from historical and modern datasets and pre-regulation historical topographic maps. Channel slope values were systematically modified to estimate figures valid before the river regulation works. The bankfull discharges were estimated from the average discharges using a robust yet complex method. The "classical" graphs of Leopold and Wolman (1957), Ackers and Charlton (1970b) and Schumm and Khan (1972) were compiled to a set up a theoretical surface, whose parameters are estimated by the real values of the above database, containing characteristics of the Pannonian Basin rivers. As a result it occurred that there is a two-dimensional function of the bankfull discharges, which provides a good estimation of the most probable sinuosity values of the rivers with the given slope and discharge characteristics. The average RMS error of this estimation is around 15% on this dataset and believed to be the effect of the non-analysed changes in the sediment discharge and size distribution.

Description: Technical Note: Field experiences using UV/VIS sensors for high-resolution monitoring of nitrate in groundwater Hydrology and Earth System Sciences Discussions, 11, 12291-12314, 2014 Author(s): M. Huebsch, F. Grimmeisen, M. Zemann, O. Fenton, K. G. Richards, P. Jordan, A. Sawarieh, P. Blum, and N. Goldscheider Two different in-situ spectrophotometers are compared that were used in the field to determine nitrate-nitrogen (NO 3 -N) concentrations at two distinct spring discharge sites. One sensor was a double wavelength spectrophotometer (DWS) and the other a multiple wavelength spectrophotometer (MWS). The objective of the study was to review the hardware options, determine ease of calibration, accuracy, influence of additional substances and to assess positive and negative aspects of the two sensors as well as troubleshooting and trade-offs. Both sensors are sufficient to monitor highly time-resolved NO 3 -N concentrations in emergent groundwater. However, the chosen path length of the sensors had a significant influence on the sensitivity and the range of detectable NO 3 -N. The accuracy of the calculated NO 3 -N concentrations of the sensors can be affected, if the content of additional substances such as turbidity, organic matter, nitrite or hydrogen carbonate significantly varies after the sensors have been calibrated to a particular water matrix. The MWS offers more possibilities for calibration and error detection, but requires more expertise compared with the DWS.

Description: Potential of the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor for the monitoring of terrestrial chlorophyll fluorescence Atmospheric Measurement Techniques Discussions, 7, 12545-12588, 2014 Author(s): L. Guanter, I. Aben, P. Tol, J. M. Krijger, A. Hollstein, P. Köhler, A. Damm, J. Joiner, C. Frankenberg, and J. Landgraf Global monitoring of sun-induced chlorophyll fluorescence (SIF) can improve our knowledge about the photosynthetic functioning of terrestrial ecosystems. The feasibility of SIF retrievals from spaceborne atmospheric spectrometers has been demonstrated by a number of studies in the last years. In this work, we investigate the potential of the upcoming TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor satellite mission for SIF retrieval. TROPOMI will sample the 675–775 nm spectral window with a spectral resolution of 0.5 nm and a pixel size of 7 km × 7 km. We use an extensive set of simulated TROPOMI data in order to assess the uncertainty of single SIF retrievals and subsequent spatio-temporal composites. Our results illustrate the enormous improvement in SIF monitoring achievable with TROPOMI with respect to comparable spectrometers currently in-flight, such as the Global Ozone Monitoring Experiment-2 (GOME-2) instrument. We find that TROPOMI can reduce global uncertainties in SIF mapping by more than a factor 2 with respect to GOME-2, which comes together with an about 5-fold improvement in spatial sampling. Finally, we discuss the potential of TROPOMI to accurately map other important vegetation parameters, such as leaf photosynthetic pigments and proxies for canopy structure, which will complement SIF retrievals for a self-contained description of vegetation condition and functioning.

Description: Complex networks, streamflow, and hydrometric monitoring system design Hydrology and Earth System Sciences Discussions, 11, 13663-13710, 2014 Author(s): M. Halverson and S. Fleming Network theory is applied to an array of streamflow gauges located in the Coast Mountains of British Columbia and Yukon, Canada. The goal of the analysis is to assess whether insights from this branch of mathematical graph theory can be meaningfully applied to hydrometric data, and more specifically, whether it may help guide decisions concerning stream gauge placement so that the full complexity of the regional hydrology is efficiently captured. The streamflow data, when represented as a complex network, has a global clustering coefficient and average shortest path length consistent with small-world networks, which are a class of stable and efficient networks common in nature, but the results did not clearly suggest a scale-free network. Stability helps ensure that the network is robust to the loss of nodes; in the context of a streamflow network, stability is interpreted as insensitivity to station removal at random. Community structure is also evident in the streamflow network. A community detection algorithm identified 10 separate communities, each of which appears to be defined by the combination of its median seasonal flow regime (pluvial, nival, hybrid, or glacial, which in this region in turn mainly reflects basin elevation) and geographic proximity to other communities (reflecting shared or different daily meteorological forcing). Betweenness analyses additionally suggest a handful of key stations which serve as bridges between communities and might therefore be highly valued. We propose that an idealized sampling network should sample high-betweenness stations, as well as small-membership communities which are by definition rare or undersampled relative to other communities, while retaining some degree of redundancy to maintain network robustness.

Description: Monitoring and modelling of soil–plant interactions: the joint use of ERT, sap flow and Eddy Covariance data to characterize the volume of an orange tree root zone Hydrology and Earth System Sciences Discussions, 11, 13353-13384, 2014 Author(s): G. Cassiani, J. Boaga, D. Vanella, M. T. Perri, and S. Consoli Mass and energy exchanges between soil, plants and atmosphere control a number of key environmental processes involving hydrology, biota and climate. The understanding of these exchanges also play a critical role for practical purposes e.g. in precision agriculture. In this paper we present a methodology based on coupling innovative data collection and models in order to obtain quantitative estimates of the key parameters of such complex flow system. In particular we propose the use of hydro-geophysical monitoring via 4-D Electrical Resistivity Tomography (ERT) in conjunction with measurements of plant transpiration via sap flow and evapotranspiration from Eddy Covariance (EC). This abundance of data is fed to a spatially distributed soil model in order to characterize the distribution of active roots. We conducted experiments in an orange orchard in Eastern Sicily (Italy), characterized by the typical Mediterranean semi-arid climate. The subsoil dynamics, particularly influenced by irrigation and root uptake, were characterized mainly by the ERT setup, consisting of 48 buried electrodes on 4 instrumented micro boreholes (about 1.2 m deep) placed at the corners of a square (about 1.3 m in side) surrounding the orange tree, plus 24 mini-electrodes on the surface spaced 0.1 m on a square grid. During the monitoring, we collected repeated ERT and TDR soil moisture measurements, soil water samples, sap flow measurements from the orange tree and EC data. We conducted a laboratory calibration of the soil electrical properties as a function of moisture content and pore water electrical conductivity. Irrigation, precipitation, sap flow and ET data are available allowing knowledge of the system's long term forcing conditions on the system. This information was used to calibrate a 1-D Richards' equation model representing the dynamics of the volume monitored via 3-D ERT. Information on the soil hydraulic properties was collected from laboratory and field experiments. The successful results of the calibrated modeling exercise allow the quantification of the soil volume interested by root water uptake. This volume is much smaller (with a surface area less than 2 m 2 , and about 40 cm thickness) than expected and assumed in the design of classical drip irrigation schemes that prove to be losing at least half of the irrigated water that is not uptaken by the plants.

Description: The Level 2 research product algorithms for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) Atmospheric Measurement Techniques Discussions, 4, 3593-3645, 2011 Author(s): P. Baron, J. Urban, H. Sagawa, J. Möller, D. P. Murtagh, J. Mendrok, E. Dupuy, T. O. Sato, S. Ochiai, K. Suzuki, T. Manabe, T. Nishibori, K. Kikuchi, R. Sato, M. Takayanagi, Y. Murayama, M. Shiotani, and Y. Kasai This paper describes the algorithms of the level-2 research (L2r) processing chain developed for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES). The chain has been developed in parallel to the operational chain for conducting researches on calibration and retrieval algorithms. L2r chain products are available to the scientific community. The objective of version 2 is the retrieval of the vertical distribution of trace gases in the altitude range of 18–90 km. An theoretical error analysis is conducted to estimate the retrieval feasibility of key parameters of the processing: line-of-sight elevation tangent altitudes (or angles), temperature and O 3 profiles. The line-of-sight tangent altitudes are retrieved between 20 and 50 km from the strong ozone (O 3 ) line at 625.371 GHz, with low correlation with the O 3 volume-mixing ratio and temperature retrieved profiles. Neglecting the non-linearity of the radiometric gain in the calibration procedure is the main systematic error. It is large for the retrieved temperature (between 5–10 K). Therefore, atmospheric pressure can not be derived from the retrieved temperature, and, then, in the altitude range where the line-of-sight tangent altitudes are retrieved, the retrieved trace gases profiles are found to be better represented on pressure levels than on altitude levels. The error analysis for the retrieved HOCl profile demonstrates that best results for inverting weak lines can be obtained by using narrow spectral windows. Future versions of the L2r algorithms will improve the temperature/pressure retrievals and also provide information in the upper tropospheric/lower stratospheric region (e.g., water vapor, ice content, O 3 ) and on stratospheric and mesospheric line-of-sight winds.

Description: Near infrared nadir sounding of vertical column densities: methodology and application to SCIAMACHY Atmospheric Measurement Techniques Discussions, 4, 3685-3737, 2011 Author(s): S. Gimeno García, F. Schreier, G. Lichtenberg, and S. Slijkhuis Nadir observations with the shortwave infrared channels of SCIAMACHY onboard the ENVISAT satellite can be used to derive information on atmospheric gases such as CO, CH 4 , N 2 O, CO 2 , and H 2 O. For the operational level 1b–2 processing of SCIAMACHY data a new retrieval code BIRRA (Beer InfraRed Retrieval Algorithm) has been developed: BIRRA performs a nonlinear least squares fit of the measured radiance, where molecular concentration vertical profiles are scaled to fit the observed data. Here we present the forward modeling (radiative transfer) and inversion (least squares optimization) fundamentals of the code along with the further processing steps required to generate higher level products such as global distributions and time series. Moreover, various aspects of level 1 (observed spectra) and auxiliary input data relevant for successful retrievals are discussed. BIRRA is currently used for operational analysis of carbon monoxide vertical column densities from SCIAMACHY channel 8 observations, and is being prepared for methane retrievals using channel 6 spectra. A set of representative CO retrievals and first CH 4 results are presented to demonstrate BIRRA's capabilities.

Description: Volatilizable biogenic organic compounds (VBOCs) with two dimensional gas chromatography-time of flight mass spectrometry (GC × GC-TOFMS): sampling methods, VBOC complexity, and chromatographic retention data Atmospheric Measurement Techniques Discussions, 4, 3647-3684, 2011 Author(s): J. F. Pankow, W. Luo, A. N. Melnychenko, K. C. Barsanti, L. M. Isabelle, C. Chen, A. B. Guenther, and T. N. Rosenstiel Two dimensional gas chromatography (GC × GC) with detection by time-of-flight mass spectrometry (TOFMS) was applied in the rapid analysis of air samples containing highly complex mixtures of volatilizable biogenic organic compounds (VBOCs). VBOC analytical methodologies are briefly reviewed, and optimal conditions are discussed for sampling with both adsorption/thermal desorption (ATD) cartridges and solid-phase microextraction (SPME) fibers. Air samples containing VBOC emissions from leaves of two tree species ( Cedrus atlantica and Calycolpus moritzianus ) were obtained by both ATD and SPME. The optimized gas chromatographic conditions utilized a 45 m, 0.25 mm I.D. low-polarity primary column (DB-VRX, 1.4 μm film) and a 1.5 m, 0.25 mm I.D. polar secondary column (Stabilwax® 0.25 μm film). Excellent separation was achieved in a 36 min temperature programmed GC × GC chromatogram. Thousands of VBOC peaks were present in the sample chromatograms; hundreds of tentative identifications by NIST mass spectral matching are provided. Very few of the tentatively identified compounds are currently available as authentic standards. Method detection limit values for a 5 l ATD sample were 3.5 pptv (10 ng m −3 ) for isoprene, methyl vinyl ketone, and methacrolein, and ~1.5 pptv (~10 ng m −3 ) for monoterpenes and sesquiterpenes. Kovats-type chromatographic retention index values on the primary column and relative retention time values on the secondary column are provided for 21 standard compounds and for 417 tentatively identified VBOCs. 19 of the 21 authentic standard compounds were found in one of the Cedrus atlantica SPME samples. In addition, easily quantifiable levels of at least 13 sesquiterpenes were found in an ATD sample obtained from a branch enclosure of Calycolpus moritzianus . Overall, the results obtained via GC × GC-TOFMS highlight an extreme, and largely uncharacterized diversity of VBOCs, consistent with the hypothesis that sesquiterpenes and other compounds beyond the current list of typically determined VBOC analytes may well be important contributors to global atmospheric levels of organic particulate matter.

Description: Domestic wells have high probability of pumping septic tank leachate Hydrology and Earth System Sciences Discussions, 8, 5701-5732, 2011 Author(s): J. E. Horn and T. Harter Onsite wastewater treatment systems such as septic systems are common in rural and semi-rural areas around the world; in the US, about 25–30 % of households are served by a septic system and a private drinking water well. Site-specific conditions and local groundwater flow are often ignored when installing septic systems and wells. Particularly in areas with small lots, thus a high septic system density, these typically shallow wells are prone to contamination by septic system leachate. Typically, mass balance approaches are used to determine a maximum septic system density that would prevent contamination of the aquifer. In this study, we estimate the probability of a well pumping partially septic system leachate. A detailed groundwater and transport model is used to calculate the capture zone of a typical drinking water well. A spatial probability analysis is performed to assess the probability that a capture zone overlaps with a septic system drainfield depending on aquifer properties, lot and drainfield size. We show that a high septic system density poses a high probability of pumping septic system leachate. The hydraulic conductivity of the aquifer has a strong influence on the intersection probability. We conclude that mass balances calculations applied on a regional scale underestimate the contamination risk of individual drinking water wells by septic systems. This is particularly relevant for contaminants released at high concentrations, for substances which experience limited attenuation, and those being harmful even in low concentrations.

Description: Use of ENVISAT ASAR Global Monitoring Mode to complement optical data in the mapping of rapid broad-scale flooding in Pakistan Hydrology and Earth System Sciences Discussions, 8, 5769-5809, 2011 Author(s): D. O'Grady, M. Leblanc, and D. Gillieson Envisat ASAR Global Monitoring Mode (GM) data are used to produce maps of the extent of the flooding in Pakistan which are made available to the rapid response effort within 24 h of acquisition. The high temporal frequency and independence of the data from cloud-free skies makes GM data a viable tool for mapping flood waters during those periods where optical satellite data is unavailable, which may be crucial to rapid response disaster planning, where thousands of lives are affected. Image differencing techniques are used, with pre-flood baseline image backscatter values being deducted from target values to eliminate regions with a permanent flood-like radar response due to volume scattering and attenuation, and to highlight the low response caused by specular reflection by open flood water. The effect of local incidence angle on the received signal is mitigated by ensuring that the deducted image is acquired from the same orbit track as the target image. Poor separability of the water class with land in areas beyond the river channels is tackled using a region-growing algorithm which seeks threshold-conformance from seed pixels at the center of the river channels. The resultant mapped extents are tested against MODIS SWIR data where available, with encouraging results.

Description: Soil weathering rates in 21 catchments of the Canadian Shield Hydrology and Earth System Sciences Discussions, 8, 5743-5768, 2011 Author(s): D. Houle, P. Lamoureux, N. Bélanger, M. Bouchard, C. Gagnon, S. Couture, and A. Bouffard Soil mineral weathering represents an essential source of nutrient base cation (Ca, Mg and K) for forest growth in addition to provide a buffering power against precipitation acidity for soils and surface waters. Weathering rates of base cations were obtained for 21 catchments located within the temperate and the boreal forest of the Canadian Shield with the geochemical model PROFILE. Weathering rates ranged from 0.58 to 4.46 kmol c ha −1 yr −1 and their spatial variation within the studied area was mostly in agreement with spatial variations in soil mineralogy. Weathering rates of Ca and Mg were significantly correlated ( r = 0.80 and 0.64) with their respective lake concentrations. Weathering rates of K and Na did not correlate with lake concentrations of K and Na. The modeled weathering rates for each catchment were also compared with estimations of net catchment exportations. The result show that modeled weathering rates of Ca were not significantly different than the net catchment exportations while modeled weathering rates of Mg were higher by 51 %. Larger differences were observed for K and Na weathering rates that were significantly different than net catchment exportations being 6.9 and 2.2 times higher than net exportations, respectively. The results for K were expected given its high reactivity with biotic compartments and suggest that most of the K produced by weathering reactions was retained within soil catchments and/or above ground biomass. This explanation does not apply to Na, however, which is a conservative element in forest ecosystems because of the insignificant needs of Na for soil microorganisms and above ground vegetations. It raises concern about the liability of the PROFILE model to provide reliable values of Na weathering rates. Overall, we concluded that the PROFILE model is powerful enough to reproduce spatial geographical gradients in weathering rates for relatively large areas as well as adequately predict absolute weathering rates values for the sum of base cations, Ca and Mg.

Description: Correcting orbital drift signal in the time series of AVHRR derived convective cloud fraction using rotated empirical orthogonal function Atmospheric Measurement Techniques Discussions, 4, 3877-3890, 2011 Author(s): A. Devasthale, K. Karlsson, J. Quaas, and H. Grassl The AVHRRs instruments onboard the series of NOAA satellites offer the longest available meteorological data records from space. These satellites have drifted in orbit resulting in shifts in the local time sampling during the life span of sensors onboard. Depending on the amplitude of a diurnal cycle of the geophysical parameters derived, orbital drift may cause spurious trends in their time series. We investigate tropical deep convective clouds, which show pronounced diurnal cycle amplitude, to bracket an upper bound of the impact of orbital drift on their time series. We carry out a rotated empirical orthogonal function analysis and show that the REOFs are useful in delineating orbital drift signal and, more importantly, in correcting this signal in the time series of convective cloud amount. These results will help facilitate the derivation of homogenized data series of cloud amount from NOAA satellite sensors and ultimately analyzing trends from them. However, we suggest detailed comparison of various methods and their rigorous testing before applying final orbital drift corrections.

Description: The role of catchment classification in rainfall-runoff modeling Hydrology and Earth System Sciences Discussions, 8, 6113-6153, 2011 Author(s): Y. He, A. Bárdossy, and E. Zehe A sound catchment classification scheme is a fundamental step towards improved catchment hydrology science and prediction in ungauged basins. Two categories of catchment classification methods are presented in the paper. The first one is based directly on physiographic properties and climatic conditions over a catchment and regarded as a Linnaean type or natural classification scheme. The second one is based on numerical clustering and regionalization methods and considered as a statistical or arbitrary classification scheme. This paper reviews each category including what has been done since recognition of the intrinsic value of catchment classification, what is being done in the current research, as well as what is to be done in the future.

Description: Infiltration-soil moisture redistribution under natural conditions: experimental evidence as a guideline for realizing simulation models Hydrology and Earth System Sciences Discussions, 8, 6199-6225, 2011 Author(s): R. Morbidelli, C. Corradini, C. Saltalippi, A. Flammini, and E. Rossi The evolution in time, t , of the experimental soil moisture vertical profile under natural conditions is investigated in order to address the corresponding simulation modelling. The measurements were conducted in a plot with a bare silty loam soil. The soil water content, θ, was continuously monitored at different depths, z , using a Time Domain Reflectometry (TDR) system. For each profile four buriable three-rod waveguides were inserted horizontally at different depths (5, 15, 25 and 35 cm). In addition, we used sensors of air temperature and relative humidity, wind speed, solar radiation, evaporation and rain as supports for the application of selected simulation models, as well as for the detection of elements leading to their improvement. The results indicate that, under natural conditions, very different trends of the θ( z , t ) function can be observed in the given fine-textured soil, where the formation of a sealing layer over the parent soil requires an adjustment of the simulation modelling commonly used for hydrological applications. In particular, because of the considerable variations in the shape of the moisture content vertical profile as a function of time, a generalization of the existing models should incorporate a representation of the variability in time of the saturated hydraulic conductivity of the uppermost soil. This conclusion is supported by the fact that the observed shape of θ( z ) can be appropriately reproduced by adopting this approach, however the observed rainfall rate and the occurrence of freeze-thaw cycles with high soil moisture contents have to be explicitly incorporated.

Description: Sensitivity studies for a space-based methane lidar mission Atmospheric Measurement Techniques Discussions, 4, 3545-3592, 2011 Author(s): C. Kiemle, M. Quatrevalet, G. Ehret, A. Amediek, A. Fix, and M. Wirth Methane is the third most important greenhouse gas in the atmosphere after water vapour and carbon dioxide. A major handicap to quantify the emissions at the Earth's surface in order to better understand biosphere-atmosphere exchange processes and potential climate feedbacks is the lack of accurate and global observations of methane. Space-based integrated path differential absorption (IPDA) lidar has potential to fill this gap, and a Methane Remote Lidar Mission (MERLIN) on a small satellite in Polar orbit was proposed by DLR and CNES in the frame of a German-French climate monitoring initiative. System simulations are used to identify key performance parameters and to find an advantageous instrument configuration, given the environmental, technological, and budget constraints. The sensitivity studies use representative averages of the atmospheric and surface state to estimate the measurement precision, i.e. the random uncertainty due to instrument noise. Key performance parameters for MERLIN are average laser power, telescope size, orbit height, surface reflectance, and detector noise. A modest-size lidar instrument with 0.45 W average laser power and 0.55 m telescope diameter on a 506 km orbit could provide 50-km averaged methane column measurement along the sub-satellite track with a precision of about 1 % over vegetation. The use of a methane absorption trough at 1.65 μm improves the near-surface measurement sensitivity and vastly relaxes the wavelength stability requirement that was identified as one of the major technological risks in the pre-phase A studies for A-SCOPE, a space-based IPDA lidar for carbon dioxide at the European Space Agency. Minimal humidity and temperature sensitivity at this wavelength position will enable accurate measurements in tropical wetlands, key regions with largely uncertain methane emissions. In contrast to actual passive remote sensors, measurements in Polar Regions will be possible and biases due to aerosol layers and thin ice clouds will be minimised.

Description: Information operator approach applied to the retrieval of the vertical distribution of atmospheric constituents from ground-based high-resolution FTIR measurements Atmospheric Measurement Techniques Discussions, 4, 3739-3785, 2011 Author(s): C. Senten, M. De Mazière, G. Vanhaelewyn, and C. Vigouroux The analysis of high spectral resolution Fourier Transform infrared (FTIR) solar absorption spectra is an important issue in remote sensing. If this is done carefully, one can obtain information, not only about the total column abundances, but also about the vertical distribution of various constituents in the atmosphere. This work introduces the application of the information operator approach for extracting vertical profile information from ground-based FTIR measurements. The algorithm is implemented and tested within the well-known retrieval code SFIT2, adapting the optimal estimation method such as to take into account only the significant contributions to the solution. In particular, we demonstrate the feasibility of the method in an application to ground-based FTIR spectra taken in the frame of the Network for the Detection of Atmospheric Composition Change (NDACC) at Ile de La Réunion (21° S, 55° E). A thorough comparison is made between the original optimal estimation method and this alternative retrieval algorithm, regarding information content, retrieval robustness and corresponding full error budget evaluation for the target species ozone (O 3 ), nitrous oxide (N 2 O), methane (CH 4 ), and carbon monoxide (CO). For O 3 and CH 4 , a comparison with the Tikhonov regularization method has also been made. It is shown that the information operator approach performs well and in most cases yields both a better accuracy and stability than the optimal estimation method. Additionally, the information operator approach has the advantage of being less sensitive to the choice of a priori information. The Tikhonov regularization results seem to be situated between both methods' results, as to profile retrievals, error budgets and column stability.

Description: Opportunistic validation of sulfur dioxide in the Sarychev Peak volcanic eruption cloud Atmospheric Measurement Techniques Discussions, 4, 3861-3875, 2011 Author(s): S. A. Carn and T. M. Lopez We report attempted validation of Ozone Monitoring Instrument (OMI) sulfur dioxide (SO 2 ) retrievals in the stratospheric volcanic cloud from Sarychev Peak (Kurile Islands) in June 2009, through opportunistic deployment of a ground-based ultraviolet (UV) spectrometer (FLYSPEC) as the volcanic cloud drifted over Central Alaska. The volcanic cloud altitude (~12–14 km) was constrained using coincident CALIPSO lidar observations. By invoking some assumptions about the spatial distribution of SO 2 , we derive averages of FLYSPEC vertical SO 2 columns for comparison with OMI SO 2 measurements. Despite limited data, we find minimum OMI-FLYSPEC differences of ~5–6 % which support the validity of the operational OMI SO 2 algorithm. These measurements represent the first attempt to validate SO 2 in a stratospheric volcanic cloud using a mobile ground-based instrument, and demonstrate the need for a network of rapidly deployable instruments for validation of space-based volcanic SO 2 measurements.

Description: Inversion of tropospheric profiles of aerosol extinction and HCHO and NO 2 mixing ratios from MAX-DOAS observations in Milano during the summer of 2003 and comparison with independent data sets Atmospheric Measurement Techniques Discussions, 4, 3891-3964, 2011 Author(s): T. Wagner, S. Beirle, T. Brauers, T. Deutschmann, U. Frieß, C. Hak, J. D. Halla, K. P. Heue, W. Junkermann, X. Li, U. Platt, and I. Pundt-Gruber We present aerosol and trace gas profiles derived from MAX-DOAS observations. Our inversion scheme is based on simple profile parameterisations used as input for an atmospheric radiative transfer model (forward model). From a least squares fit of the forward model to the MAX-DOAS measurements, two profile parameters are retrieved including integrated quantities (aerosol optical depth or trace gas vertical column density), and parameters describing the height and shape of the respective profiles. From these results, the aerosol extinction and trace gas mixing ratios can also be calculated. We apply the profile inversion to MAX-DOAS observations during a measurement campaign in Milano, Italy, September 2003, which allowed simultaneous observations from three telescopes (directed to north, west, south). Profile inversions for aerosols and trace gases were possible on 23 days. Especially in the middle of the campaign (17–20 September 2003), enhanced values of aerosol optical depth and NO 2 and HCHO mixing ratios were found. The retrieved layer heights were typically similar for HCHO and aerosols. For NO 2 , lower layer heights were found, which increased during the day. The MAX-DOAS inversion results are compared to independent measurements: (1) aerosol optical depth measured at an AERONET station at Ispra; (2) near-surface NO 2 and HCHO (formaldehyde) mixing ratios measured by long path DOAS and Hantzsch instruments at Bresso; (3) vertical profiles of HCHO and aerosols measured by an ultra light aircraft. Depending on the viewing direction, the aerosol optical depths from MAX-DOAS are either smaller or larger than those from AERONET observations. Similar comparison results are found for the MAX-DOAS NO 2 mixing ratios versus long path DOAS measurements. In contrast, the MAX-DOAS HCHO mixing ratios are generally higher than those from long path DOAS or Hantzsch instruments. The comparison of the HCHO and aerosol profiles from the aircraft showed reasonable agreement with the respective MAX-DOAS layer heights. From the comparison of the results for the different telescopes, it was possible to investigate the internal consistency of the MAX-DOAS observations. As part of our study, a cloud classification algorithm was developed (based on the MAX-DOAS zenith viewing directions), and the effects of clouds on the profile inversion were investigated. Different effects of clouds on aerosols and trace gas retrievals were found: while the aerosol optical depth is systematically underestimated and the HCHO mixing ratio is systematically overestimated under cloudy conditions, the NO 2 mixing ratios are only slightly affected. These findings are in basic agreement with radiative transfer simulations.

Description: Modeling the ascent of sounding balloons: derivation of the vertical air motion Atmospheric Measurement Techniques Discussions, 4, 3965-4012, 2011 Author(s): A. Gallice, F. G. Wienhold, C. R. Hoyle, F. Immler, and T. Peter A new model to describe the ascent of sounding balloons in the troposphere and lower stratosphere (up to ~30–35 km altitude) is presented. Contrary to previous models, detailed account is taken of both the variation of the drag coefficient with altitude and the heat imbalance between the balloon and the atmosphere. To compensate for the lack of data on the drag coefficient of sounding balloons, a reference curve for the relationship between drag coefficient and Reynolds number is derived from a dataset of flights launched during the Lindenberg Upper Air Methods Intercomparisons (LUAMI) campaign. The transfer of heat from the surrounding air into the balloon is accounted for by solving the radial heat diffusion equation inside the balloon. The potential applications of the model include the forecast of the trajectory of sounding balloons, which can be used to increase the accuracy of the match technique, and the derivation of the air vertical velocity. The latter is obtained by subtracting the ascent rate of the balloon in still air calculated by the model from the actual ascent rate. This technique is shown to provide an approximation for the vertical air motion with an uncertainty error of 0.5 m s −1 in the troposphere and 0.2 m s −1 in the stratosphere. An example of extraction of the air vertical velocity is provided in this paper. We show that the air vertical velocities derived from the balloon soundings in this paper are in general agreement with small-scale atmospheric velocity fluctuations related to gravity waves, mechanical turbulence, or other small-scale air motions measured during the SUCCESS campaign (Subsonic Aircraft: Contrail and Cloud Effects Special Study) in the orographically unperturbed mid-latitude middle troposphere.

Description: Sand box experiments to evaluate the influence of subsurface temperature probe design on temperature based water flux calculation Hydrology and Earth System Sciences Discussions, 8, 6155-6197, 2011 Author(s): M. Munz, S. E. Oswald, and C. Schmidt Quantification of subsurface water fluxes based on the one dimensional solution to the heat transport equation depends on the accuracy of measured subsurface temperatures. The influence of temperature probe setup on the accuracy of vertical water flux calculation was systematically evaluated in this experimental study. Four temperature probe setups were installed into a sand box experiment to measure temporal highly resolved vertical temperature profiles under controlled water fluxes in the range of ±1.3 m d −1 . Pass band filtered time series provided amplitude and phase of the diurnal temperature signal varying with depth depending on water flux. Amplitude ratios of setups directly installed into the saturated sediment significantly varied with sand box hydraulic gradients. Amplitude ratios provided an accurate basis for the analytical calculation of water flow velocities, which matched measured flow velocities. Calculated flow velocities were sensitive to thermal properties of saturated sediment and to probe distance, but insensitive to thermal dispersivity equal to solute dispersivity. Amplitude ratios of temperature probe setups indirectly installed into piezometer pipes were influenced by thermal exchange processes within the pipes and significantly varied with water flux direction only. Temperature time lags of small probe distances of all setups were found to be insensitive to vertical water flux.

Description: The use of LIDAR as a data source for digital elevation models – a study of the relationship between the accuracy of digital elevation models and topographical attributes in northern peatlands Hydrology and Earth System Sciences Discussions, 8, 5497-5522, 2011 Author(s): A. Hasan, P. Pilesjö, and A. Persson It is important to study the factors affecting estimates of wetness since wetness is crucial in climate change studies. The availability of digital elevation models (DEMs) generated with high resolution data is increasing, and their use is expanding. LIDAR earth elevation data have been used to create several DEMs with different resolutions, using various interpolation parameters, in order to compare the models with collected surface data. The aim is to study the accuracy of DEMs in relation to topographical attributes such as slope and drainage area, which are normally used to estimate the wetness in terms of topographic wetness indices. Evaluation points were chosen from the high-resolution LIDAR dataset at a maximum distance of 10 mm from the cell center for each DEM resolution studied, 0.5, 1, 5, 10, 30 and 90 m. The interpolation method used was inverse distance weighting method with four search radii: 1, 2, 5 and 10 m. The DEM was evaluated using a quantile-quantile test and the normalized median absolute deviation. The accuracy of the estimated elevation for different slopes was tested using the DEM with 0.5 m resolution. Drainage areas were investigated at three resolutions, with coinciding evaluation points. The ability of the model to generate the drainage area at each resolution was obtained by pairwise comparison of three data subsets. The results show that the accuracy of the elevations obtained with the DEM model are the same for different resolutions, but vary with search radius. The accuracy of the values (NMAD of errors) varies from 29.7 mm to 88.9 mm, being higher for flatter areas. It was also found that the accuracy of the drainage area is highly dependent on DEM resolution. Coarse resolution yielded larger estimates of the drainage area but lower slope values. This may lead to overestimation of wetness values when using a coarse resolution DEM.

Description: Multivariate design via Copulas Hydrology and Earth System Sciences Discussions, 8, 5523-5558, 2011 Author(s): G. Salvadori, C. De Michele, and F. Durante Calculating return periods and design quantiles in a multivariate framework is a difficult problem: essentially, this is due to the lack of a natural total order in multi-dimensional Euclidean spaces. This paper tries to make the issue clear. First, we outline a possible way to introduce a coherent notion of multivariate total order, and discuss its consequences on the calculation of multivariate return period: in particular, the latter is based on Copulas and the Kendall's measure, which provides a consistent notion of multivariate quantile. Secondly, we introduce several approaches for the identification of critical design events: these latter quantities are of utmost importance in practical applications, but their calculation is yet limited, due to the lack of a suitable theoretical setting where to embed the problem. Throughout the paper, a case study involving the behavior of a dam is used to illustrate the new concepts outlined in this work.

Description: Influence of initial heterogeneities and recharge limitations on the evolution of aperture distributions in carbonate aquifers Hydrology and Earth System Sciences Discussions, 8, 5631-5666, 2011 Author(s): B. Hubinger and S. Birk Karst aquifers evolve where the dissolution of soluble rocks causes the enlargement of discrete pathways along fractures or bedding planes, thus creating highly conductive solution conduits. To identify general interrelations between hydrogeological conditions and the properties of the evolving conduit systems the aperture-size frequency distributions resulting from generic models of conduit evolution are analysed. For this purpose, a process-based numerical model coupling flow and rock dissolution is employed. Initial protoconduits are represented by tubes with log-normally distributed aperture sizes with a mean of 0.5 mm. Apertures are spatially uncorrelated and widen up to the metre range due to dissolution by chemically aggressive waters. Several examples of conduit development are examined focussing on influences of the initial heterogeneity and the available amount of recharge. If the available recharge is sufficiently high the evolving conduits compete for flow and those with large apertures and high hydraulic gradients attract more and more water. As a consequence, the positive feedback between increasing flow and dissolution causes the breakthrough of a conduit pathway connecting the recharge and discharge sides of the modelling domain. Under these competitive flow conditions dynamically stable bimodal aperture distributions are found to evolve, i.e. a certain percentage of tubes continues to be enlarged while the remaining tubes stay small-sized. The percentage of strongly widened tubes is found to be independent of the breakthrough time and decreases with increasing heterogeneity of the initial apertures and decreasing amount of available water. If the competition for flow is suppressed because the availability of water is strongly limited breakthrough of a conduit pathway is inhibited and the conduit pathways widen very slowly. The resulting aperture distributions are found to be unimodal covering some orders of magnitudes in size. Under these suppressed flow conditions the entire range of apertures continues to be enlarged. Hence, the number of tubes reaching aperture sizes in the order of centimetres or decimetres continues to increase with time and in the long term may exceed the number of large-sized tubes evolving under competitive flow conditions. This suggests that conduit development under suppressed flow conditions may significantly enhance the permeability of the formation e.g. in deep-seated carbonate settings.

Description: Skewness as measure of the invariance of instantaneous renormalized drop diameter distributions – Part 1: Convective vs. stratiform precipitation Hydrology and Earth System Sciences Discussions, 8, 5605-5629, 2011 Author(s): M. Ignaccolo and C. De Michele We investigate the variability of the instantaneous distribution shape of the renormalized drop diameter making use of the third order central moment: the skewness . Disdrometer data, collected at Darwin Australia, are considered either as whole or as divided in convective and stratiform precipitation intervals. We show that in all cases the distribution of the skewness is strongly peaked around 0.64. This allows to identify a most common distribution of renormalized drop diameters and two main variations, one with larger and one with smaller skewness. The distributions' shapes are independent from the stratiform vs. convective classification.

Description: The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties Hydrology and Earth System Sciences Discussions, 8, 5667-5699, 2011 Author(s): H. Norouzi, M. Temimi, W. B. Rossow, C. Pearl, M. Azarderakhsh, and R. Khanbilvardi Microwave observations at low frequencies exhibit more sensitivity to surface and subsurface properties with little interference from the atmosphere. The objective of this study is to develop a global land emissivity product using passive microwave observations from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and to investigate its sensitivity to land surface properties. The developed product complements existing land emissivity products from SSM/I and AMSU by adding land emissivity estimates at two lower frequencies, 6.9 and 10.65 GHz (C- and X-band, respectively). Observations at these low frequencies penetrate deeper into the soil layer. Ancillary data used in the analysis, such as surface skin temperature and cloud mask, are obtained from International Satellite Cloud Climatology Project (ISCCP). Atmospheric properties are obtained from the TIROS Operational Vertical Sounder (TOVS) observations to determine the small upwelling and downwelling atmospheric emissions as well as the atmospheric transmission. A sensitivity test confirms the small effect of the atmosphere but shows that skin temperature accuracy can significantly affect emissivity estimates. Retrieved emissivities at C- and X-bands and their polarization differences exhibit similar patterns of variation with changes in land cover type, soil moisture, and vegetation density as seen at SSM/I-like frequencies (Ka and Ku bands). The emissivity maps from AMSR-E at these higher frequencies agree reasonably well with the existing SSM/I-based product. The inherent but small discrepancy introduced by the difference between SSM/I and AMSR-E frequencies and incidence angles has been examined and found to be small. Large differences between emissivity estimates from ascending and descending overpasses were found at the lower frequencies due to the inconsistency between the thermal IR skin temperatures and passive microwave brightness temperatures which can come from below the surface. This issue must be addressed in future studies to improve the accuracy of the emissivity estimates at lower frequencies.

Description: Influence of soil parameters on the skewness coefficient of the annual maximum flood peaks Hydrology and Earth System Sciences Discussions, 8, 5559-5604, 2011 Author(s): A. Gioia, V. Iacobellis, S. Manfreda, and M. Fiorentino Understanding the spatial variability of key parameters of flood probability distributions represents a strategy to provide insights on hydrologic similarity and building probabilistic models able to reduce the uncertainty in flood prediction in ungauged basins. In this work, we exploited the theoretically derived distribution of floods TCIF (Gioia et al., 2008), based on two different threshold mechanisms associated respectively to ordinary and extraordinary events. The model is based on the hypotheses that ordinary floods are generally due to rainfall events exceeding a threshold infiltration rate in a small source area, while the so-called outlier events, responsible of the high skewness of flood distributions, are triggered when severe rainfalls exceed a storage threshold over a large portion of the basin. Within this scheme, a sensitivity analysis was performed in order to analyze the effects of climatic and geomorphologic parameters on the skewness coefficient. In particular, the analysis was conducted investigating the influence on flood distribution of physical factors such as rainfall intensity, soil infiltration capacity, and basin area, in order to provide insights in catchment classification and process conceptualization.

Description: Technical note: Towards a continuous classification of climate using bivariate colour mapping Hydrology and Earth System Sciences Discussions, 8, 5733-5742, 2011 Author(s): A. J. Teuling Climate is often defined in terms of discrete classes. Here I use bivariate colour mapping to show that the global distribution of Köppen-Geiger climate classes can largely be reproduced by combining the simple means of two key states of the climate system (i.e., air temperature and relative humidity). This allows for a classification that is not only continuous in space, but can be applied at and transferred between timescales ranging from minutes to decades.

Description: The importance of parameter resampling for soil moisture data assimilation into hydrologic models using the particle filter Hydrology and Earth System Sciences Discussions, 8, 5849-5890, 2011 Author(s): D. A. Plaza, R. De Keyser, G. J. M. De Lannoy, L. Giustarini, P. Matgen, and V. R. N. Pauwels The Ensemble Kalman filter (EnKF) and the Sequential Importance Resampling (SIR) particle filter are evaluated for their performance in soil moisture assimilation and the consequent effect on discharge. With respect to the resulting soil moisture time series, both filters perform similarly. However, both filters have a negative effect on the discharge due to inconsistency between the parameter values and the states after the assimilation. In order to overcome this inconsistency, parameter resampling is applied along with the SIR filter, to obtain consistent parameter values with the analyzed soil moisture state. Extreme parameter replication, which could lead to a particle collapse, is avoided by the perturbation of the parameters with white noise. Both the modelled soil moisture and discharge are improved if the complementary parameter resampling is applied. The SIR filter with parameter resampling offers an efficient way to deal with biased observations. The robustness of the methodology is evaluated for 3 model parameter sets and 3 assimilation frequencies. Overall, the results in this paper indicate that the particle filter is a promising tool for hydrologic modelling purposes, but that an additional parameter resampling may be necessary to consistently update all state variables and fluxes within the model.

Description: Spatial moments of catchment rainfall: rainfall spatial organisation, basin morphology, and flood response Hydrology and Earth System Sciences Discussions, 8, 5811-5847, 2011 Author(s): D. Zoccatelli, M. Borga, A. Viglione, G. B. Chirico, and G. Blöschl This paper provides a general analytical framework for assessing the dependence existing between spatial rainfall organisation, basin morphology and runoff response. The analytical framework builds upon a set of spatial rainfall statistics (termed " spatial moments of catchment rainfall ") which describe the spatial rainfall organisation in terms of concentration and dispersion statistics as a function of the distance measured along the flow routing coordinate. The introduction of these statistics permits derivation of a simple relationship for the quantification of storm velocity at the catchment scale. The paper illustrates the development of the analytical framework and explains the conceptual meaning of the statistics by means of application to five extreme flash floods occurred in various European regions in the period 2002–2007. High resolution radar rainfall fields and a distributed hydrologic model are employed to examine how effective are these statistics in describing the degree of spatial rainfall organisation which is important for runoff modelling. This is obtained by quantifying the effects of neglecting the spatial rainfall variability on flood modelling, with a focus on runoff timing. The size of the study catchments ranges between 36 to 982 km 2 . The analysis reported here shows that the spatial moments of catchment rainfall can be effectively employed to isolate and describe the features of rainfall spatial organization which have significant impact on runoff simulation. These statistics provide essential information on what space-time scales rainfall has to be monitored, given certain catchment and flood characteristics, and what are the effects of space-time aggregation on flood response modeling.

Description: Evaluation of the transferability of hydrological model parameters for simulations under changed climatic conditions Hydrology and Earth System Sciences Discussions, 8, 5891-5915, 2011 Author(s): S. Bastola, C. Murphy, and J. Sweeney Conceptual hydrological models are widely used for climate change impact assessment. The implicit assumption in most such work is that the parameters estimated from observations remain valid for future climatic conditions. This paper evaluates a simple threshold based approach for testing this assumption, where a set of behavioural simulators are identified for different climatic conditions for the future simulation i.e. wet, average and dry conditions. These simulators were derived using three different data sets that are generated by sampling a block of one year of data without replacement from the observations such that they define the different climatic conditions. The simulators estimated from the wet climatic data set showed the tendency to underestimate flow when applied to dry data set and vice versa. However, the performances of the three sets of basin simulators on chronologically coherent data are identical to the simulators identified from a sufficiently long data series that contains both wet and dry climatic conditions. The results presented suggest that the issue of time invariance in the value of parameters has a minimal effect on the simulation if the change in precipitation is less than 10 % of the data used for calibration.

Description: Mass transfer effects in 2-D dual-permeability modeling of field preferential bromide leaching with drain effluent Hydrology and Earth System Sciences Discussions, 8, 5917-5967, 2011 Author(s): H. H. Gerke, J. Dusek, and T. Vogel Subsurface drained experimental fields are frequently used for studying preferential flow (PF) in structured soils. Considering two-dimensional (2-D) transport towards the drain, however, the relevance of mass transfer coefficients, apparently reflecting small-scale soil structural properties, for the water and solute balances of the entire drained field is largely unknown. This paper reviews and analyzes effects of mass transfer reductions on Br − leaching for a subsurface drained experimental field using a numerical 2-D dual-permeability model (2D-DPERM). The sensitivity of the "diffusive" mass transfer component on bromide (Br − ) leaching patterns is discussed. Flow and transport is simulated in a 2-D vertical cross-section using parameters, boundary conditions (BC), and data of a Br − tracer irrigation experiment on a subsurface drained field (5000 m 2 area) at Bokhorst (Germany), where soils have developed from glacial till sediments. The 2D-DPERM simulation scenarios assume realistic irrigation and rainfall rates, and Br-application in the soil matrix (SM) domain. The mass transfer reduction controls preferential tracer movement and can be related to physical and chemical properties at the interface between flow path and soil matrix in structured soil. A reduced solute mass transfer rate coefficient allows a better match of the Br − mass flow observed in the tile drain discharge. The results suggest that coefficients of water and solute transfer between PF and SM domains have a clear impact on Br − effluent from the drain. Amount and composition of the drain effluent is analyzed as a highly complex interrelation between temporally and spatially variable mass transfer in the 2-D vertical flow domain that depends on varying "advective" and "diffusive" transfer components, the spatial distribution of residual tracer concentrations, and the lateral flow fields in both domains from plots of the whole subsurface drained field. The local-scale soil structural effects (e.g., such as macropore wall coatings), here conceptualized as changes in mass transfer coefficients, can have a clear effect on leaching at the plot and field-scales.

Description: Evaluation and bias correction of satellite rainfall data for drought monitoring in Indonesia Hydrology and Earth System Sciences Discussions, 8, 5969-5997, 2011 Author(s): R. R. E. Vernimmen, A. Hooijer, Mamenun, and E. Aldrian The accuracy of satellite rainfall data from different sources, TRMM 3B42RT, CMORPH and PERSIANN, was investigated through comparison with reliable ground station rainfall data in Indonesia, with a focus on their ability to detect patterns of low rainfall that may lead to drought conditions. It was found that all sources underestimated rainfall in dry season months. The CMORPH and PERSIANN data differed most from ground station data and are also very different from the TRMM data. However, it proved possible to improve TRMM data to yield sufficiently accurate estimates, both for dry periods ( R 2 0.65–0.92) and annually ( R 2 0.84–0.96), applying a single parameterized bias correction equation that is constant in space and time. It is proposed that these bias corrected TRMM data be used in real-time drought monitoring, in Indonesia and probably in other countries where similar conditions exist. This will yield major advantages, in terms of accuracy, spatial coverage, timely availability and cost efficiency, over drought monitoring with only ground stations.

Description: Potential and limitations of the MAX-DOAS method to retrieve the vertical distribution of tropospheric nitrogen dioxide Atmospheric Measurement Techniques Discussions, 4, 4013-4072, 2011 Author(s): T. Vlemmix, A. J. M. Piters, A. J. C. Berkhout, L. F. L. Gast, P. Wang, and P. F. Levelt Muliple Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments can measure from the ground the absorption by nitrogen dioxide (NO 2 ) of scattered sunlight seen in multiple viewing directions. This paper studies the potential of this technique to derive the vertical distribution of NO 2 in the troposphere. Such profile information is essential in validation studies in which MAX-DOAS retrievals play a role. The retrieval algorithm used is based on a pre-calculated look-up table and assumes homogeneous mixing of aerosols and NO 2 in layers extending from the surface to a variable height. Two retrieval models are compared: one including and one excluding an elevated NO 2 layer at a fixed altitude in the free troposphere. An ensemble technique is applied to derive retrieved model uncertainties. Sensitivity studies demonstrate that MAX-DOAS based retrievals can make a distinction between an NO 2 layer that extends from the surface to a certain height (having a constant mixing ratio, or a mixing ratio that decreases with altitude) and an elevated NO 2 layer. The height of the elevated NO 2 layer can only be retrieved accurately when the aerosol extinction profile is known and the measurement noise is low. The uncertainty in this elevated NO 2 layer height provides the main source of uncertainty in the retrieval of the free tropospheric contribution to the tropospheric NO 2 column. A comparison was performed with independent data, based on observations done at the CINDI campaign, held in the Netherlands in 2009. Comparison with lidar partial tropospheric NO 2 columns showed a correlation of 0.78, and an average difference of 0.1× 10 15 molec cm −2 . The diurnal evolution of the NO 2 volume mixing ratio measured by in-situ monitors at the surface and averaged over five days with cloud-free mornings, compares quite well to the MAX-DOAS retrieval: a correlation was found of 0.8, and an average difference of 0.2 ppb.

Description: Water isotopic ratios from a continuously melted ice core sample Atmospheric Measurement Techniques Discussions, 4, 4073-4104, 2011 Author(s): V. Gkinis, T. J. Popp, T. Blunier, M. Bigler, S. Schüpbach, and S. J. Johnsen A new technique for on-line high resolution isotopic analysis of liquid water, tailored for ice core studies is presented. We build an interface between an Infra Red Cavity Ring Down Spectrometer (IR-CRDS) and a Continuous Flow Analysis (CFA) system. The system offers the possibility to perform simultaneuous water isotopic analysis of δ 18 O and δD on a continuous stream of liquid water as generated from a continuously melted ice rod. Injection of sub μl amounts of liquid water is achieved by pumping sample through a fused silica capillary and instantaneously vaporizing it with 100 % efficiency in a home made oven at a temperature of 170 °C. A calibration procedure allows for proper reporting of the data on the VSMOW scale. We apply the necessary corrections based on the assessed performance of the system regarding instrumental drifts and dependance on humidity levels. The melt rates are monitored in order to assign a depth scale to the measured isotopic profiles. Application of spectral methods yields the combined uncertainty of the system at below 0.1 ‰ and 0.5 ‰ for δ 18 O and δD, respectively. This performance is comparable to that achieved with mass spectrometry. Dispersion of the sample in the transfer lines limits the resolution of the technique. In this work we investigate and assess these dispersion effects. By using an optimal filtering method we show how the measured profiles can be corrected for the smoothing effects resulting from the sample dispersion. Considering the significant advantages the technique offers, i.e. simultaneuous measurement of δ 18 O and δD, potentially in combination with chemical components that are traditionally measured on CFA systems, notable reduction on analysis time and power consumption, we consider it as an alternative to traditional isotope ratio mass spectrometry with the possibility to be deployed for field ice core studies. We present data acquired in the framework of the NEEM deep ice core drilling project in Greenland, during the 2010 field season.

Description: Effects of antecedent soil moisture on runoff modeling in small semiarid watersheds of southeastern Arizona Hydrology and Earth System Sciences Discussions, 8, 6227-6256, 2011 Author(s): Y. Zhang, H. Wei, and M. A. Nearing Antecedent soil moisture prior to a rain event influences the rainfall-runoff relationship. Very few studies have looked at the effects of antecedent soil moisture on runoff modeling sensitivities in arid and semi-arid areas. This study examines the influence of initial soil moisture on model runoff prediction capability in small semiarid watersheds using model sensitivity and by comparing the use of antecedent vs. average long term soil water content for defining the model initial conditions for the modified Green-Ampt Mein-Larson model within the Rangeland Hydrology and Erosion Model (RHEM). Measured rainfall, runoff, and soil moisture data from four semiarid rangeland watersheds ranging in size from 0.34 to 4.53 ha on the Walnut Gulch Experimental Watershed in southeastern Arizona, USA, were used. Results showed that: (a) there were no significant correlations between measured runoff ratio and antecedent soil moisture in any of the four watersheds; (b) average sensitivities of simulated runoff amounts and peaks to antecedent soil moisture were 0.05 mm and 0.18 mm h −1 , respectively, with each 1 % change in antecedent soil moisture; (c) runoff amounts and peaks simulated with long term average soil moisture were statistically equivalent to those simulated with measured antecedent soil moisture. The relative lack of sensitivity of modeled runoff to antecedent soil moisture in this case is contrary to results reported in other studies, and is largely due to the fact that the surface soil is nearly always very dry in this environment.