ALBERT

Description: Snow availability in Alpine catchments plays an important role in water resources management. In this paper we propose a method for an optimal estimation of snow depth (areal extension and thickness) in Alpine systems from point data and satellite observations by using significant explanatory variables deduced from a digital terrain model. It is intended to be a parsimonious approach that may complement physical-based methodologies. Different techniques (multiple regression, multi-criteria analysis and kriging) are integrated to address the following issues: We identify the explanatory variables that could be helpful based on a critical review of the scientific literature. We study the relationship between ground observations and explanatory variables using a systematic procedure for a complete multiple regression analysis. Multiple regression models are calibrated combining all suggested model structures and explanatory variables. We also propose an evaluation of the models (using indices to analyze the goodness of fit) and select the best approaches (models and variables) based on multi-criteria analysis. Estimation of the snow depth is performed with the selected regression models. The residual estimation is improved by applying kriging in cases with spatial correlation. The final estimate is obtained by combining regression and kriging results, and constraining the snow domain in accordance with satellite data. The method is illustrated using the case study of the Sierra Nevada mountain range (Southern Spain). A cross validation experiment has confirmed the efficiency of the proposed procedure. Finally, although it is not the scope of this work, the snow depth is used to asses a first estimation of snow water equivalent (SWE) resources.

Description: The urban environment modifies the hydrologic cycle resulting in increased runoff rates, volumes and peak flows. Green Infrastructure, which uses best management practices (BMPs), is a natural system approach used to mitigate the impacts of urbanization onto stormwater runoff. Patterns of stormwater runoff from urban environments are complex and it is unclear how efficiently green infrastructure will improve the urban water cycle. These challenges arise from issues of scale, the merits of BMPs depend on changes to small scale hydrologic processes aggregated up from the neighborhood to the urban watershed. Here, we use a hyper-resolution (1 m), physically-based hydrologic model of the urban hydrologic cycle with explicit inclusion of the built environment. This model represents the changes to hydrology at the BMP scale (~1 m), represents each individual BMP explicitly to represent response over the urban watershed. Our study varies both the percentage of BMP emplacement and their spatial location for storm events of increasing intensity in an urban watershed. We develop a metric of effectiveness that indicates a nonlinear relationship is seen between percent BMP emplacement and storm intensity. Results indicate that BMP effectiveness varies with spatial location and that type and emplacement within the urban watershed may be more important than overall percent.

Description: A number of extensive droughts and destructive floods have occurred in Poland in the last 25 years, hence projections of low and high river flows are of considerable interest and importance. In the first part of this paper, projections of low and high flows in the rivers of the Vistula and the Odra basins (VOB region), for two future time horizons, are presented. Projections are based on the SWAT hydrological model simulations driven by results of the EURO-CORDEX experiment under Representative Concentration Pathways 4.5 and 8.5. The VOB region covers most of Poland and parts of five neighboring countries, giving this study an international relevance. In the second part of the paper a review of projections of low and high flows in rivers in Central and Eastern Europe is presented. Despite a substantial spread of flow projections, the main message of the modelling part is that increases of both low and high flows are dominating. The magnitude of increase of low flow is considerably higher than that of high flow. In other words, future streamflow droughts are projected to be less severe, while, in contrast, river floods are projected to increase, which is a challenge for flood risk reduction, water management and climate change adaptation. There is an overall agreement of our findings for the VOB region with projections of hydrological extremes from large-scale models forced by EURO-CORDEX results in the European-scale studies.

Description: Transformations of precipitation into groundwater and streamflow are fundamental hydrological processes, critical to irrigated agriculture, hydroelectric power generation and ecosystem health. Our understanding of the timing of groundwater recharge and streamflow generation remains incomplete, limiting our ability to predict fresh water, nutrient, and contaminant fluxes, especially in large basins. Here we analyze thousands of rain, snow, groundwater and streamflow δ 18 O and δ 2 H values in the Nelson River basin, which covers 1.2 million km 2 of central Canada. We show that the fraction of precipitation that recharges aquifers is ~1.3-5 times higher for precipitation falling during cold months with subzero mean monthly temperatures than for precipitation falling during warmer months. The near-ubiquity of cold-season-biased groundwater recharge implies that changes to winter water balances may have disproportionate impacts on annual groundwater recharge rates. We also show that young streamflow—defined as precipitation that enters a river in less than ~2.3 months—comprises ~27 % of annual streamflow, but varies widely among tributaries in the Nelson River basin (1-59 %). Young streamflow fractions are lower in steep catchments, and higher in flatter catchments such as the transboundary Red River basin. Our findings imply that flat, lower-permeability, heavily-tiled landscapes favor more rapid transmission of precipitation into rivers, possibly mobilizing excess soluble fertilizers and exacerbating eutrophication events in Lake Winnipeg.

Description: Saturation-excess runoff is the major runoff mechanism in humid well-vegetated areas where infiltration rates often exceed rainfall intensity. While the Soil and Water Assessment Tool (SWAT) is one of the most widely used models, it predicts runoff based mainly on soil and land use characteristics, and is implicitly an infiltration-excess runoff type of model. Previous attempts to incorporate the saturation-excess runoff mechanism in SWAT fell short due to the inability to distribute water from one Hydrological Response Unit (HRU) to another. This paper introduces a modified version of SWAT, referred to as SWAT-Hillslope (SWAT-HS). This modification improves the simulation of saturation-excess runoff by redefining HRUs based on wetness classes, and by introducing a surface aquifer with the ability to route interflow from “drier” to “wetter” wetness classes. Mathematically, the surface aquifer is a non-linear reservoir that generates rapid subsurface stormflow as the water table in the surface aquifer rises. The SWAT-HS model was tested in the Town Brook watershed in the upper reaches of the West Branch Delaware River in the Catskill region of New York, USA. SWAT-HS predicted discharge well with a Nash-Sutcliffe Efficiency of 0.68 and 0.87 for daily and monthly time steps. Compared to the original SWAT model, SWAT-HS predicted less surface runoff and groundwater flow and more lateral flow. The saturated areas predicted by SWAT-HS were concentrated in locations with a high topographic index and were in agreement with field observations. With the incorporation of topographic characteristics and the addition of the surface aquifer, SWAT-HS improved streamflow simulation and gave a good representation of saturated areas on the date that measurements were available. SWAT-HS is expected to improve water quality model predictions where the location of the surface runoff matters.

Description: Precipitation and temperature time series suffer from many problems, such as short time period, inadequate spatial coverage, missing data and biases from various causes, which is particularly critical in remote areas such as Northern Canada. The development of alternative datasets for using as proxies for inadequate/missing weather data represents a key research area. In this paper, the performance of six alternative datasets is evaluated for hydrological modeling over 12 watersheds located across Canada and the contiguous United States. The datasets can be classified into three distinct categories: (1) interpolated gridded data, (2) reanalysis data, and (3) climate model outputs. Hydrological simulations were carried out using a lumped conceptual hydrological model calibrated using standard weather data, and compared against results using a calibration specific to each alternative dataset. Prior to the hydrological simulations, the alternative datasets were all evaluated with respect to their ability to reproduce gridded daily precipitation and temperature characteristics over North America. The results show that both the reanalysis data and climate model data adequately represent the spatial pattern of daily precipitation and temperature over North America. The North American Regional Reanalysis (NARR) dataset consistently shows the best performance. With respect to hydrological modeling, the observed discharges are accurately represented by both the gridded and NARR datasets, and more so for the NARR data. The National Centers for Environmental Prediction (NCEP) dataset consistently performs worst as it is unable to even capture the seasonal pattern of observed streamflow for three out of the 12 watersheds. These results indicate that the NARR dataset could be used as a proxy for gauged precipitation and temperature for hydrological modeling over watersheds where observational datasets are deficient. The results also illustrate the ability of climate model data to be used for performing hydrological modeling when driven by reanalysis data at their boundaries, and especially so for high-resolution models.

Description: Flat terrain and soils with variable permeability make it difficult to assess the relative importance of surface and subsurface runoff in the Canadian Prairies, especially in riparian areas that are critical for water transmission and solute transport. The main objective of this study is therefore to determine whether patterns of hydrologic state variables, namely near-surface soil moisture (SM), soil electrical conductivity (SEC) and soil temperature (ST), can help infer riparian-to-stream soil water movement in Prairie landscapes. Focus is on the near-level Catfish Creek Watershed (south-eastern Manitoba, Canada) where three riparian sites were monitored: a naturally vegetated grassland site, a headwater forested site, and a highly impacted grassed site adjacent to an engineered drainage dyke and a man-made drainage channel. Data from nine to twelve SM, SEC and ST surveys completed at each site in 2015 using a 75-point grid are matched with riparian water table data, surface water level data from adjacent drainage channels and indicators of antecedent moisture conditions. Pattern characteristics, in the form of descriptive statistics and variogram parameters, are estimated for each state variable and then correlated to indicators of antecedent moisture conditions (AMCs), stream and subsurface water level data to infer soil water movement. Results show that potential evapotranspiration, depth to water table and antecedent precipitation have a significant yet variable impact on SM, SEC and ST patterns. A switching behaviour, between dry and wet conditions, is present in riparian areas characterized by grassland vegetation and well-drained soils. The occurrence of shallow subsurface flow is inferred during the wettest conditions. While riparian SM conditions are useful for predicting streamflow response in adjacent channels, such is not the case for riparian SEC and ST. Further investigations are however necessary to confirm the usefulness of SM spatial patterns for predicting streamflow response in other landscapes across the Canadian Prairies.

Description: Bioretention cells, which are generally effective in controlling surface runoff and recharging groundwater, have been widely adopted as low impact development practices. However, shallow groundwater has limited their implementation in some locations due to the potential problems of a reduction in surface runoff control, groundwater pollution, and continuous groundwater drainage through the underdrain. Many guidelines have established minimum requirements for the groundwater depth below bioretention cells, but they may not be optimized for certain environmental conditions and bioretention cell designs. This study made use of a variably saturated flow model to examine the hydrologic performance of a single bioretention cell in shallow groundwater with event-based simulations, considering a wide range of initial groundwater depths, media and in situ soil types, surface runoff loads, and underdrain sizes. Performance indicators (e.g., runoff reduction, time for infiltrated water to reach the bioretention cell bottom and the groundwater table, height and dissipation time of groundwater mound) were evaluated to examine the processes of runoff generation, the formation and dissipation of groundwater mounds, and the bioretention cell's performance in a shallow groundwater environment. The most influential factors were the initial groundwater depth, the hydraulic conductivity of the media soil, and the rainfall runoff load. With a deeper initial groundwater table, infiltrated water took longer to reach the bioretention cell bottom and groundwater table. Groundwater mounds, however, took longer to dissipate even though they were smaller. The groundwater quality can be better protected if relatively less-permeable soil types (e.g., sandy loam) are used as the media, although it may compromise the performance in runoff quantity control. However, only very high surface runoff loads would cause concerns regarding a reduction in runoff quantity control and possible groundwater contamination due to the shallow groundwater. A distance of 1.5-3 m between the bioretention cell bottom and the groundwater table is generally sufficient. The results of this study could help to guide the planning and design of bioretention cells in areas of shallow groundwater.

Description: Northern peatlands are a vital component of the global carbon cycle, containing large stores of soil organic carbon and acting as a long-term carbon sink. Moss productivity is an important factor in determining whether these wetlands will retain this function under future climatic conditions. Research on unsaturated water flow in peatlands, which controls moss productivity during periods of evaporative stress, has focused on relatively deep bog systems. However, shallower peatlands and marginal connective wetlands can be essential components of many landscape mosaics. In order to better understand factors influencing moss productivity, water balance simulations using Hydrus 1-D were run for different soil profile depths, compositions and antecedent moisture conditions. Our results demonstrate a bimodal distribution of peatland realizations; either primarily conserving water by limiting evapotranspiration or, maximizing moss productivity. For sustained periods of evaporative stress, both deep water storage and a shallow initial water table delay the onset of high vegetative stress, thus maximizing moss productivity. A total depth of sand and peat of 0.8 m is identified as the threshold above which increasing peat depth has no effect on changing vegetative stress response. In contrast, wetlands with shallow peat deposits (less than 0.5 m thick) are least able to buffer prolonged periods of evaporation due to limited labile water storage, and will thus quickly experience vegetative stress and so limit evaporation and conserve water. With a predicted increase in the frequency and size of rain events in continental North America the moss productivity of shallow wetland systems may increase, but also greater moisture availability will increase the likelihood they remain as wetlands in a changing climate.

Description: Shallow aquifers typically have greater hydrologic connectivity and response to recharge and changes in surface water management practices than deeper aquifers, and are therefore often managed to reduce the risk of flooding. Quantification of the water table elevation response under different management scenarios provides valuable information in shallow aquifer systems to assess indirect influences of such modifications. The Episodic Master Recession method was applied to the 15-min water table elevation and NEXRAD rainfall data for six wells to identify water table response and individual rainfall events. The objectives of this study were to evaluate the effects of rainfall, water table elevation, canal stage, site specific characteristics, and canal structure modification/ water management practice on the fluctuations in water table elevations using multiple/stepwise multiple linear regression techniques. With the modification of canal structure and operation adjustment, significant difference existed in water table response in the southern wells due to its relative downstream position regarding the general groundwater flow direction and the structural modification locations. On average, water table response height and flood risk were lower after than before the structure modification to canals. The effect of rainfall event size on the height of water table response was greater than the effect of antecedent water table elevation and canal stage on the height of water table response. Other factors including leakance of the canal bed sediment, specific yield, and rainfall on i-1 day had significant effects on the height of water table response as well. Antecedent water table elevation and canal stage had greater and more linear effects on the height of water table response after the management changes to canals. Variation in water table response height/rainfall event size ratio was attributed to difference in S y , antecedent soil water content, hydraulic gradient, rainfall size and runoff ratio. After the structure modification, water table response height/rainfall event size ratio demonstrated more linear and proportional relationship with antecedent water table elevation and canal stage.

Description: The proper management of coastal aquifers commonly requires an understanding of regional mass flow and complete seawater-freshwater circulation. In this study, time series observations of seawater intrusion and refreshing were conducted using a column experiment based on natural flow conditions in coastal groundwater and a sampled medium from a coastal sandy aquifer without chemical treatment. Ranges of hydrodynamical and hydrochemical variables were tested and analyzed. The results showed that the zeta potential of suspended colloids in aqueous solution in an aquifer polluted with 0.5 g/kg of heavy metals exhibited an isoelectric point for pH values ranging from 5.70 to 6.07 when freshwater or seawater completely occupied the aquifer pores, which is representative of natural hydrochemical conditions. In this scenario, a high background concentration of heavy metals induced colloidal immobilization. Otherwise, seawater-freshwater circulation enabled colloid mobilization due to ionic strength and pH fluctuations. The migration of multiple heavy metals occurred at a characteristic time of approximately 1 pore volume after each intrusion stage began and when the peak rate of colloid release was reached. At these times, the colloid behavior determined the quantity and pathway of heavy metal transport. Based on the influences of seawater and freshwater interactions, the quantity of mobilized particles generally decreased and was uniformly distributed in each fraction due to particle loss and decreased porous connectivity. We speculate that the decrease in the total surface area of the migratory colloids may cause colloid-associated heavy metal transport to decrease. The experimental results provide a useful basis for testing coastal groundwater flow and mass transport models because these phenomena require full characterization to precisely evaluate the associated fluxes from the field scale to the microscopic dimension.

Description: The Natural Resources Conservation Service Curve Number model (NRCS CN) is one of the most recognizable procedures in the field of rainfall-runoff estimation. It has been widely applied for different purposes in hydrological models. In spite of its widespread use, some uncertainties have not even clarified, and must be examined for its proper application. Particularly, choosing the most representative rainfall-runoff events, and the coefficient λ which relates the parameters of the model (curve number CN and initial abstraction I a ). In this research, an advanced analysis is developed to evaluate the influence of λ for a set of representative watersheds of the ARS-USDA (Agricultural Research Service of the United Stated Department of Agriculture). They are characterized by different soil properties, land uses and climatic conditions. Finally, two novel methodologies for the selection of the most representative rainfall-runoff events and for the adaptation of coefficient λ are included, based on the pattern of rainfall distribution.

Description: Modeling nutrient transport during snowmelt in cold regions remains a major scientific challenge. A key limitation of existing nutrient models for application in cold regions is the inadequate representation of snowmelt, including hydrological and biogeochemical processes. This brief period can account for more than 80% of the total annual surface runoff in the Canadian Prairies and Northern Canada and processes such as atmospheric deposition, over-winter redistribution of snow, ion exclusion from snow crystals, frozen soils, and snowcovered area depletion during melt influence the distribution and release of snow and soil nutrients, thus affecting the timing and magnitude of snowmelt runoff nutrient concentrations. Research in cold regions suggests that nitrate (NO3) runoff at the field scale can be divided into five phases during snowmelt. In the first phase, water and ions originating from ion-rich snow layers travel and diffuse through the snowpack. This process causes ion concentrations in runoff to gradually increase. The second phase occurs when this snow ion meltwater front has reached the bottom of the snowpack and forms runoff to the edge-of-the-field (EOF). During the third and fourth phases, the main source of NO3 transitions from the snowpack to the soil. Finally, the fifth and last phase occurs when the snow has completely melted, and the thawing soil becomes the main source of NO3 to the stream. In this research, a process-based model was developed to simulate hourly export based on this five-phase approach. Results from an application in the Red River Basin of southern Manitoba, Canada shows that the model can adequately capture the dynamics and rapid changes of NO3 concentrations during this period at relevant temporal resolutions. This is a significant achievement to advance the current nutrient modeling paradigm in cold climates, which is generally limited to satisfactory results at monthly or annual resolutions. The approach can inform catchment-scale nutrient models to improve simulation of this critical snowmelt period. Nutrient exports Winter Snow Nitrate Agriculture Nutrient model

Description: Upland agricultural land management activities such as grazing, vegetation burning and bare ground restoration impact hydrological elements of headwater catchments, many of which may be important for downstream flood peaks (e.g. overland flow and soil water storage). However, there is poor understanding of how these management practices affect river flow peaks during high magnitude rainfall events. Using the distributed TOPMODEL, spatial configurations of land management were modelled to predict flood response in an upland catchment which contains different regions operating subsidised agricultural stewardship schemes. Heavy grazing leading to soil compaction and loss of vegetation cover in stewardship regions covering 79.8% of the catchment gave a 42 min earlier flow peak which was 82.2% higher (under a 1-hr 15 mm storm) than the current simulated hydrograph. Light grazing over the same regions of the catchment had much less influence on river flow peaks (18 min earlier and 32.9% increase). Rotational burning (covering 8.8% of the catchment), most of which is located in the headwater areas, increased the peak by 3.2% in the same rainfall event. Vegetation restoration with either Eriophorum or Sphagnum (higher density) in bare areas (5.8%) of the catchment provided a reduction of flood peak (3.9% and 5.2% in the 15 mm storm event); while, the same total area revegetated with Sphagnum in riparian regions delivered a much larger decrease (15.0%) in river flow peaks. We show that changes of vegetation cover in highly-sensitive areas (e.g. near-stream zones) generate large impacts on flood peaks. Thus it is possible to design spatially distributed management systems for upland catchments which reduce flood peaks while at the same time ensuring economic viability for upland farmers.

Description: Fluvial organic carbon (OC) transformations are an important component of carbon cycling and greenhouse gas production in inland waters resulting in considerable recent interest in the fate of fluvial OC exported from carbon rich soils such as peatlands. Additionally, peatland catchments are important drinking water collection areas, where high OC concentrations in runoff have water treatment implications. This analysis presents the results from a year-round intensive study within a water treatment catchment draining an area of peatland, considering carbon transformations along a continuum from headwater river, through a storage reservoir and pipe, to a water treatment works. The study uses a unique combination of methods (colourmetric, ultrafiltration and 14 C radiocarbon dating) to assess catchment wide changes in fluvial carbon composition (colour, size and age) alongside concentration measures. The results indicate clear patterns of carbon transformations in the river and reservoir, and dissolved Low Molecular Weight (LMW) coloured carbon to be most subject to change, with both loss and replacement within the catchment residence time. Whilst the evidence suggests Dissolved OC (DOC) gains are from Particulate OC (POC) breakdown, the mechanisms of DOC loss are less certain and may represent greenhouse gas losses or conversions to POC. The transformations presented here appear to have minimal impact on the amount of harder to treat (〈10 kDa) dissolved carbon, although they do have implications for total DOC loading to water treatment works. This paper has shown that reservoirs in peatland systems are not passive receptors of particulate and dissolved organic carbon from headwater systems but that this fluvial carbon load is actively cycled in the reservoir system with implications for understanding peatland carbon cycling and water treatment in peatland catchments.

Description: Soil pipes, continuous macropores parallel to the soil surface, are an important factor in hillslope hydrological processes. However, the water flow dynamics in soil pipes, especially closed soil pipes, is not well understood. In this study, the water and air dynamics within closed soil pipes have been investigated in a bench-scale laboratory experiment by using a soil box with an artificial acrylic soil pipe. In order to grasp the state of water and air within the soil pipe, the existing soil pipe flow and air pressure in the soil pipe were directly measured. The laboratory experiment showed that air in the soil pipe had an important role in the water flow in the closed soil pipe. When air entrapment occurred in the soil pipe before the soil matrix around the soil pipe was saturated with water, water intrusion in the soil pipe was prevented by air entrapped in the pipe, which inhibited the soil pipe flow. This air entrapment in the soil pipe was controlled by the soil water and air flow. Moreover, after the soil pipe flow started, the soil pipe was not filled completely with water even when the soil pipe was completely submerged under the groundwater table. The entrapped air in the soil pipe prevented further water intrusion in the soil pipe.

Description: It is often assumed that the net groundwater flow direction is toward the channel in headwater streams in humid climates, with magnitudes dependent on flow state. However, studies that characterize stream-groundwater interactions in ephemeral and intermittent streams in humid landscapes remain sparse. Here, we examined seasonally-driven stream-groundwater interactions in response to temporary streamflow based on field observations of streamflow and groundwater on an adjacent hillslope. The direction of hydraulic head gradients between the stream and groundwater shifted seasonally. The stream gained water (head gradients were toward the stream) when storage state was high. During this period, streamflow was persistent. The stream lost water to the groundwater system (head gradients were away from the stream) when storage state was low. During this period, streamflow only occurred in response to precipitation events and head gradients remained predominantly away from the stream during events. This suggested mechanisms other than deep groundwater contributions produced runoff when storage was low, such as surface and perched subsurface flowpaths above the water table. Analysis of the annual water balance for the study period showed that the residual between precipitation inputs and streamflow and evapotranspiration outputs, which were attributed to the loss of water to the deeper, regional groundwater system, was similar in magnitude to streamflow. This, coupled with results that showed bi-directionality in stream-groundwater head gradients, indicated that headwaters comprised of temporary (e.g. ephemeral and intermittent) streams can be important focal areas for regional groundwater recharge and both contribute to and receive water, solutes, and materials from the groundwater system.

Description: To understand the moisture regime at the southern slopes of Mt. Kilimanjaro, we analyzed the isotopic variability of oxygen (δ 18 O) and hydrogen (δD) of rainfall, throughfall and fog from a total of 2,140 samples collected weekly over two years at nine study sites along an elevation transect ranging from 950 m a.s.l. to 3,880 m a.s.l.. Precipitation in the Kilimanjaro tropical rainforests consists of a combination of rainfall, throughfall and fog. We defined Local Meteoric Water Lines (LMWL) for all three precipitation types individually and the overall precipitation (δD prec = 7.45(± 0.05) x δ 18 O prec + 13.61(± 0.20) ( n = 2,140; R 2 = 0.91, p 〈 0.001). We investigated the precipitation type specific stable isotope composition and analyzed effects of amount, altitude and temperature. Aggregated annual mean values revealed isotope composition of rainfall as most depleted and fog water as most enriched in heavy isotopes at the highest elevation research site. We found an altitude effect of δ 18 O rain = -0.11 ‰ * 100 m -1 , which varied according to precipitation type and season. The relatively weak isotope/altitude gradient may reveal two different moisture sources in the research area: (i) local moisture recycling and (ii) regional moisture sources. Generally, the seasonality of δ 18 O rain values follows the bimodal rainfall distribution under the influences of south- and northeasterly trade winds. These seasonal patterns of isotopic composition were linked to different regional moisture sources by analyzing HYSPLIT backward trajectories. Seasonality of d excess values revealed evidence of enhanced moisture recycling after the onset of the rainy seasons. This comprehensive dataset is essential for further research using stable isotopes as a hydrological tracer of sources of precipitation that contribute to water resources of the Kilimanjaro region.

Description: In this paper, we addressed a sensitivity analysis of the snow module of the GEOtop2.0 model at point and catchment scale in a small high-elevation catchment in the Eastern Italian Alps (catchment size: 61 km 2 ). Simulated snow depth and snow water equivalent at the point scale were compared with measured data at four locations from 2009–2013. At the catchment scale, simulated snow covered area was compared with binary snow cover maps derived from MODIS and Landsat satellite imagery. Sensitivity analyses were used to assess the effect of different model parameterisations on model performance at both scales and the effect of different thresholds of simulated snow depth on the agreement with MODIS data. Our results at point scale indicated that modifying only the snow correction factor resulted in substantial improvements of the snow model and effectively compensated inaccurate winter precipitation by enhancing snow accumulation. SCA inaccuracies at catchment scale during accumulation and melt period were affected little by different snow depth thresholds when using calibrated winter precipitation from point scale. However, inaccuracies were strongly controlled by topographic characteristics and model parameterisations driving snow albedo ( snow aging coefficient and extinction of snow albedo ) during accumulation and melt period. While highest accuracies (OA = 1 in 86% of the catchment area) were observed during winter, lower accuracies (OA 〈 0.7) occurred during the early accumulation and melt period (in 29% and 23%, respectively), mostly present in areas with grassland and forest, slopes of 20°– 40°, areas exposed NW or areas with a roughness index of -0.25 to 0 m. These findings may give recommendations for defining more effective model parameterisation strategies and guide future work, in which simulated and MODIS SCA may be combined to generate improved products for SCA monitoring in Alpine catchments.

Description: The seasonally-dry climate of Northern California imposes significant water stress on ecosystems and water resources during the dry summer months. Frequently during summer, the only water inputs occur as non-rainfall water, in the form of fog and dew. However, due to spatially heterogeneous fog interaction within a watershed, estimating fog water fluxes to understand watershed-scale hydrologic effects remains challenging. In this study, we characterized the role of coastal fog, a dominant feature of Northern Californian coastal ecosystems, in a San Francisco Peninsula watershed. To monitor fog occurrence, intensity, and spatial extent, we focused on the mechanisms through which fog can affect the water balance: throughfall following canopy interception of fog, soil moisture, streamflow, and meteorological variables. A stratified sampling design was used to capture the watershed's spatial heterogeneities in relation to fog events. We developed a novel spatial averaging scheme to upscale local observations of throughfall inputs and evapotranspiration suppression and make watershed-scale estimates of fog water fluxes. Inputs from fog water throughfall (10–30 mm/year) and fog suppression of evapotranspiration (125 mm/year) reduced dry season water deficits by 25% at watershed scales. Evapotranspiration suppression was much more important for this reduction in water deficit than were direct inputs of fog water. The new upscaling scheme was analyzed to explore the sensitivity of its results to the methodology (data type and interpolation method) employed. This evaluation suggests that our combination of sensors and remote sensing allows an improved incorporation of spatially-averaged fog fluxes into the water balance than traditional interpolation approaches.

Description: ABSTRACT The occurrence of water ponding on soil surfaces during and after heavy rainfall produces surface runoff or surface water accumulation in low-lying areas, which might reduce the water supply to soils and result in a reduction of the soil water that plants can use, especially in arid climates. On Mongolian rangeland, we observed ponded water on the surface of a specific soil condition subjected to a heavy rainfall of 30 mm/h. By contrast, ponded water was not observed for the same type of soil where livestock grazing had been removed for 6 years via a fence or for nearby soil containing less clay. We measured the infiltration rate (the saturated hydraulic conductivity of the surface soil; K s ) of the three sites by applying ponded water on the soil surface (an intake rate test). The results showed that K s in the rangeland was lower than the rainfall intensity in the site where water ponded on the soil surface; however, K s of the soil inside of the fence has recovered to three times that of the soil outside of the fence to exceed the rainfall intensity. Heavy rainfall that exceeds the infiltration rate occurs several times a year at the livestock-grazing site where we observed ponded water. Slight water repellency of the soil reduces rain infiltration to increase the possibility of surface ponding for the soil.

Description: Although it has been increasingly acknowledged that groundwater flow pattern is complicated in the three-dimensional (3-D) domain, two-dimensional (2-D) water table-induced flow models are still widely used to delineate basin-scale groundwater circulation. However, the validity of 2-D cross-sectional flow field induced by water table has been seldom examined. Here, we derive the analytical solution of 3-D water table-induced hydraulic head in a Tóthian basin and then examine the validity of 2-D cross-sectional models by comparing the flow fields of selected cross sections calculated by the 2-D cross-sectional model with those by the 3-D model, which represents the “true” cases. For cross sections in the recharge or discharge area of the 3-D basin, even if head difference is not significant, the 2-D cross-sectional models result in flow patterns absolutely different from the true ones. For the cross section following the principal direction of groundwater flow, although 2-D cross-sectional models would overestimate the penetrating depth of local flow systems and underestimate the recharge/discharge flux, the flow pattern from the cross-sectional model is similar to the true one and could be close enough to the true one by adjusting the decay exponent and anisotropy ratio of permeability. Consequently, to determine whether a 2-D cross-sectional model is applicable, a comparison of hydraulic head difference between 2-D and 3-D solutions is not enough. Instead, the similarity of flow pattern should be considered to determine whether a cross-sectional model is applicable. This study improves understanding of groundwater flow induced by more natural water table undulations in the 3-D domain and the limitations of 2-D models accounting for cross-sectional water table undulation only.

Description: Some conceptual models suggest that baseflow in agriculturally-fragmented watersheds may contain little, if any, groundwater. This has critical implications for stream quality and ecosystem functioning. Here we: 1) identify the sources and flowpaths contributing to baseflow using 222 Rn and 87 Sr/ 86 Sr, and 2) quantify mean apparent ages of groundwater and baseflow using multiple isotopic tracers (CFC, SF 6 , 36 Cl, and 3 H) in four small (0.08 to 0.64 km 2 ) tributary catchments to the Wabash River in Indiana, USA. 222 Rn activities and 87 Sr/ 86 Sr ratios indicate that baseflow in three catchments is sourced primarily from groundwater; baseflow in the fourth is dominated by a source similar to agricultural runoff. CFC-12 data indicate that springs in one catchment are discharging significant proportions of water that recharged between 1974 (42 +/- 2 years) and 1961 (55 +/- 2 years). Those same springs have 36 Cl/Cl ratios between 1381.08 +/- 29.37 (x 10 -15 ) and 1530.64 +/- 27.65 (x 10 -15 ) indicating that a substantial proportion of the discharge likely recharged between 1975 (41 years) and 1950 (66 years). Groundwater collected from streambed mini-piezometers in a separate catchment have CFC-12 concentrations indicating that a large proportion of the recharge occurred between 1948 (68 +/- 2 years) to 1950 (66 +/- 2 years). Samples collected in September 2015 after above-average summer rainfall did not show significant decreases in mean apparent age. The relatively old ages observed in three of the catchments can be explained by geological complexities which are likely present in all four catchments, but overwhelmed by flow from the shallow phreatic aquifer in the fourth catchment.

Description: With high spatiotemporal resolution and wide coverage, satellite-based precipitation products can potentially fill the deficiencies of traditional in situ gauge precipitation observations and provide an alternative data source for ungauged areas. However, due to the relatively poor accuracy and high uncertainty of satellite-based precipitation products, it remains necessary to assess the quality and applicability of the products for each investigated area. This study evaluated the accuracy and error of the latest Tropical Rainfall Measuring Mission (TRMM) Multi-satellites Precipitation Analysis (TMPA) 3B42-V7 satellite-based precipitation product and validated the applicability of the product for the Beijiang and Dongjiang River Basin, downstream of the Pearl River Basin in China. The study first evaluated the accuracy, error, and bias of the 3B42-V7 product during 1998-2006 at daily and monthly scale via comparison with in situ observations. The study further validated the applicability of the product via hydrologic simulation using the Variable Infiltration Capacity (VIC) hydrological model for three hydrological stations in the Beijiang River Basin, considering two scenarios: a streamflow simulation with gauge-calibrated parameters (scenario I) and a simulation after recalibration with the 3B42-V7 product (scenario II). The results revealed that: 1) the 3B42-V7 product produced acceptable accuracy both at the daily scale and high accuracy at the monthly scale, while generally tending to overestimate precipitation; 2) the product clearly overestimated the frequency of no rainfall events at the gridcell-scale and light rainfall (〈 1 mm/day) events at the region-scale, and also overestimated the amount of heavy rain (25-50 mm/day) and hard rain (≥ 50 mm/day) events; 3) under scenario I, the 3B42-V7 product performed poorly at three stations with gauge-calibrated parameters; under scenario II, the recalibrated model provided significantly improved performance of streamflow simulation with the 3B42-V7 product; 4) the VIC model has the ability to reveal the hydrological characteristics of the karst landform in the Beijiang Basin when using the 3B42-V7 product.

Description: Over a four-month summer period, we monitored how forest ( Pinus sylvestris ) and heather moorland ( Calluna spp. and Erica spp.) vegetation canopies altered the volume and isotopic composition of net precipitation (NP) in a southern boreal landscape in northern Scotland. During that summer period, interception (I) losses were relatively high, and higher under forests compared to moorland (46% of gross rainfall (GR) compared with 35%, respectively). Throughfall (TF) volumes exhibited marked spatial variability in forests, depending upon local canopy density, but were more evenly distributed under heather moorland. In the forest stands, stemflow (StF) was a relatively small canopy flow path accounting for only 0.9-1.6% of NP and only substantial in larger events. Overall, the isotopic composition of NP was not markedly affected by canopy interactions; temporal variation of stable water isotopes in TF closely corresponded to that of GR with differences of TF-GR being -0.52 ‰ for δ 2 H and -0.14 ‰ for δ 18 O for forests and 0.29 ‰ for δ 2 H and -0.04 ‰ for δ 18 O for heather moorland. These differences were close to, or within, analytical precision of isotope determination, though the greater differences under forest were statistically significant. Evidence for evaporative fractionation was generally restricted to low rainfall volumes in low intensity events, though at times subtle effects of liquid-vapour moisture exchange and/or selective transmission though canopies were evident. Fractionation and other effects were more evident in StF but only marked in smaller events. The study confirmed earlier work that increased forest cover in the Scottish Highlands will likely cause an increase in interception and green water fluxes at the expenses of blue water fluxes to streams. However, the low energy, humid environment means that isotopic changes during such interactions will only have a minor overall effect on the isotopic composition of NP.

Description: The accurate measurement ofprecipitation is essential to understanding regional hydrological processes and hydrological cycling.Quantification of precipitation over remote regions such as the Tibetan plateau (TP) is highly unreliable because of thescarcity of rain gauges.The objective of this study is to evaluate the performance of the satellite precipitation product of Tropical Rainfall Measuring Mission (TRMM) 3B42 v7 at daily, weekly, monthly and seasonalscales. Comparison between TRMM grid precipitation and point-based rain gauge precipitation was conducted using nearest neighbor (NN) and bilinear weighted interpolation (BWI) methods. The results showed that the TRMM product could not capture daily precipitation well due to some rainfall events being missed at short time scalesbut provided reasonably good precipitation data at weekly, monthly and seasonal scales. TRMM tended to underestimate the precipitation of small rainfallevents (less than 1 mm/day), while it overestimated the precipitation of large rainfall events (greater than 20 mm/day). Consequently, TRMM showedbetter performance in the summer monsoon season than in the winter season.Through comparison, it was also found that the bilinear weighted interpolation method performs better than the nearest neighbor method in TRMM precipitation extraction.

Description: Using water budget data from published literature, we demonstrate how hydrologic processes govern the function of various stormwater infrastructure technologies. Hydrologic observations are displayed on a Water Budget Triangle, a ternary plot tool developed to visualize simplified water budgets, enabling side-by-side comparison of green and grey approaches to stormwater management. The tool indicates ranges of hydrologic function for green roofs, constructed wetlands, cisterns, bioretention and other stormwater control management structures. Water budgets are plotted for several example systems to provide insight on structural and environmental design factors, and seasonal variation in hydrologic processes of stormwater management systems. Previously published water budgets and models are used to suggest appropriate operational standards for several green and grey stormwater control structures and compare between conventional and low-impact development approaches. We compare models, characterize and quantify water budgets and expected ranges for green and grey infrastructure systems, and demonstrate how the Water Budget Triangle tool may help users to develop a data-driven approach for understanding design and retrofit of green stormwater infrastructure.

Description: This study investigates 72 catchments across the federal state of Baden-Wuerttemberg, Germany, for changes in water quality during low-flow events. Data from the state's water quality monitoring network provided seven water quality parameters (water temperature, electrical conductivity, concentrations of chloride, sodium, sulfate, nitrate and phosphate), which were statistically related to streamflow variability. Water temperatures increased during low-flow in summer but decreased during low-flow in winter. Nitrate concentrations revealed high spatial heterogeneity with about one third of the stations showing decreasing values during low discharge. For most other parameters concentrations increased during low-flow. Despite consistent trend directions, the magnitudes of changes with streamflow differed markedly across the state. Both multiple linear regression and a multiple analysis of variances were applied to explain these differences with the help of catchment characteristics. Results indicated that for sulfate and conductivity, geology of the catchments was the most important control, whereas for chloride, sodium and nitrate sewage treatment plants had largest influence. For phosphate no clear control could be identified. Independent from the applied method, land use was a less important control on river water quality during low-flow than geology or inflow from sewage treatment plants. These results show that the effects of diffuse and point sources, as well as those of natural and anthropogenic sources differ for different water quality parameters. Overall, a high diversity of potential water quality deterioration signals needs to be considered when the ecological status of rivers is to be protected during low-flow events.

Description: The deeply buried river-connected Xishan Karst Aquifer (XKA)in western Beijing, China, hasbeen suffering from diminishing recharge for several decades, which in turn leads to the disappearing of spring water outflows and continuously lowering of groundwater level in the area. Thus, it is important to correctly recognize the groundwater recharge and flow paths for the sustainable development of the XKA. To investigate these issues, thehydrochemical and isotopic compositions are analysed for both surface water and groundwater samples collected over an area of about 280 km 2 . Results show that, (1)the river wateris characterized by high Na contents; (2) theδ 2 H and δ 18 O values in the river water are distinctively higher than those of groundwater samples, afterexperiencingthe long-time evaporative enrichment in the upstream reservoir; (3)theSr concentrations and 87 Sr/ 86 Sr ratios of groundwater clearly indicated the interaction between water and carbonate minerals, but excluded the water–silicate interaction; and (4) the groundwater samples in the direct recharge area of the XKA has the lowest Na concentrations as well as the δ 2 H and δ 18 O values. Based on the large differences in the Na contents and 18 O values of groundwater and surface water, a simple two-component mixing model is developed for the study area andthe fractions of the river water are estimated for groundwater samples. We find that the distribution pattern of the river water fractions in the XKA clearly shows a change of directions in the preferential flow path of the groundwaterfrom its source zone to the discharge area.Overall, our results suggest that the recharged surface water can be a useful evidence for delineation the groundwater flow path in river-connected karst aquifer.This studyimprovesour understanding of the heterogeneity in karst groundwater systems.

Description: Testing hydrological models over different spatio-temporal scales is important both for evaluating diagnostics and aiding process understanding. High-frequency (6hr) stable isotope sampling of rainfall and runoff was undertaken during 3 week periods in summer and winter within 12 months of daily sampling in a 3.2 km 2 catchment in the Scottish Highlands. This was used to calibrate and test a tracer-aided model to assess the: (1) information content of high resolution data; (2) effect of different calibration strategies on simulations and inferred processes; (3) model transferability to 〈1 km 2 sub-catchment. The 6-hourly data were successfully incorporated without loss of model performance, improving the temporal resolution of the modelling, and making it more relevant to the time dynamics of the isotope and hydrometric response. However, this added little new information due to old-water dominance and riparian mixing in this peatland catchment. Time variant results, from differential split sample testing, highlighted the importance of calibrating to a wide range of hydrological conditions. This also provided insights into the non-stationarity of catchment mixing processes, in relation to storage and water ages, which varied markedly depending on the calibration period. Application to the nested sub-catchment produced equivalent parameterisation and performance, highlighting similarity in dominant processes. The study highlighted the utility of high-resolution data in combination with tracer-aided models, applied at multiple spatial scales, as learning tools to enhance process understanding and evaluation of model behaviour across non-stationary conditions. This helps reveal more fully the catchment response in terms of the different mechanistic controls on both wave celerites and particle velocities.

Description: Rain-on-snow events have generated major floods around the world, particularly in coastal, mountainous regions. Most previous studies focused on a limited number of major rain-on-snow events or were based primarily on model results, largely due to a lack of long-term records from lysimeters or other instrumentation for quantifying event water balances. In this analysis we used records from five Automated Snow Pillow (ASP) sites in south coastal British Columbia, Canada, to reconstruct event water balances for 286 rain-on-snow events over a 10-year period. For large rain-on-snow events (event rainfall 〉 40 mm), snowmelt enhanced the production of water available for runoff (WAR) by approximately 25% over rainfall alone. For smaller events, a range of antecedent and meteorological factors influenced WAR generation, particularly the antecedent liquid water content of the snowpack. Most large events were associated with atmospheric rivers. Rainfall dominated WAR generation during autumn and winter events, while snowmelt dominated during spring and summer events. In the majority of events, the sensible heat of rain contributed less than 10% of the total energy consumed by snowmelt. This analysis illustrated the importance of understanding the amount of rainfall occurring at high elevations during rain-on-snow events in mountainous regions.

Description: Arctic deltas, such as the Mackenzie Delta, are expected to face major climate change and increased human influence in the near future. Deltas are characterised by highly dynamic fluvial processes and changing climate will cause considerable evolution of the riverine environment. The changes are difficult to predict with existing knowledge and data. This study quantified channel planform change of the Mackenzie Delta (1983-2013), analysing its temporal and spatial patterns. We addressed the main obstacle of research on large remote areas, the lack of data, by developing a unique work flow that utilised Landsat satellite imagery, hydrological time series, remote sensing-based change analysis and automatic vectorisation of channels. Our results indicate that the Mackenzie Delta experienced constant evolution but at a highly varying rate over the 30 years. The study demonstrates that the magnitude and duration of flood peaks and the presence of spring ice breakup floods determine the rate of Arctic delta planform change. Changing winter conditions and spring flood magnitudes may therefore affect the stability of Arctic deltas. However, no clear trends towards decreased recurrence or magnitude of spring floods or increased instability of the delta plain have yet been observed in the Mackenzie Delta. The delta plain was most dynamic at the beginning and at the end of the examined period, corresponding to intense flooding, while the rates of change were subtle during the low-flood period 1994-2007. Largest changes have occurred along the wide Middle Channel and in the outermost delta. Relative to their size however, smaller meandering channels have been highly dynamic. Hotspots of change in the delta plain are located in anastomosing and braiding channel segments and, at the local scale, in point bars and cut-banks along meandering channels. Our study describes how Landsat satellite data can be utilised for advancing fluvial geomorphological research in remote areas. However, cloudiness in the delta restricts production of dense time series with simultaneous coverage of the whole area and requires manual pre-processing.

Description: Deleterious effects of urban stormwater are widely recognized. In several countries, regulations have been put into place to improve the conditions of receiving water bodies, but planning and engineering of stormwater control is typically carried out at smaller scales. Quantifying cumulative effectiveness of many stormwater control measures on a watershed-scale is critical to understanding how small-scale practices translate to urban river health. We review 100 empirical and modeling studies of stormwater management effectiveness at the watershed-scale in diverse physiographic settings. Effects of networks with stormwater control measures (SCMs) that promote infiltration and harvest have been more intensively studied than have detention-based SCM networks. Studies of peak flows and flow volumes are common, whereas baseflow, groundwater recharge and evapotranspiration have received comparatively little attention. Export of nutrients and suspended sediments have been the primary water quality focus in the United States, while metals, particularly those associated with sediments, have received greater attention in Europe and Australia. Often, quantifying cumulative effects of stormwater management is complicated by needing to separate its signal from the signal of urbanization itself, innate watershed characteristics that lead to a range of hydrologic and water quality responses, and the varying functions of multiple types of SCMs. Biases in geographic distribution of study areas, and size and impervious surface cover of watersheds studied also limit our understanding of responses. We propose hysteretic trajectories for how watershed function responds to increasing imperviousness and stormwater management. Even where impervious area is treated with SCMs, watershed function may not be restored to its pre-development condition because of: the lack of treatment of all stormwater generated from treated impervious surfaces; non-additive effects of individual SCMs; and persistence of urban effects beyond impervious surfaces. In most cases, pollutant load decreases largely result from runoff reductions rather than lowered solute or particulate concentrations. Understanding interactions between natural and built landscapes, including stormwater management strategies, is critical for successfully managing detrimental impacts of stormwater at the watershed scale.

Description: Multi-scene Landsat 5 TM imagery, Principal Component Analysis (PCA), and the Normalised Difference Vegetation Index (NDVI) were used to produce the first region-scale map of riparian vegetation for the Pilbara (230,000 km2), Western Australia. Riparian vegetation is an environmentally important habitat in the arid and desert climate of the Pilbara. These habitats are supported by infrequent flow events and in some locations by groundwater discharge. Our analysis suggests that riparian vegetation covers less than 4% of the Pilbara region, while almost 10.5% of this area is comprised of Groundwater Dependent Vegetation (GDV). GDV is often associated with open water (river pools), providing refugia for a variety of species. GDV has an extremely high ecological value and are often important Indigenous sites. This paper demonstrates how Landsat data calibrated to Top of Atmosphere (TOA) reflectance can be used to delineate riparian vegetation across 16 Landsat Scenes and two UTM (Universal Transverse Mercator) spatial zones. The proposed method is able to delineate riparian vegetation and GDV, without the need for Bidirectional Reflectance Distribution Function (BRDF) correction. Results were validated using ground-truth data from local and regional scale vegetation surveys.

Description: A hydraulic invariance (HI)-based methodology was developed as a tool to support implementation of storm flow control measures into land use master-plans (LUMPs) for urban catchments. The methodology is based on the use of simple hydrologic analysis to compare pre- and post-development catchment flow release scenarios. Differently from previous literature examples, for which the parcel scale is usually considered for the analysis, HI was pursued assuming the LUMP areas of transformation as the basic units for assigning storm water control measures in the form of flow release restrictions. The methodology was applied to a case study catchment in the southern part of the City of Catania (Italy), for which the LUMP re-design has been recently proposed. Simulations were run based on the use of the EPA-SWMM model and allowed deriving flow release restrictions in order to achieve HI at the sub-catchment level for design events of different return period.

Description: Climatic changes have altered surface water regimes worldwide, and climate projections suggest that such alterations will continue. To inform management decisions, climate projections must be paired with hydrologic models to develop quantitative estimates of watershed scale water regime changes. Such modeling approaches often involve downscaling climate model outputs, which are generally presented at coarse spatial scales. In this study, Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model projections were analyzed to determine models representing severe and conservative climate scenarios for the study watershed. Based on temperature and precipitation projections, output from GFDL-ESM2G (RCP2.6) and MIROC-ESM (RCP8.5) were selected to represent conservative (Δ C ) and severe (Δ S ) change scenarios, respectively. Climate data were used as forcing for the Soil and Water Assessment Tool (SWAT) to analyze the potential effects of climate change on hydrologic processes in a mixed-use watershed in central Missouri, USA. Results showed annual streamflow decreases ranging from -5.9% to -26.8% and evapotranspiration (ET) increases ranging from +7.2% to +19.4%. During the mid-21 st century, sizeable decreases to summer streamflow were observed under both scenarios, along with large increases of fall, spring, and summer ET under Δ S . During the late 21 st century period, large decreases of summer streamflow under both scenarios, and large increases to spring (Δ S ), fall (Δ S ) and summer (Δ C ) ET were observed. This study demonstrated the sensitivity of a Midwestern watershed to future climatic changes utilizing projections from CMIP5 models, and presented an approach that used multiple climate model outputs to characterize potential watershed scale climate impacts.

Description: Recent advances in sediment fingerprinting research have seen Bayesian mixing models being increasingly employed as an effective method to coherently translate component uncertainties into source apportionment results. Here, we advance earlier work by presenting an extended Bayesian mixing model capable of providing a full Bayes treatment of geochemical uncertainties. The performance of the extended full Bayes model was assessed against the equivalent empirical Bayes model and traditional frequentist optimisation. The performance of models coded in different Bayesian software (‘JAGS’ and ‘Stan’) was also evaluated, alongside an assessment of model sensitivity to reduced source representativeness and non-conservative fingerprint behaviour. Results revealed comparable accuracy and precision for the full and empirical Bayes models across both synthetic and real sediment geochemistry datasets, demonstrating that the empirical treatment of source data here represents a close approximation of the full Bayes treatment. Contrasts in the performance of models coded in JAGS and Stan revealed that the choice of software employed can impact significantly upon source apportionment results. Bayesian models coded in Stan were the least sensitive to both reduced source representativeness and non-conservative fingerprint behaviour, indicating Stan as the preferred software for future Bayesian sediment fingerprinting studies. Whilst the frequentist optimisation generally yielded comparable accuracy to the Bayesian models, uncertainties around apportionment estimates were substantially greater and the frequentist model was less effective at dealing with non-conservative behaviour. Overall, the effective performance of the extended full Bayes mixing model coded in Stan represents a notable advancement in source apportionment modelling relative to previous approaches. Both the mixing model and the software comparisons presented here should provide useful guidelines for future sediment fingerprinting studies.

Description: Rainfall interception loss plays an important role in eco-hydrological processes in dryland shrub ecosystems, but its drivers still remain poorly understood. In this study, a statistical model was developed to simulate interception loss based on the mass balance measurements arising from the partitioning of rainfall in two dominant xerophytic shrub ( Hippophae rhamnoides and Spiraea pubescens ) communities in the Loess Plateau. We measured throughfall and stemflow in the field under natural rainfall, calculated the canopy storage capacity in the laboratory, and identified key factors controlling these components for the two shrubs. We quantified and scaled-up the stemflow and the canopy storage capacity measurements from the branches and/or leaves to stand level. The average interception loss, throughfall, and stemflow fluxes account for 24.9%, 72.2%, and 2.9% of the gross rainfall for H. rhamnoides , and 19.2%, 70.7%, and 10.1% for S. pubescens , respectively. Throughfall increased with increasing rainfall for both shrubs, however, it was only correlated with the leaf area index (LAI) for S. pubescens . For stemflow measured from individual branches, we found that the rainfall amount and basal diameter are the best predictors for H. rhamnoides , while rainfall amount and branch biomass appear to be the best predictors for S. pubescens . At the stand level, stemflow production is affected by the rainfall amount for H. rhamnoides , and it is affected by both the rainfall amount and the LAI for S. pubescens . The canopy storage capacity of H. rhamnoides (1.07–1.28 mm) was larger than S. pubescens (0.88–1.07 mm), and it is mainly determined by the branches and stems of H. rhamnoides , and the leaves of S. pubescens . The differences in interception loss between the two shrub stands are mainly attributed to different canopy structures induced differences in stemflow production and canopy storage. We evaluated the effects of canopy structure on rainfall interception loss, and our developed model provides a better understanding of the effects of the canopy structure on the water cycles in dryland shrub ecosystems.

Description: Evaluating the flow paths that contribute to solute flux in stream networks can lead to greater understanding of the linkages between biogeochemistry and hydrology. We compared the contributions of groundwater in spring brooks and in seepage through the stream bed to nitrate flux in the Emmons Creek network in the Wisconsin sand plains. We predicted that spring brooks would contribute disproportionately to nitrate flux due to the presumed higher advection rates in springs and less opportunity for nitrate removal relative to seeps. Nitrate flux was measured in 15 spring brooks that entered Emmons Creek. Nitrate flux from seepage was measured at the locations of 30 piezometers, based on Darcy's Law, and by a reach-scale injection of Rhodamine WT (RWT). When seepage discharge was estimated from the RWT release groundwater inputs from seepage and springs accounted for the discharge gain in the Emmons Creek channel. Springs brooks and seepage (based on the RWT release) contributed 37% and 63%, respectively, to nitrate flux inputs in the study reach. Contrary to our prediction, seeps contributed disproportionately to nitrate flux relative to their discharge. Relatively high rates of seepage discharge and higher than anticipated nitrate concentrations in the shallow pore water at seepage locations contributed to the unanticipated result.

Description: Although fractional integration and differentiation have found many applications in various fields of science, such as physics, finance, bioengineering, continuum mechanics and hydrology, their engineering applications, especially in the field of fluid flow processes, are rather limited. In this study, a finite difference numerical approach is proposed to solve the time-space fractional governing equations of one-dimensional unsteady/non-uniform open channel flow process. By numerical simulations, results of the proposed fractional governing equations of the open channel flow process were compared with those of the standard Saint-Venant equations. Numerical simulations showed that flow discharge and water depth can exhibit heavier tails in downstream locations as space and time fractional derivative powers decrease from 1. The fractional governing equations under consideration are generalizations of the well-known Saint-Venant equations, which are written in the integer differentiation framework. The new governing equations in the fractional order differentiation framework have the capability of modeling nonlocal flow processes both in time and in space by taking the global correlations into consideration. Furthermore, the generalized flow process may possibly shed light on understanding the theory of the anomalous transport processes and observed heavy-tailed distributions of particle displacements in transport processes.

Description: Street and garden trees in urban areas are often exposed to advection of strong vapor pressure deficit ( VPD ) air that can raise the whole-tree transpiration rate ( E T ), known as the oasis effect . However, urban trees tend to have small soil volume compared with natural conditions and so they are believed to strongly regulate stomata. E T characteristics of such urban trees have not been well understood because of a lack of reliable measurement methods. Therefore, we propose a novel weighing lysimeter method and investigate the whole-tree water balance of an isolated container-grown Zelkova serrata to examine (a) which biotic and abiotic factors determine E T , and (b) which spatial and temporal information is needed to predict E T under urban conditions. Whole-tree water balance and environmental conditions were measured from 2010 to 2012. Although leaf area substantially increased in the study period, daily E T did not vary much. E T increased with VPD almost linearly in 2010, but showed saturation in 2011 and 2012. Root water uptake lagged E T by 40 min in 2012. These results suggest that the small planter box interfered with root growth and that hydraulic supply capacities did not increase sufficiently to support leaf area increase. From analysis of water balance, we believe that neglecting soil drought effects on street trees without irrigation in Japan will overestimate E T over 4–5 sunny days at the longest. This is unlike previous studies of forest.

Description: The saturated hydraulic conductivity, K s , is a soil property that has a key role in the partitioning of rainfall into surface runoff and infiltration. The commonly used instruments and methods for in-situ measurements of K s have frequently provided conflicting results. Comparison of K s estimates obtained by three classical devices – namely the Double Ring Infiltrometer (DRI), the Guelph version of the Constant-Head Well Permeameter (GUELPH-CHP) and the CSIRO version of the Tension Permeameter (CSIRO-TP) is presented. A distinguishing feature in this study is the use of steady deep flow rates, obtained from controlled rainfall-runoff experiments, as benchmark values of K s at local and field-plot scales, thereby enabling an assessment of these methods in reliably reproducing repeatable values and in their capability of determining plot-scale variation of K s . We find that the DRI grossly overestimates K s , the GUELPH-CHP gives conflicting estimates of K s with substantial overestimation in laboratory experiments and underestimation at the plot scale, while the CSIRO-TP yields average K s values with significant errors of 24% in the plot scale experiment and 66% in laboratory experiments. While the DRI would likely yield a better estimate of the nature of variability than the GUELPH-CHP and CSIRO-TP, a separate calibration may be warranted to correct for the overestimation of K s values. The reasons for such discrepancies within and between the measurement methods are not yet fully understood, and serve as motivation for future work to better characterize the uncertainty associated with individual measurements of K s using these methods and the characterization of field scale variability from multiple local measurements.

Description: This study explores rainfall spatial variability and its influence on runoff modelling. A novel assessment scheme integrated with coefficients of variance (CV) and Moran's I is introduced to describe effective rainfall spatial variability. CV is widely accepted to identify rainfall variability through rainfall intensity, whereas Moran's I reflects rainfall spatial autocorrelation. This new assessment framework combines these two indicators to assess the spatial variability derived from both rainfall intensity and distribution, which are crucial in determining the time and magnitude of runoff generation. Four model structures embedded in the Variable Infiltration Capacity (VIC) model are adopted for hydrological modelling in the Brue catchment of England. The models are assigned with 1, 3, 8 and 27 hydrological response units (HRUs) respectively and diverse rainfall spatial information for 236 events are extracted from 1995. This study investigates the model performance of different partitioning based on rainfall spatial variability through peak volume (Q p ) and time to peak (T p ), along with the rainfall event process. The results show that models associated with dense spatial partitioning are broadly capable of capturing more spatial information with better performance. It is unnecessary to utilize models with high spatial density for simple rainfall events, though they show distinct advantages on complex events. With additional spatial information, Q p experiences a notable improvement over T p . Moreover, seasonal patterns signified by the assessment scheme implies the feasibility of seasonal models.

Description: Soil erosion by water in abandoned dry terraces is one of the most important environmental problems in semi-arid areas, enhancing biological degradation and reducing possible resources that can be obtained. However, little is known about the effects of the types of lithology and soil properties on the early stages of soil erosion. Therefore, the main aim of this research was to assess the effect of different lithologies (marls, limestones and metamorphic –phyllites, schists and greywackes- materials) and soil properties on the early stages of soil erosion by water in abandoned dry terraces, compared with similar terraces still in agricultural use. Soil analyses (texture, aggregate stability and bulk density) and 22 rainfall simulations were carried out under dry conditions. During the experiments, local inclination, vegetation and stone cover, total organic matter and antecedent soil moisture were also quantified. The results showed that the highest soil loss (41.41 g m -2 in cultivated plots and 17.05 g m -2 in the abandoned plots) and runoff (3.79 L m -2 in the abandoned plot) occurred on marl substrata. Marls also showed the shallowest infiltration front (9 cm) and lowest infiltration rate (4.3 cm min -1 ). Limestones and, especially, metamorphic areas, showed a lower degree of soil erosion, higher infiltration rates and deeper infiltration fronts.

Description: Historically, paired watershed studies have been used to quantify the hydrological effects of land use and management practices by concurrently monitoring two similar watersheds during calibration (pre-treatment) and post-treatment periods. This study characterizes seasonal water table and flow response to rainfall during the calibration period and tests a change detection technique of moving sums of recursive residuals (MOSUM) to select calibration periods for each control-treatment watershed pair when the regression coefficients for daily water table elevation (WTE) were most stable to minimize regression model uncertainty. The control and treatment watersheds were one watershed of 3−4 year-old intensely managed loblolly pine ( Pinus taeda L. ) with natural understory, one watershed of 3−4 year-old loblolly pine intercropped with switchgrass ( Panicum virgatum) , one watershed of 14−15 year-old thinned loblolly pine with natural understory (control), and one watershed of switchgrass only. The study period spanned from 2009 to 2012. Silvicultural operational practices during this period acted as external factors, potentially shifting hydrologic calibration relationships between control and treatment watersheds. MOSUM results indicated significant changes in regression parameters due to silvicultural operations and were used to identify stable relationships for WTE. None of the calibration relationships developed using this method were significantly different from the classical calibration relationship based on published historical data. We attribute that to the similarity of historical and 2010−2012 leaf area index (LAI) on control and treatment watersheds as moderated by the emergent vegetation. While the MOSUM approach does not eliminate the need for true calibration data or replace the classic paired watershed approach, our results show that it may be an effective alternative approach when true data is unavailable, as it minimizes the impacts of external disturbances other than the treatment of interest.

Description: ABSTRACT The Sierra Gorda aquifer is one of the most extensive of southern Spain. The main groundwater discharge is produced at its northern boundary through several high-flow springs. In this study, stable isotopes of dissolved sulfate (δ 34 S and δ 18 O) and groundwater chemistry were used to determine the origin of the sulfate and to characterize the groundwater flow. We sampled the main springs, as well as other minor outlets related to perched water tables, in order to determine the different sources of SO 4 2- (e.g. dissolution of evaporites, atmospheric deposition, etc.). The substantial difference in the amount of dissolved SO 4 2- between the springs located in its northwestern part (≈25 mg/l) and those elsewhere in the northern part (≈60 mg/l) suggests zones with separate groundwater flow systems. A third group of springs, far from the northeastern boundary of the permeable outcrops, shows higher SO 4 2- content than the rest (≈125 mg/l). The isotopic range of sulfate (-0.3 to 14.82‰ V-CTD) points to several sources, including dissolution of Triassic or Miocene evaporites, atmospheric deposition, and decomposition of organic material in the soil. Among these, the dissolution of Triassic gypsum —which overlies the saturated zone as a consequence of the folds and faults that deform the aquifer— is the main source of SO 4 2- (range from 12.79 to 14.82‰ V-CTD). This range is typical for Triassic gypsum. The higher karstification in the western sector, together with important differences in the saturated thickness between the western and eastern sectors, would also be due to the tectonic structure and could explain the difference in SO 4 2- contents in the water. This singular arrangement may cause a higher residence time of groundwater in the eastern sector; thus, a higher contact time with Triassic evaporitic rocks is inferred. Accordingly, the stable isotopes of SO 4 2- are found to be a valuable tool for identifying areas with different flow systems in the saturated zone of karstic aquifers, as well as for evaluating aspects such as the degree of karstification.

Description: This study examines the 1914-2015 runoff trends and variability for 136 rivers draining British Columbia's (BC's) Coast and Insular Mountains. Rivers are partitioned into eastward and westward flowing rivers based on flow direction from the Coast Mountains. Thus, eastward and westward runoff trends and influence of topography on runoff are explored. Our findings indicate that rivers flowing eastward to the Nechako and Chilcotin plateaus contribute the lowest annual runoff compared to westward rivers where runoff is high. Low interannual runoff variability is evident in westward rivers and their alpine watersheds, whereas eastward rivers exhibit high interannual runoff variability. On Vancouver Island, some of the rivers with the highest annual runoff exhibit high interannual variability. A significant ( p 〈 0.05) negative correlation exists between mean annual runoff ( R m ) and latitude, gauged area, mean elevation, and its corresponding coefficient of variation (CV). However, a significant positive correlation was found between the glacierized area of mountainous regions and R m . The mean CV in annual runoff is significantly negatively correlated with latitude and glacierized area, but significantly positively correlated with longitude. Annual and seasonal runoff trend analyses of each river were performed for an early (1936-2015), a middle (1966-2015), and a late (1986-2015) period using the Mann-Kendall test. Trend analyses revealed a shift towards more positive detectable (signal-to-noise ratio 〉 1) trends in annual and seasonal runoff from the middle to the late period across the study domain. Most positive detectable seasonal runoff trends in the middle period occur in spring in glacierized westward rivers located 〉1200 m, whereas in the late period they all occur in fall and are regionally coherent around Vancouver Island and south coastal BC. Rivers draining eastward exhibit more positive trends over 1986-2015 compared to westward rivers. This study provides crucial information on the hydrology of mountain watersheds across BC's coast in response to Pacific Decadal Oscillation phase changes, the elevational amplification of regional climate change, and their influences on precipitation and glacier retreat.

Description: The hydrology and water balance of megadunes and lakes have been investigated in the Badain Jaran Desert of China. Field observations and analyses of sand layer water content, field capacity, secondary salt content and grain size reveal three types of important natural phenomenon: (1) Vegetation bands on the leeward slope of the megadunes reflect the hydrological regime within the sandy vadose zone. (2) Seepage, wet sand deposits and secondary salt deposits indicate the pattern of water movement within the sandy vadose zone. (3) Zones of groundwater seeps and descending springs around the lakes reflect the influence of the local topography on the hydrological regime of the megadunes. The seepage exposed on the sloping surface of the megadunes and gravity water contained within the sand layer confirm the occurrence of preferential flow within the vadose zone of the megadunes. Alternating layers of coarse and fine sand create the conditions for the formation of preferential flows. The preferential flows promote movement of water within the sand layer water that leads to deep penetration of water within the megadunes and ultimately to the recharging of groundwater and lake water. Our results indicate that a positive water balance promotes recharge of the megadunes, which depends on the high permeability of the megadune material, the shallow depth of the surface sand layer affected by evaporation, the occurrence of rainfall events exceeding 15 mm, and the sparse vegetation cover. Water balance estimates indicate that the annual water storage of the megadunes is about 7.5 mm, accounting for only 8% of annual precipitation; however, the shallow groundwater per unit area under the megadunes receives only 3.6% of annual precipitation but it is still able maintain a dynamic balance of the lake water. From a water budget perspective, the annual water storage in the megadunes is sufficient to serve as a recharge source for lake water, thereby enabling the long-term persistence of the lakes. Overall, our findings demonstrate that precipitation is a significant component of the hydrological cycle in arid deserts.

Description: Concentration-discharge (c-Q) plots are routinely used as an integrated signal of watershed response to infer solute sources and travel pathways. However, the interpretation of c-Q data can be difficult unless these data are fitted using statistical models. Such models are frequently applied for geogenic solutes, but it is unclear to what extent they might aid in the investigation of nutrient export patterns, particularly for total dissolved phosphorus (TDP), which is a critical driver of downstream eutrophication problems. The goal of the present study was therefore to statistically model c-Q relations (where c is TDP concentrations) in a set of contrasting watersheds in the Northern Great Plains – ranging in size from 0.2 to 1000+ km 2 – to assess the controls of landscape properties on TDP transport dynamics. Six statistical models were fitted to c-Q data, notably (i) one linear model, (ii) one model assuming that c-Q relations are driven by the mixing of end-member waters from different landscape locations (i.e., hydrograph separation), (iii) one model relying on a biogeochemical stationarity hypothesis (i.e., power law), (iv) one model hypothesizing that c-Q relations change as a function of the solute subsurface contact time (i.e., hyperbolic model), and (v) two models assuming that solute fluxes are mostly dependent on reaction rates (i.e., chemical models). Model performance ranged from mediocre (R 2 〈 0.2) to very good (R 2 〉 0.9), but the hydrograph separation model seemed most universal. No watershed was found to exhibit chemostatic behavior but many showed signs of dilution or enrichment behavior. A tendency toward a multi-model fit and better model performance was observed for watersheds with moderate slope and higher effective drainage area. The relatively poor model performance obtained outside these conditions illustrates the likely importance of controls on TDP concentrations in the region that are independent of flow dynamics.

Description: Groundwater resources of the Republic of the Maldives are threatened by a variety of factors including variable future rainfall patterns, continued population growth and associated pumping demands, rising sea level, and contamination from the land surface. This study assesses changes in groundwater availability due to variable rainfall patterns and sea level rise (SLR) in the coming decades, a key component of water resources management for the country. Using a suite of two-dimensional density-dependent groundwater flow models, time-dependent thickness of the freshwater lens is simulated for a range of island sizes (200 m to 1100 m) during the time period of 2011 to 2050, with recharge to the freshwater lens calculated using rainfall patterns provided by General Circulation Models (GCM) for the three distinct geographic regions of the Maldives. The effect of SLR on the freshwater lens is quantified using estimates of shoreline recession and associated decreases in island width. If rainfall is solely considered, groundwater availability is projected to increase, as lens thickness during the 2031-2050 time periods is slightly greater (1-5%) than during the 2011-2030 time period. However, including the impact of SLR indicates an overall decrease in lens thickness, with drastic decreases (60% to 100%) projected for small islands (200 m) and moderate decreases (12% to 14%) expected for 400 m islands, which accommodate one-third of the national population. Similar methodologies can be used for other atoll island nations, such as the Republic of Marshall Islands, Federated States of Micronesia and the Republic of Kiribati. For the Maldives, results from this study can be used in conjunction with population growth estimates to determine the feasibility of including groundwater in water resources planning and management for the country.

Description: We introduce a new representation of coupled solute and water- age dynamics at the catchment scale, which shows how the contributions of young runoff waters can be directly referenced to observed water-quality patterns. The methodology stems from recent trends in hydrologic transport that acknowledge the dynamic nature of streamflow age and explores the use of water age fractions as an alternative to the mean age. The approach uses a travel-time-based transport model to compute the fractions of streamflow that are younger than some thresholds (e.g. younger than a few weeks) and compares them to observed solute concentration patterns. The method is here validated with data from the Hubbard Brook Experimental Forest (HBEF) during spring 2008, where we show that the presence of water younger than roughly 2 weeks, tracked using a hydrologic transport model and deuterium measurements, mimics the variation in dissolved silicon concentrations. Our approach suggests that an age-discharge relationship can be coupled to classic concentration-discharge relationship, to identify the links between transport timescales and solute concentration. Our results highlight that the younger streamflow components can be crucial for determining water quality variations and for characterizing the dominant hydrologic transport dynamics.

Description: The study applies the improved cloud-free Moderate Resolution Imaging Spectral radiometer (MODIS) daily snow cover product (MODMYD_MC) to investigate the snow cover variations from snow Hydrologic Year (HY) HY2000 to HY2013 in the Amur River Basin (ARB), Northeast Asia. The fractions of forest cover were 38%, 63% and 47% in 2009 in China (the southern ARB), Russia (the northern ARB) and ARB, respectively. Validation results show that MODMYD_MC has a snow agreement of 88% against in situ snow depth (SD) observations (SD≥4cm). The agreement is about 10% lower at the forested stations than at the non-forested stations. Snow Cover Durations (SCD) from MODMYD_MC are 20 days shorter than ground observations (SD≥1cm) at the forested stations, while they are just 8 days shorter than ground observations (SD≥1cm) at the non-forested stations. Annual mean SCDs from MODMYD_MC in the forested areas are 21 days shorter than those in the nearby farmland in the SanJiang Plain. This indicates forest has a complex influence on the snow accumulation and melting processes and even on optical satellite snow cover mapping. Meanwhile, SCD and mean snow cover are negatively correlated with air temperature in ARB, especially in the snow melting season, when mean air temperature in March and April can explain 86% and 74% of the mean snow cover variations in China ARB and Russia ARB, respectively. From 1961 to 2015, the annual mean air temperature presented an increase trend by 0.33°C/decade in both China ARB and Russia ARB, while it had a decrease trend from HY2000 to HY2013. The decrease of air temperature led to an increase of snow cover, which is different from the global decrease trend of snow cover variations. SCD and snow cover had larger increase rates in China ARB than in Russia ARB, and they were larger in the forested areas than in the nearby farmland in the SanJiang Plain.

Description: This study aimed to understand changes in the biogeochemical processing of organic matter (OM) in response to multiple stressors (e.g., littoral area expansion, wastewater input and hydrological regulation) in East Dongting Lake (central China) over the past sixty years, using analyses of total organic carbon (TOC), total nitrogen (TN), C/N ratios, δ 13 C, δ 15 N and diatoms from two sediment cores collected from the littoral and central parts of the lake. OM mainly originated from phytoplankton and C 3 plant-derived soil OM based on the ranges of C/N ratios (from 7 to 11) and δ 13 C (between -27 and -23‰). Littoral area expansion due to siltation caused an increasing influx of terrestrial soil OM in the 1980s and the 1990s, subsequently lowering δ 13 C values and rising C/N ratios in both sediment cores. Meanwhile, higher δ 15 N was linked to a high influx of isotopically heavy nitrate from urban and agricultural wastewaters. After 2000, slight decreases in TOC and TN in the littoral area were attributable to reducing inputs of external OM, likely linked to declining sediment influx from the upper reaches resulting from the Three Gorges Dam impoundment. Contrasting increases in TOC, TN and C/N ratios in the central part indicated a high influx of terrestrial soil OM due to the declining distance from the shoreline with littoral area expansion. Declining δ 15 N values after 2000 indicated an increase in N 2 -fixing cyanobacteria with eutrophication. Changes in diatom assemblages in both the littoral and central zones reflected nutrient enrichment and hydrological alterations. These results indicate that littoral expansion, declining riverine influx and anthropogenic nutrient inputs are potential driving forces for the biogeochemical processing of OM in floodplain lakes. This study provides sedimentary biogeochemical clues for tracking past limnological conditions of floodplain lakes that are subjected to increasing disturbances from hydrological regulation and eutrophication.

Description: River confluences and their associated tributaries are key morphodynamic nodes that play important roles in controlling hydraulic geometry and hyporheic water exchange in fluvial networks. However, the existing knowledge regarding hyporheic water exchange associated with river confluence morphology is relatively scarce. On the 14 th and 15 th of January, 2016, the general hydraulic and morphological characteristics of the confluent meander bend (CMB) between the Juehe River and the Haohe River in the southern region of Xi’an City, Shaanxi Province, China were investigated. The patterns and magnitudes of vertical hyporheic water exchange (VHWE) were estimated based on a one-dimension heat steady-state model while the sediment vertical hydraulic conductivity ( K v ) was calculated via in situ permeameter tests. The results demonstrated that six hydrodynamic zones and their extensions were observed at the CMB during the test period. These zones were likely controlled by the obtuse junction angle and low momentum flux ratio, influencing the sediment grain size distribution of the CMB. The VHWE patterns at the test site during the test period mostly showed upwelling flow dominated by regional groundwater discharging into the river. The occurrence of longitudinal downwelling and upwelling patterns along the meander bend at the CMB was likely subjected to the comprehensive influences of the local sinuosity of the meander bend and regional groundwater discharge, and finally formed regional and local flow paths. Additionally, in dominated upwelling areas, the change in VHWE magnitudes was nearly consistent with that in K v values, and higher values of both variables generally occurred in erosional zones near the thalweg paths of the CMB, which were mostly made up of sand and gravel. This was potentially caused by the erosional and depositional processes subjected to confluence morphology. Furthermore, lower K v values observed in downwelling areas at the CMB were attributed to sediment clogging caused by local downwelling flow. The confluence morphology and sediment K v are thus likely the driving factors which cause local variations in the VHWE of fluvial systems.

Description: Hydrologic model development and calibration has continued in most cases to focus only on accurately reproducing streamflows. However, complex models, for example so-called physically-based models, possess large degrees of freedom that, if not constrained properly, may lead to poor model performance when used for prediction. We argue that constraining a model to represent streamflow, which is an integrated resultant of many factors across the watershed, is necessary but by no means sufficient to develop a high-fidelity model. To address this problem, we develop a framework to utilize the Gravity Recovery and Climate Experiment's (GRACE) total water storage anomaly data as a supplement to streamflows for model calibration, in a multi-objective setting. The VARS method (Variogram Analysis of Response Surfaces) for global sensitivity analysis is used to understand the model behavior with respect to streamflow and GRACE data, and the BORG multi-objective optimization method is applied for model calibration. Two sub-basins of the Saskatchewan River Basin in Western Canada are used as a case study. Results show that the developed framework is superior to the conventional approach of calibration only to streamflows, even when multiple streamflow-based error functions are simultaneously minimized. It is shown that a range of (possibly false) system trajectories in state variable space can lead to similar (acceptable) model responses. This observation has significant implications for land-surface and hydrologic model development, and, if not addressed properly, may undermine the credibility of the model in prediction. The framework effectively constrains the model behavior (by constraining posterior parameter space) and results in more credible representation of hydrology across the watershed.

Description: Rainfall-runoff models are widely used to predict flows using observed (instrumental) time series of air temperature and precipitation as inputs. Poor model performance is often associated with difficulties in estimating catchment scale meteorological variables from point observations. Readily available gridded climate products are an underutilized source of temperature and precipitation time series for rainfall-runoff modelling which may overcome some of the performance issues associated with poor quality instrumental data in small headwater monitoring catchments. Here we compare the performance of instrumental measured and E-OBS gridded temperature and precipitation time series as inputs in the rainfall-runoff models PERSiST and HBV for flow prediction in six small Swedish catchments. For both models and most catchments, the gridded data produced statistically better simulations than those obtained using instrumental measurements. Despite the high correspondence between instrumental and gridded temperature, both temperature and precipitation were responsible for the difference. We conclude that (1) gridded climate products such as the E-OBS dataset could be more widely used as alternative input to rainfall-runoff models, even when instrumental measurements are available and (2) the processing applied to gridded climate products appear to provide a more realistic approximation of small catchment scale temperature and precipitation patterns needed for flow simulations. Further research on this issue is needed and encouraged.

Description: ABSTRACT Urbanization threatens headwater stream ecosystems globally. Watershed restoration practices, such as infiltration-based stormwater management, are implemented to mitigate the detrimental effects of urbanization on aquatic ecosystems. However, their effectiveness for restoring hydrologic processes and watershed storage remains poorly understood. Our study used a comparative hydrology approach to quantify the effects of urban watershed restoration on watershed hydrologic function in headwater streams, located within the Coastal Plain of Maryland, USA. We selected 11 headwater streams that spanned an urbanization–restoration gradient (4 forested; 4 urban-degraded, 3 urban-restored) to evaluate changes in watershed hydrologic function from both urbanization and watershed restoration. Discrete discharge and continuous, high-frequency rainfall-stage monitoring were conducted in each watershed. These datasets were used to develop six hydrologic metrics describing changes in watershed storage, flowpath connectivity, or the resultant stream flow regime. The hydrological effects of urbanization were clearly observed in all hydrologic metrics, but only one of the three restored watersheds exhibited partially restored hydrologic function. At this site, a larger minimum runoff threshold was observed relative to urban-degraded watersheds, suggesting enhanced infiltration of stormwater runoff within the restoration structure. However, baseflow in the stream draining this watershed remained low compared to the forested reference streams, suggesting that enhanced infiltration of stormwater runoff did not recharge subsurface storage zones contributing to stream baseflow. The highly variable responses among the three restored watersheds were likely due to the spatial heterogeneity of urban development, including the level of impervious cover and extent of the storm sewer network. This study yielded important knowledge on how restoration strategies, such as infiltration-based stormwater management, modulated – or failed to modulate – hydrological processes affected by urbanization, which will help improve the design of future urban watershed management strategies. More broadly, we highlighted a multi-metric approach that can be used to monitor restoration of headwater stream ecosystems in disturbed landscapes.

Description: Hydrological and hydrochemical processes in the critical zone of karst environments are controlled by the fracture-conduit network. Modelling hydrological and hydrochemical dynamics in such heterogeneous hydrogeological settings remains a research challenge. In this study, water and solute transport in the dual flow system of the karst critical zone were investigated in a 73.5km 2 catchment in southwest China. We developed a dual reservoir conceptual runoff model combined with an ARMA model with algorithms to assess dissolution rates in the “fast flow” and “slow flow” systems. This model was applied to three catchments with typical karst critical zone architectures, to show how flow exchange between fracture and conduit networks changes in relation to catchment storage dynamics. The flux of bidirectional water and solute exchange between the fissure and conduit system increase from the headwaters to the outfall due to the large area of the developed conduits and low hydraulic gradient in the lower catchment. Rainfall amounts have a significant influence on partitioning the relative proportions of flow and solutes derived from different sources reaching the underground outlet. The effect of rainfall on catchment function is modulated by the structure of the karst critical zone (e.g. epikarst and sinkholes). Thin epikarst and well-developed sinkholes in the headwaters divert more surface water (younger water) into the underground channel network, leading to a higher fraction of rainfall recharge into the fast flow system and total outflow. Also, the contribution of carbonate weathering to mass export is also higher in the headwaters due to the infiltration of younger water with low solute concentrations through sinkholes.

Description: Mapping saturation areas during rainfall events is important for understanding the dynamics of overland flow. In this study we evaluate the potential of high temporal resolution time lapse photography for mapping the dynamics of saturation areas (i.e. areas where water is visually ponding on the surface) on the hillslope scale during natural rainfall. We take one image per minute over a 100 x 15 m 2 depression area on an agricultural field in the Hydrological Open Air Laboratory (HOAL), Austria. The images are georectified and classified by an automated procedure, using grey intensity as a threshold to identify saturation area. The optimum threshold T is obtained by comparing saturation areas from the automated analysis with the manual analysis of 149 images. T is found to be highly correlated with an image brightness characteristic defined as the greyscale image histogram mode M (Pearson correlation r = 0.91). We estimate T as T = M + C where C is a calibration parameter assumed to be constant during each event. The automated procedure estimates the total saturation area close to the manual analysis with mean NRMSE of 9% and 21% if C is calibrated for each event and taken constant for all events, respectively. The spatial patterns of saturation are estimated with a geometric mean accuracy index of 94% as compared to the manual analysis of the same photos. The patterns are tested against field observations for one date as a preliminary demonstration, which yields an RMSE of the shortest distance between the measured boundary points and the automatically classified boundary as 23 cm. The usefulness of the patterns is illustrated by exploring runoff generation processes of an example event. Overall, the proposed classification method based on grey intensity is found to process images with highly varying brightnesses well. It is more efficient than the manual tracing for a large number of images, which allows the exploration of surface flow processes at high temporal resolution.

Description: A better understanding of solute transport and retention mechanism in rock fractures has been challenging due to difficulty in their direct observations in microscale rough-walled fractures. Six representative troughs in a rough-walled fracture were selected for microscale observations of eddy formation with increasing flow velocity and its effect on spatiotemporal changes of solute concentration. This experimental study was enabled by a microscale visualization technique of micro Particle Image Velocimetry (micro-PIV). With increasing flow velocity (Re ≤ 2.86), no eddies were generated, and solutes along the main streamlines transported rapidly, while those near the wall moved slowly. A larger amount of solutes remained trapped at all troughs at Re = 2.86 than Re 〈 1. For Re = 8.57, weak eddies started to be developed at the troughs on the lee side, which little contributed to overall solute flushing in the fracture. Accordingly, a large of amount of water was needed for solute flushing. The flow condition of 1 〈 Re 〈 10, before a full development of eddies, was least favorable in terms of time and amount of remediation fluid required to reach a target concentration. After large eddies were fully developed at troughs on the lee side for Re = 17.13, solutes were substantially reduced by eddies with less amount of water. Fully-developed eddies were found to enhance solute transport and recovery, as opposed to a general consensus that eddies trap and delay solutes. Direct inflow into troughs on the stoss side also made a great contribution to solute flushing out of the troughs. This study indicates that fully-developed eddies or strong inflows at troughs are highly possible to form for Re 〉 10 and this flow range could be favorable for efficient remediation.

Description: This study demonstrates the spatial variation in hydrological processes across the Upper Mississippi River Basin (UMRB) by the end of 21st century, by ingesting FOREcasting SCEnarios (FORE-SCE) of Land-use Change projections into a physics based hydrologic model - Soil and Water Assessment Tool (SWAT). The model is created for UMRB (440,000 km 2 ), using the National Landcover Database of year 2001 and climate data of 1991-2010. Considering 1991-2010 as the baseline reference period, FORE-SCE projections of year 2091 under three scenarios (A1B, A2 and B1 from the Intergovernmental Panel on Climate Change) are separately assimilated into the calibrated model, while climate input is kept the same as in the baseline. Modeling results suggest an increase of 0.5% and 3.5% in the average annual streamflow at the basin outlet (Grafton, Illinois) during 2081-2100, respectively for A1B and A2; while for B1, streamflow would decrease by 1.5%. Under the “worst-case” A2 scenario, 6% and 133% increase respectively in agricultural and urban areas with 30% depletion of forest and grassland would result into 70% increase in surface runoff, 20% decrease in soil moisture and 4% decrease in evapotranspiration in certain parts of the basin. Conversion of cropland, forest or grassland to perennial hay/pasture areas would lower surface runoff by 25% especially in the central region, while persistent forest cover in the northern region would cause up to 7% increase in evapotranspiration. The ecosystem in the lower half of UMRB is likely to become adverse, as dictated by a composite water-energy balance indicator. Future land use change extents and resultant hydrologic responses are found significantly different under A2, A1B and B1 scenarios, which resonates the need for multi-scenario ensemble assessments towards characterizing a probable future. The spatial variation of hydrologic processes as shown here helps to identify potential “hot spots”, giving ways to adopt more effective policy alternatives at regional level.

Description: Frozen ground hydrological effects on runoff, storage and release have been observed in the field and tested in numerical models, but few physical models of frozen slopes (at scales from 1-15 m) exist partly because the design of such an experiment requires new engineering design for realistic whole-slope freezing and physical model innovation. Here we present a new freezable tilting hillslope physical model for hydrological system testing under a variety of climate conditions with the ability to perform multiple (up to 20 per year) freeze/thaw cycles. The 4 by 2 m hillslope is mobile and tiltable based on a modified tri-axle 4.88 m (16’) dump trailer to facilitate testing multiple configurations. The system includes controllable boundary conditions on all surfaces; examples of side and base flow boundary conditions include permeable membranes, impermeable barriers, semi-permeable configurations as well as constant head conditions. To simulate cold regions and to freeze the hillslope in a realistic and controlled manner, insulation and a removable freezer system are incorporated onto the top boundary of the hillslope. The freezing system is designed to expedite the freezing process by the addition of a 10,130 KJ (9,600 BTU) refrigeration coil to the top-center of the insulated ceiling. Center placement provides radial freezing of the hillslope in a top-down fashion, similar to what natural systems encounter in the environment. The perimeter walls are insulated with 100 mm of spray-foam insulation, while the base of the hillslope is not insulated to simulate natural heat fluxes beneath the frozen layer of soil. Our preliminary testing shows that covers can be frozen down to -10°C in approximately 7 days, with subsequent thaw on a similar timeframe.

Description: Glaciers are significant fresh water storage systems in western China and contribute substantially to the summertime runoff of many large rivers in the Tibetan Plateau. Under the scenario of climate change, discussions of glacier variability and melting contributions in alpine basins are important for understanding the runoff composition and ensuring that water resources are adequately managed and protected in the downstream areas. Based on the multi-source spatial data and long-term ground observation of climatic and hydrologic data, using the remote sensing interpretation, degree-day model, and ice volume method, we presented a comprehensive study of the glacier changes in number, area, and termini and their impacts on summertime runoff and water resource in the Tuotuo River basin, located in the source region of the Yangtze River. The results indicated that climate change, especially rising temperature, accelerated the glacier melting and consequently led to hydrological change. From 1969 to 2009, the glacier retreat showed an absolutely dominant tendency with 13 reduced glaciers and lost glacier area of 45.05 km 2 , accompanied by limited growing glaciers in the study area. Meanwhile, it indicated that annual glacial runoff was averagely 0.38×10 8 m 3 , accounting for 4.96 % of the total summertime runoff, followed by the supply from precipitation and snowmelt. The reliability of this magnitude was assessed by the classic volume method, which also showed that the water resources from glacier melting in the Tuotuo River basin increased by approximate 17.11×10 8 m 3 , accounting for about 3.77 % of the total runoff over the whole period of 1969 - 2009. Findings from this study will serve as a reference for future research about glacier hydrology in regions where observational data are deficient. Also it can help the planning of future water management strategies in the source region of the Yangtze River.

Description: Daily precipitation/temperature data collected at 74 weather stations across the Pearl River basin of China (PRBC), for the years 1952-2013 were used to analyze extreme precipitation (EP) processes at annual and seasonal scales in terms of precipitation magnitude, occurrence rates and timing. Peak-over-Threshold (POT) sampling, Modified Mann-Kendall (MMK) trend tests and Poisson regression model (PRM) were utilized in this study. Causes driving the observed statistical behaviors of EP were investigated, focusing particularly on the impacts of temperature change and the El Niño-Southern Oscillation (ENSO). EP events, which occur mainly during April and September, are most frequent in June. At an annual scale they are subject to relatively even inter-annual distributions during the wet season. Significant trends were observed in the magnitude, frequency and timing of EP events during the dry seasons, although no such trends were seen during the wet seasons. Seasonal shifts in EP can easily trigger sudden flood or drought events and warming temperatures and ENSO events also have significant impacts on EP processes across the PRBC, as reflected by their increased magnitude and frequency in the western PRBC and decreased precipitation magnitudes in the eastern PRBC during ENSO periods. These results provide important evidence of regional hydrological responses to global climate changes in terms of EP regimes in tropical and subtropical zones.

Description: Freshwater resources in the arid Arabian Peninsula, especially transboundary aquifers shared by Saudi Arabia, Jordan, and Iraq, are of critical environmental and geopolitical significance. Monthly Gravity Recovery and Climate Experiment (GRACE) satellite-derived gravity field solutions acquired over the expansive Saq transboundary aquifer system were analyzed and spatiotemporally correlated with relevant land surface model outputs, remote sensing observations, and field data to quantify temporal variations in regional water resources and to identify the controlling factors affecting these resources. Our results show substantial GRACE-derived Terrestrial Water Storage (TWS) and Groundwater Storage (GWS) depletion rates of -9.05 ± 0.25 mm/year (-4.84 ± 0.13 km 3 /year) and -6.52 ± 0.29 mm/year (-3.49 ± 0.15 km 3 /year), respectively. The rapid decline is attributed to both climatic and anthropogenic factors; observed TWS depletion is partially related to a decline in regional rainfall, while GWS depletions are highly correlated with increasing groundwater extraction for irrigation and observed water level declines in regional supply wells.

Description: Scale- and location-dependent relationships between soil water content (SWC) and individual environmental factors have been widely explored. SWC is controlled by multiple factors concurrently; however, the multivariate relationship is rarely explored at different scales and locations. Multivariate controls of SWC at different scales and locations in two seasons within a hummocky landscape of North America were identified using bivariate wavelet coherency and multiple wavelet coherence. Results showed that depth to CaCO 3 layer which was correlated with elevation over all locations at scales of 36–144 m and cos(aspect) provided the best individual factor for explaining SWC variations in the spring (May 2) and summer (August 23), respectively. Although spatial patterns of SWC were temporally stable, different topographic indices affected spatial distribution of SWC in different seasons (elevation in spring and aspect in summer) due to different dominating hydrological processes. These varying hydrological processes also resulted in the distinct role of soil organic carbon (SOC) content in different seasons: a positive correlation in the spring and a negative correlation in the summer. Multiple wavelet coherence identified a combination of depth to CaCO 3 layer and SOC in spring and a combination of cos(aspect) and SOC in summer that controlled SWC at different scales and locations, respectively. This indicated a combined effect of soil and topographic properties on SWC distribution and a clear need for these two factors in developing scale-dependent prediction of SWC in the hummocky landscape of North America.

Description: Hydrologic recovery after wildfire is critical for restoring the ecosystem services of protecting of human lives and infrastructure from hazards and delivering water supply of sufficient quality and quantity. Recovery of soil-hydraulic properties, such as field-saturated hydraulic conductivity ( K fs ), is a key factor for assessing the duration of watershed-scale flash flood and debris flow risks after wildfire. Despite the crucial role of K fs in parameterizing numerical hydrologic models to predict the magnitude of post-wildfire runoff and erosion, existing quantitative relations to predict K fs recovery with time since wildfire are lacking. Here we conduct meta-analyses of five datasets from the literature that measure or estimate K fs with time since wildfire for longer than three years duration. The meta-analyses focus on fitting two quantitative relations (linear and nonlinear logistic) to explain trends in K fs temporal recovery. The two relations adequately described temporal recovery except for one site where macropore flow dominated infiltration and K fs recovery. This work also suggests that K fs can have low hydrologic resistance (large post-fire changes), and moderate to high hydrologic stability (recovery time relative to disturbance recurrence interval) and resilience (recovery of hydrologic function and provision of ecosystem services). Future K fs relations could more explicitly incorporate processes like soil-water repellency, ground cover and soil structure regeneration, macropore recovery, and vegetation regrowth.

Description: Surface soil moisture has been extensively studied for various land uses and landforms. Although many studies have reported potential factors that control surface soil moisture over space or time, the findings have not always been consistent, indicating a need for identification of the main factors. This study focused on the static controls of topographic, soil, and vegetation features on surface soil moisture in a steep natural forested headwater catchment consisting of three hillslope units of a gully area, side slope, and valley-head slope. Using a simple correlation analysis to investigate the effects of the static factors on surface soil moisture at 0–20 cm depths at 470 points in 13 surveys, we addressed the characteristics of surface soil moisture and its main controlling factors. The results indicated that the mean of surface soil moisture was in the decreasing order of gully area 〉 valley-head 〉 side slope. The relationship between the mean and standard deviation of surface soil moisture showed a convex-upward shape in the headwater catchment, a negative curvilinear shape in the gully area, and positive curvilinear shapes at the side and valley-head slopes. At the headwater catchment and valley-head slope, positive contributions of soil porosity and negative contributions of slope gradient and saturated hydraulic conductivity were the main controlling factors of surface soil moisture under wetter conditions, whereas positive contributions of topographic wetness index and negative contributions of vegetation density were the main controlling factors of surface soil moisture under drier conditions. At the side slope underlain by fractured bedrocks, only saturated hydraulic conductivity and vegetation density were observed to be controlling factors. Surface soil moisture in the gully area was mainly affected by runoff rather than static features. Thus, using hillslope units is effective for approximately estimating the hydrological behaviors of surface moisture on a larger scale, whereas dependency between the main static factors and moisture conditions is helpful for estimating the spatial distributions of surface moisture on a smaller scale.

Description: Stable isotopes of water are known to provide information on mean altitudes of spring recharge areas which is an important parameter for groundwater resources management especially in karstic environments. Very often a lack of precipitation input data is limiting the possibility for an appropriate estimation of mean catchment altitudes. In the Jeita spring catchment, Lebanon, a characterization of precipitation input was possible with samples collected at six stations at varying altitudes (88 amount weighed monthly samples). A local meteoric water line for the Jeita spring catchment (Jeita-LMWL) was characterized as δ 2 H = 6.04 * δ 18 O + 8.45 (R 2 = 0.92) for a two year observation period between October 2012 and September 2014. Integral samples from the snow layer were collected at 22 sites at altitudes ranging from 1,000 to 2,300 m above sea level at the end of February 2012 and February 2013, when snow height reached a maximum of more than 6 m at the highest peak in the catchment. Water samples were continuously collected from six springs (Jeita, Kashkoush, Labbane, Assal, Afqa, and Rouaiss). Jeita spring water samples were collected additionally in daily time steps during the snowmelt season in 2012. Mean isotope values of the sampled springs range from -6.8 ‰ to -8.2 ‰, and from -33 ‰ to -44 ‰, for δ 18 O and δ 2 H, respectively. The stable isotope data shows that input variability (space and time, snow cover and rainfall) has direct impacts on mean altitude estimates of spring catchments. A more profound interpretation of spring response to rainfall for six local springs in the Lebanon Mountains was possible in comparison to four earlier described springs collected in the Anti-Lebanon Mountains in Syria.

Description: Litter layers develop across a diverse array of vegetated ecosystems and undergo significant temporal compositional changes due to canopy phenological phases and disturbances. Past research on temporal dynamics of litter interception have focused primarily on litter thickness and leaf fall, yet forest phenophases can change many more litter attributes (e.g., woody debris, bark shedding, and release of reproductive materials). In this study, weekly changes in litter composition over 1 year were used to estimate litter water storage dynamics and model event-based litter interception. Litter interception substantially reduced throughfall (6-43%) and litter water storage capacity ranged from 1-3 mm, peaking when megastrobili release and liana leaf senescence occurred simultaneously during fall 2015. Tropical storm disturbances occurred during the sampling period, allowing evaluation of how meteorological disturbances altered litter interception. High wind speeds and intense rainfall from two tropical storms increased litter interception by introducing new woody debris which, in this study, stored more water than the pre-existing woody debris. After two extreme weather events, a third (Hurricane Hermine) did not increase woody debris (or litter interception), suggesting that the canopy pool of branches susceptible to breakage had been largely depleted. Needle and bark shedding had minor effects on litter interception. Results suggest that the release of reproductive materials and meteorological disturbances appear to be the major compositional drivers of litter interception beyond their obvious contribution to litter thickness.

Description: The Arctic hydrologic cycle is intensifying, as evidenced by increased rates of precipitation, evapotranspiration, and riverine discharge. However, the controls on water fluxes from terrestrial to aquatic systems in upland Arctic landscapes are poorly understood. Upland landscapes account for 1/3 rd of the Arctic land surface and are often drained by zero-order geomorphic flowpath features called water tracks. Previous work in the region attributed rapid runoff response at larger stream orders to water tracks, but models suggest water tracks are hydrologically disconnected from the surrounding hillslope. To better understand the role of water tracks in upland landscapes, we investigated the surface and subsurface hydrologic responses of six water tracks and their hillslope watersheds to natural patterns of rainfall, soil thaw, and drainage. Between storms, both water track discharge and the water table in the hillslope watersheds exhibited diel fluctuations that, when lagged by five hours, were temporally correlated with peak evapotranspiration rate. Water track soils remained saturated for more of the summer season than soils in their surrounding hillslope watersheds. When rainfall occurred, the subsurface response was nearly instantaneous, but the water tracks took significantly longer than the hillslopes to respond to rainfall, and longer than the responses previously observed in nearby larger order Arctic streams. There was also evidence for antecedent soil water storage conditions controlling the magnitude of runoff response. Based on these observations, we used a broken stick model to test the hypothesis that runoff production in response to individual storms was primarily controlled by rainfall amount and antecedent water storage conditions near the water track outlet. We found that the relative importance of the two factors varied by site and that water tracks with similar watershed geometries and at similar landscape positions had similar rainfall-runoff model relationships. Thus, the response of terrestrial water fluxes in the upland Arctic to climate change depends on the non-linear interactions between rainfall patterns and subsurface water storage capacity on hillslopes. Predicting these interactions across the landscape remains an important challenge.

Description: ABSTRACT Snow and glaciers are known to be important sources for fresh water; nevertheless, our understanding of the hydrological functioning of glacial catchments remains limited when compared with lower-altitude catchments. In this study, a temperate glacial region located in the southeast margin of the Tibetan Plateau is selected to analyze the characteristics of δ 18 O and δ D in different water sources and the contribution of glacier-snow meltwater to streamflow. The results indicate that the δ 18 O of river water ranges from -16.2‰ to -10.2‰ with a mean of -14.1‰ and that the δ D values range from -117.0‰ to -68.0‰ with a mean of -103.1‰. These values are more negative than those of glacier-snow meltwater but less negative than those of precipitation. The d -excess values are found to decrease from meltwater to river to lake/reservoir water as a result of evaporation. Based on hydrograph separation, glacier-snow meltwater accounts for 51.5% of river water in the Baishui catchment in the melting season. In the Yanggong catchment, snow meltwater contributes 47.9% to river water in the pre-monsoon period and glacier meltwater contributes only 6.8% in the monsoon period. The uncertainty in hydrograph separation is sensitive to the variation of tracer concentrations of streamflow components. The input of meltwater to a water system varies with local climate and glacier changes. The results confirm that hydrograph separation using water isotopes is valuable for evaluating the recharge sources of rivers, especially in ungauged glacial regions. This study provides insights into the hydrological processes of glacial catchments on the Tibetan Plateau, which is important for water resource management.

Description: Lakes are a prominent geographic feature in northern landscapes and play an important role in understanding regional climate systems. In order to better model changes within climate systems it is important to study lake ice processes. Although the availability of records for lake ice through ground measurements has declined in recent years, the increased use of remote sensing provides an alternative to this. Using a pre-classified snow and ice remote sensing product with a 500 metre resolution, based on images from the Moderate Resolution Imaging Spectroradiometer (MODIS/MOD10A1), and the use of measured and reanalysis temperature data, this study evaluated lake ice phenology dates in connection to recent trends in temperature and 0°C isotherms within Ontario and Manitoba between 2001 and 2014. Temperature trends indicated both regional warming and cooling, with significant cooling observed in Southern Ontario (p 〈 0.05) and significant warming in Southern Manitoba (p 〈 0.1) during the fall. Spatial analysis of the trends in the lake ice data showed significant clustering of significant trends in ice on dates (p 〈 0.01). When analyzing the trends in ice phenology in connection to the trends in temperature, it was found that 70% of lakes experienced a change in the ice on date with the expected change in temperature and 85% of lakes for ice off date. When shifting ice on and ice off dates are investigated in relation to 0°C isotherms, it was seen that 80% of ice on dates and 100% of ice off dates shifted in sync with the isotherm dates. This demonstrates that the ice phenology of lakes in Ontario and Manitoba, Canada is responding to short-term variability in temperature. The MODIS product could be used to investigate ice phenology on a large scale and contribute towards expanding existing records of ice phenology. Establishing long term ice records could be a valuable asset for other research ranging from water balance studies to the response of lake biota under changing climate.

Description: This paper presents the use of stable isotopes of water for hydrological characterization and flow component partitioning in the Red River Delta (RRD), the downstream section of the Red River. Water samples were collected monthly during 2015 from the mainstream section of the river and its right bank tributaries flowing through the RRD. In general, δ 18 O and δ 2 H river signatures were depleted in summer-autumn (May-October) and elevated in winter-spring (November-April), displaying seasonal variation in response to regional monsoon air mass contest. The Pacific equatorial-maritime air mass dominates in summer and the northern Asia continental air mass controls in winter. Results show that water of the RRD tributaries stems solely from local sources and is completely separated from water arriving from upstream sub-basins. This separation is due to the extensive management of the RRD (e.g. dykes and dams) for the purposes of irrigation and inundation prevention. Mainstream river section δ 18 O and δ 2 H compositions range from -10.58 and -73.74‰ to -6.80 and -43.40‰, respectively, and the corresponding ranges inside the RRD were from -9.35 and -64.27‰ to -2.09 and -15.80‰. A combination of data analysis and hydrological simulation confirms the role of upstream hydropower reservoirs in retaining and mixing upstream water. River water inside the RRD experienced strong evaporation characterized by depleted d-excess values, becoming negative in summer. On the other hand, the main stream of the Red River has d-excess values around 10‰, indicating moderate evaporation. Hydrograph separation shows that in upstream sub-basins, the groundwater fraction dominates the river flow composition, especially during low flow regimes. Inside the RRD, the river receives groundwater during the dry season while groundwater replenishment occurs in the rainy season. Annual evaporation obtained from this hydrograph separation computation was about 6.3% of catchment discharge, the same order as deduced from the difference between sub-basin precipitation and discharge values. This study shows the necessity to re-evaluate empirical approaches in large river hydrology assessment schemes, especially in the context of climate change.

Description: Hydrologic models are useful to understand the effects of climate and land-use changes on dry-season flows. In practice, there is often a trade-off between simplicity and accuracy, especially when resources for catchment management are scarce. Here, we evaluated the performance of a monthly rainfall-runoff model (dynamic water balance model, DWBM) for dry-season flow prediction under climate and land-use change. Using different methods with decreasing amounts of catchment information to set the four model parameters, we predicted dry-season flow for 89 Australian catchments, and verified model performance with an independent dataset of 641 catchments in the United States. For the Australian catchments, model performance without catchment information (other than climate forcing) was fair; it increased significantly as the information to infer the four model parameters increased. Regressions to infer model parameters from catchment characteristics did not hold for catchments in the United States, meaning that a new calibration effort was needed to increase model performance there. Recognizing the interest in relative change for practical applications, we also examined how DWBM could be used to simulate a change in dry-season flow following land-use change. We compared results with and without calibration data, and showed that predictions of changes in dry-season flow were robust with respect to uncertainty in model parameters. Our analyses confirm that climate is a strong driver of dry-season flow and that parsimonious models such as DWBM have useful management applications: predicting seasonal flow under various climate forcings when calibration data are available, and providing estimates of the relative effect of land-use on seasonal flow for ungauged catchments.

Description: There remains continued use of non-linear, logistic regression models for predicting water temperature from air temperature. A dominant feature of these non-linear models is an upper bound on river water temperature. This upper bound is often attributed to a large increase in evaporative cooling at high air temperatures, but the exact conditions under which such an increase may occur have not been thoroughly explored. To better understand the appropriateness of the non-linear model for predicting river water temperatures, it is essential to understand the physical basis for the upper bound and when it should and should not be included in the statistical model. This paper applies and validates an energy balance model against eight river systems spread across different climate regions of the U.S. The energy balance model is then used to develop a diagram relating vapor pressure deficit and air temperature to water temperature. With knowledge of future vapor pressure deficit (difference between saturation and actual vapor content in the atmosphere) conditions in a given climate, the diagram can be used to predict the likelihood of an upper bound in the air-water temperature relationship. This investigation offers a fundamental physical explanation of the most appropriate form of statistical models that should be used for predicting future water temperature from air temperature in different geographic regions with different climate conditions. In general, climatic regions that have only a slight increase in vapor pressure deficit with increasing air temperature (typically humid regions) would not be expected to have an upper bound. Conversely, climatic regions in which vapor pressure deficit sharply increases with increasing air temperature (typically arid regions) would be expected to have an upper bound.

Description: For various hydrological applications such as flood control projects, a knowledge of stage–discharge relationship is of particular interest to river engineers. Stage-discharge curves in compound channels cannot be easily predicted in comparison with single channels due to their 3D characteristics of flow. In this paper, the concept of cross-sectional isovel contours is used for estimation of stage-discharge curves in compound channels. The multivariate Newton's method is applied to the difference between the observed and estimated data to optimize the exponent values of the governing parameters. The accuracy of the proposed model is tested successfully against available experimental results, which are taken from the FCF (Flood Channel Facility) laboratory. Then the results are compared with the Single and Divided Channel Methods ( SCM and DCM , respectively), the Weighted Divided Channel Method ( WDCM ), the Exchange Discharge Method ( EDM ) and the Coherence Method ( COHM ). The average values of MAPE (Mean Absolute Percentage Error) and NRMSE (Normalized Root Mean Square Error) in discharge estimation based on each referenced section at any level for six sections of the experimental cases are within 3.1% and 0.023, respectively. The biggest advantage of the proposed method is its inherent simplicity which does not need any calibration.

Description: In the Great Lakes basin of North America, annual runoff is dominated by snowmelt. This snowmelt-induced runoff plays an important role within the hydrologic cycle of the basin, influencing soil moisture availability and driving the seasonal cycle of spring and summer Lake levels. Despite this, relatively little is understood about the patterns and trends of snow ablation event frequency and magnitude within the Great Lakes basin. This study uses a gridded dataset of Canadian and United States surface snow depth observations to develop a regional climatology of snow ablation events from 1960-2009. An ablation event is defined as an inter-diurnal snow depth decrease within an individual grid cell. A clear seasonal cycle in ablation event frequency exists within the basin and peak ablation event probability is latitudinally dependent. Most of the basin experiences peak ablation frequency in March, while the northern and southern regions of the basin experience respective peaks in April and February. An investigation into the inter-annual frequency of ablation events reveals ablation events significantly decrease within the northeastern and northwestern Lake Superior drainage basins and significantly increase within the eastern Lake Huron and Georgian Bay drainage basins. In the eastern Lake Huron and Georgian Bay drainage basins, larger ablation events are occurring more frequently, and a larger impact to the hydrology can be expected. Trends in ablation events are attributed primarily to changes in snowfall and snow depth across the region.

Description: This study examines a method to improve a process-oriented hydrological model concept applied to another region than it was first developed for. In principle, we propose to analyse and refine each major hydrological process separately, sequentially, and iteratively. To test the method, the HYPE model concept (HYdrological Predictions for the Environment, originally developed for Sweden) was here applied to the data-sparse Niger River basin in West Africa. Errors in the baseline Niger-HYPE model were analysed to identify inadequately described processes. These process descriptions were subsequently isolated and refined through a set of experiments focusing on concept development, input data enhancement, and multi-variable calibration. The refinements were guided by in situ discharge observations, earth observations, local expert knowledge, and previous studies. The results show that the original model concept could simulate the annual cycle of discharge, but not the magnitudes or daily dynamics (56-station average Nash-Sutcliffe Efficiency (NSE) = −1). The main processes requiring improved descriptions were precipitation, evaporation, surface runoff, infiltration, soil storage, reservoir regulations, aquifer recharge, and flooding and river-atmosphere exchange in the Inner Niger Delta. Of these, evaporation, flooding and river-atmosphere exchange differ so much between Sweden and the Niger River that the model concept had to be refined. All refinements were synthesized in a new model version (Niger-HYPE2.0) performing significantly better across the basin (56-station average NSE=0.4). This study demonstrates the danger of applying a model off the shelf, and the obligation to carefully evaluate and revise process descriptions when applying a model concept to a new region. Moreover, the results indicate that our approach to separately, sequentially, and iteratively refine processes together with local experts can substantially improve process-oriented hydrological models.

Description: Environmental change resulting from intensified human interventions and climate change has impacted the hydrological function of many large river systems, largely altering the production and transport of runoff and sediment. It is thus vital to quantitatively evaluate the influence of climate change and human activities on streamflow and sediment discharge. Water balance equations, hydrological models and comparative analyses are commonly used to fulfill this need. Double mass curves (DMC), being one useful method for comparative analyses, are characterized by low data requirements and high transferability, and thus more practical than water balance equations and hydrological models for hydrologic benefic evaluations. However, the detailed derivation procedure of the DMC has, to date, yet been described in literature. Moreover, in previous studies changing points of the DMC were determined either rather empirically or as the changing point of streamflow/sediment discharge (i.e. precipitation was not considered). Hence, the changing point detected may be subject to inaccuracies. This paper, for the first time, comprehensively detailed the derivation procedure of the DMC, a new way was proposed to quantitatively examine the changing point of the DMC, an example was also given to demonstrate the use of the DMC in the hydrologic benefic evaluation. It is hopeful that the method given in our paper will be widely adopted by future studies as a standard procedure to derive and use the DMC.

Description: Fluvial flood events have substantial impacts on humans, both socially and economically, as well as on ecosystems (e.g. hydroecology, pollutant transport). Concurrent with climate change the seasonality of flooding in cold environments is expected to shift from a snowmelt-dominated to a rainfall-dominated flow regime. This would have profound impacts on water management strategies, i.e. flood risk mitigation, drinking water supply and hydro power. In addition, cold climate hydrological systems exhibit complex interactions with catchment properties and large-scale climate fluctuations making the manifestation of changes difficult to detect and predict. Understanding a possible change in flood seasonality and defining related key drivers therefore is essential to mitigate risk and to keep management strategies viable under a changing climate. This study explores changes in flood seasonality across near-natural catchments in Scandinavia using circular statistics and trend tests. Results indicate strong seasonality in flooding for snowmelt-dominated catchments with a single peak occurring in spring and early summer (March through June), whereas flood peaks are more equally distributed throughout the year for catchments located close to the Atlantic coast and in the south of the study area. Flood seasonality has changed over the past century seen as decreasing trends in summer maximum daily flows and increasing winter and spring maximum daily flows with 5-35% of the catchments showing significant changes at the 5% significance level. Seasonal mean daily flows corroborate those findings with higher percentages (5-60%) of the catchments showing statistically significant changes. Alterations in annual flood occurrence also point toward a shift in flow regime from snowmelt-dominated to rainfall-dominated with consistent changes toward earlier timing of the flood peak (significant for 25% of the catchments). Regionally consistent patterns suggest a first order climate control as well as a local second order catchment control which causes inter-seasonal variability in the streamflow response.

Description: Non-point source pollution in the impervious surface of city, which including dissolved and particulate pollutants, is a significant source of water pollution. Simple first-order decay models can generally simulate the cumulative wash-off process of the particulate pollutants. There is inadequate knowledge as to whether or not they are suitable for dissolved pollutants. This study presents a mathematical wash-off model for dissolved pollutants, which combines analytical equations for overland flows and the exponential equation for the pollutant wash-off. A series of laboratory experiments have been conducted to verify this wash-off model. It shows that the pollutant concentration and pollutant transport rate can be predicted well by the newly-developed equations. It is found that the pollutant concentration monotonically decreases to zero as the accumulated pollutants are washed off, while the pollutant transport rate first increases to the maximum value and then decreases to zero. The maximum pollutant transport rate is found to increase with the decrease of the arrival time of the maximum value. The difference between the simplified exponential model and the amended wash-off equation depends on the initial residual percentage ( P c ) , but the present equation generally provides a more accurate representation of the wash-off process of dissolved pollutants.

Description: Local governmental agencies are increasingly undertaking potentially costly “status-and-trends” monitoring to evaluate the effectiveness of stormwater control measures and land-use planning strategies, or to satisfy regulatory requirements. Little guidance is presently available for such efforts, and so we have explored the application, interpretation, and temporal limitations of well-established hydrologic metrics of runoff changes from urbanization, making use of an unusually long-duration, high-quality data set from the Pacific Northwest (USA) with direct applicability to urban and urbanizing watersheds. Three metrics previously identified for their utility in identifying hydrologic conditions with biological importance that respond to watershed urbanization—T Qmean (the fraction of time that flows exceed the mean annual discharge), the Richards-Baker Index (characterizing flashiness relative to the mean discharge), and the annual tally of wet-season day-to-day flow reversals (the total number of days that reverse the prior days’ increasing or decreasing trend)—are all successful in stratifying watersheds across a range of urbanization, as measured by total contributing area of urban development. All metrics respond with statistical significance to multi-decadal trends in urbanization, but none detect trends in watershed-scale urbanization over the course of a single decade. This suggests a minimum period over which dependable trends in hydrologic alteration (or improvement) can be detected with confidence. The metrics also prove less well suited to urbanizing watersheds in a semi-arid climate, with only flow reversals showing a response consistent with prior findings from more humid regions. We also explore the use of stage as a surrogate for discharge in calculating these metrics, recognizing potentially significant agency cost savings in data collection with minimal loss of information. This approach is feasible but cannot be implemented under current data-reporting practices, requiring measurement of water-depth values and preservation of the full precision of the original recorded data. With these caveats, however, hydrologic metrics based on stage should prove as or more useful, at least in the context of status-and-trends monitoring, as those based on subsequent calculations of discharge.