Hydrological

Despite the importance of litter on forest floor albedo and brightness, previous studies have not documented forest floor albedo or litter cover in any detail. Our objective was to describe the seasonal influence of litter on spectral albedos and nadir reflectances of a forest snowpack in a mixed-hardwood stand in the Sleepers River Research Watershed (SRRW) in Danville, Vermont (37°39′ N, 119°2′ W). Experimental measurements in a nearby open area at the Snow Research Station of the SRRW nearly duplicated the spectral trend observed in the forest. Spectral albedo and nadir reflectance measurements in the visible and near infrared (350–2500 nm) transitioned from a gently curved shape through the visible range (for finer-grained, lightly littered snow) to one having a peak in the red/near-infrared (near 760 nm) as the snowmelt season progressed (for coarser-grained, more heavily littered snow). The snowpack became optically thin as surface litter reached high percentages. A point-in-time digital photographic survey of the late-lying snowpacks of three forest stands and the open showed that median litter cover percentages in the coniferous, deciduous, mixed-forest, and an open area were 17·5, 6·1, 1·2, and 0·04 respectively. A Kruskal–Wallis ANOVA on ranks and pairwise comparisons using Dunn's test indicated that the litter covers of the three forest stands were significantly different with >95% confidence. The snowpack was relatively shallow (<1 m), as is typical for this area of Vermont. From a remote-sensing standpoint, and since shallow snow and increased grain size also lower the visible albedo, we can expect that snowpack litter will cause decreased albedo earlier in the snowmelt season, at deeper snow depths, and will tend to shift the maximum albedo peak to the red/NIR range as the melt season progresses. Published in 2001 by John Wiley & Sons, Ltd.

Recent developments in hydrological modelling of river basins are focused on prediction in ungauged basins, which implies the need to improve relationships between model parameters and easilyobtainable information, such as satellite images, and to test the transferability of model parameters. A large-scale distributed hydrological model is described, which has been used in several large river basins in Brazil. The model parameters are related to classes of physical characteristics, such as soil type, land use, geology and vegetation. The model uses two basin space units: square grids for flow direction along the basin and GRU-group response units-which are hydrological classes of the basin physical characteristics for water balance. Expected ranges of parameter values are associated with each of these classes during the model calibration. Results are presented of the model fitting in the Taquari-Antas River basin in Brazil (26 000 km 2 and 11 flow gauges). Based on this fitting, the model was then applied to the Upper Uruguay River basin (52 000 km 2 ), having similar physical conditions, without any further calibration, in order to test the transferability of the model. The results in the Uruguay basin were compared with recorded flow data and showed relatively small errors, although a tendency to underestimate mean flows was found.

In this paper, we present MHYDAS-Erosion, a dynamic and distributed single-storm water erosion model developed as a module of the existing hydrological MHYDAS model. As with many catchment erosion models, MHYDAS-Erosion is able to simulate sediment transport, erosion and deposition by rill and interrill processes. Its originality stems from its capacity to integrate the impact of land management practices (LMP) as key elements controlling the sedimentological connectivity in agricultural catchments. To this end, the water-sediment pathways are first determined by a specific process-oriented procedure defined and controlled by the user, which makes the integration of LMP easier. The LMP dynamic behaviours are then integrated into the model as a time-dependent function of hydrological variables and LMP characteristics. The first version of the model was implemented for vegetative filters and tested using water and sediment discharge measurements at three nested scales of a densely instrumented catchment (Roujan, OMERE Observatory, southern France). The results of discharge and soil loss for simulated rainfall events have been found to acceptably compare with available data. The average R 2 values for water and sediment discharge are 0Ð82 and 0Ð83, respectively. The sensitivity of the model to changes in the proportion of LMP was assessed for a single rain event by considering three scenarios of the Roujan catchment management with vegetative filters: 0% (Scenario 1), 18% (Scenario 2, real case) and 100% (Scenario 3). Compared to Scenario 2 (real case), soil losses decreased for Scenario 3 by 65% on the agricultural plot scale, 62% on the sub-catchment scale and 45% at the outlet of the catchment and increased for Scenario 1 by 0% on the plot scale, 26% on the sub-catchment scale and 18% at the outlet of the catchment.

In mountain regions, wind is known to cause inhomogeneous snow distribution. Recently, it has been shown that this snow distribution can be explained by considering high-resolution flow fields created by numerical flow models. It has also been shown that small-scale deposition features are mainly caused by saltation and snow redistribution, larger-scale inhomogeneities are caused by preferential deposition, and the total deposition patterns correlate well with the mean wind speeds. On the basis of the strong correlation between wind fields and snow depth, we develop a new empirical parameterization of preferential deposition of precipitation, which is only governed by the mean flow field. The parameterization is based on case studies, where the physically based model for wind-induced snow transport ALPINE3D was combined with wind fields from the regional atmospheric model ARPS (Advanced Regional Prediction System). The parameterization, along with ARPS wind fields and a distributed energy balance snowmelt model, is used to simulate the 2006/2007 accumulation season over the catchment of Haut Glacier d'Arolla, a glacierized Alpine basin. The results were validated with a Light Detection and Ranging (LiDAR)-derived fully distributed high-resolution dataset of snow depth and show that the developed parameterization, despite being simple, is able to largely reduce the difference between modelled and measured snow depth and is suitable for application in glacier mass balance models.

Continuous monitoring of dissolved organic matter (DOM) character and concentration at hourly resolution is rare, despite the importance of analysing organic matter variability at high-temporal resolution to evaluate river carbon budgeting, river water health by detecting episodic pollution and to determine short-term variations in chemical and ecological function. The authors report a 2-week experiment performed on DOM sampled from Bournbrook, Birmingham, UK, an urban river for which spectrophotometric (fluorescence, absorbance), physiochemical (dissolved organic carbon [DOC], electrical conductivity, pH) and isotopic (D/H) parameters have been measured at hourly frequency. Our results show that the river had sub-daily variations in both organic matter concentration and characteristics. In particular, after relatively high-magnitude precipitation events, organic carbon concentration increased, with an associated increase in intensity of both humic-like and tryptophan-like fluorescence. D/H isotopic ratio demonstrates different hydrological responses to different rainfall events, and organic matter character reflects this difference. Events with precipitation < 2 mm typically yielded isotopically heavy water with relatively hydrophilic DOM and relatively low specific absorbance. Events with precipitation > 2 mm had isotopically lighter water with higher specific absorbance and a decrease in the proportion of microbially derived to humic-like fluorescence. In our heavily urbanized catchment, we interpret these signals as one where riverine DOM is dominated by storm sewer-derived ‘old’ organic matter at low-rainfall amounts and a mixed signal at high-precipitation amounts where ‘event’ surface runoff-derived organic matter dominate during storm sewer and combined sewer overflow routed DOM. Copyright © 2009 John Wiley & Sons, Ltd.

The Great Plains landscape is less topographically complex than most other regions within North America, but diverse aquatic ecosystems, such as playas, pothole lakes, ox-bow lakes, springs, groundwater aquifers, intermittent and ephemeral streams, as well as large rivers and wetlands, are highly dynamic and responsive to extreme climatic uctuations. We review the evidence for climatic change that demonstrates the historical importance of extremes in north±south dierences in summer temperatures and east±west dierences in aridity across four large subregions. These physical driving forces alter density strati®cation, deoxygenation, decomposition and salinity. Biotic community composition and associated ecosystem processes of productivity and nutrient cycling respond rapidly to these climatically driven dynamics. Ecosystem processes also respond to cultural eects such as dams and diversions of water for irrigation, waste dilution and urban demands for drinking water and industrial uses. Distinguishing climatic from cultural eects in future models of aquatic ecosystem functioning will require more re®nement in both climatic and economic forecasting. There is a need, for example, to predict how long-term climatic forecasts (based on both ENSO and global warming simulations) relate to the permanence and productivity of shallow water ecosystems. Aquatic ecologists, hydrologists, climatologists and geographers have much to discuss regarding the synthesis of available data and the design of future interdisciplinary research.

There are still relatively few hydrochemical studies of glacial runo and meltwater routing from the high latitudes, where non-temperate glacier ice is frequently encountered. Representative samples of glacier meltwater were obtained from Scott Turnerbreen, a`cold-based' glacier at 788 N in the Norwegian high Arctic archipelago of Svalbard, during the 1993 melt season and analysed for major ion chemistry. Laboratory dissolution experiments were also conducted, using suspended sediment from the runo. Signi®cant concentrations of crustal weathering derived SO 2À 4 are present in the runo, which is characterized by high ratios of SO 2 À 4 : SO 2À 4 HCO À 3 and high pCO 2 . Meltwater is not routed subglacially, but¯ows to the glacier terminus through subaerial, ice marginal channels, and partly¯ows through a proglacial icing, containing highly concentrated interstitial waters, immediately afront the terminus. The hydrochemistry of the runo is controlled by: (1) seasonal variations in the input of solutes from snow-and icemelt; (2) proglacial solute acquisition from the icing; and (3) subaerial chemical weathering within saturated, ice-cored lateral moraine adjoining drainage channels at the glacier margins, sediment and concentrated pore water from which is entrained by¯owing meltwater. Diurnal variations in solute concentration arise from the net eects of variable sediment pore water entrainment and dilution in the ice marginal streams. Explanation of the hydrochemistry of Scott Turnerbreen requires only one major subaerial¯ow path, the ice marginal channel system, in which seasonally varying inputs of concentrated snowmelt and dilute icemelt are modi®ed by seepage or entrainment of concentrated pore waters from sediment in lateral moraine, and by concentrated interstitial waters from the proglacial icing, supplied by leaching, slow drainage at grain intersections or simple melting of the icing itself. The ice marginal channels are analogous neither to dilute supra/englacial nor to concentrated subglacial¯ow components. #

A rainfall interception measuring system was developed and tested for open-grown trees. The system includes direct measurements of gross precipitation, throughfall and stem¯ow, as well as continuous collection of micrometeorological data. The data were sampled every second and collected at 30-s time steps using pressure transducers monitoring water depth in collection containers coupled to Campbell CR10 dataloggers. The system was tested on a 9-year-old broadleaf deciduous tree ( pear, Pyrus calleryana`Bradford') and an 8-yearold broadleaf evergreen tree (cork oak, Quercus suber) representing trees having divergent canopy distributions of foliage and stems. Partitioning of gross precipitation into throughfall, stem¯ow and canopy interception is presented for these two mature open-grown trees during the 1996±1998 rainy seasons. Interception losses accounted for about 15% of gross precipitation for the pear tree and 27% for the oak tree. The fraction of gross precipitation reaching the ground included 8% by stem¯ow and 77% by throughfall for the pear tree, as compared with 15% and 58%, respectively, for the oak tree. The analysis of temporal patterns in interception indicates that it was greatest at the beginning of each rainfall event. Rainfall frequency is more signi®cant than rainfall rate and duration in determining interception losses. Both stem¯ow and throughfall varied with rainfall intensity and wind speed. Increasing precipitation rates and wind speed increased stem¯ow but reduced throughfall. Analysis of rainfall interception processes at dierent time-scales indicates that canopy interception varied from 100% at the beginning of the rain event to about 3% at the maximum rain intensity for the oak tree. These values re¯ected the canopy surface water storage changes during the rain event. The winter domain precipitation at our study site in the Central Valley of California limited our opportunities to collect interception data during non-winter seasons. This precipitation pattern makes the results more speci®c to the Mediterranean climate region.

Links between severe hydrological droughts and weather types (WTs) were explored to improve the understanding of hydroclimatological processes involved in the development of regional hydrological drought in north-western Europe. A new Regional Drought Area Index (RDAI) was developed, using daily streamflow, to represent the drought-affected area. Daily RDAI series were created for two regions with homogeneous drought behaviour in Denmark and four regions in Great Britain. An objective version of the Hess-Brezowsky Grosswetterlagen yielding 29 WTs was used. Regional drought characteristics, including duration and frequency, were found to vary considerably between regions. However, in 1976 and 1996, all regions experienced severe events, and these years were found to be the most severe drought years across the study region as a whole. The hydrological response time (i.e. the time over which WTs influence drought development) was found to vary markedly (45-210 days) between regions according to basin storage properties. WT-frequency anomalies (FAs) before and during the onset of the five most severe droughts were identified for each region. The dominant drought-yielding WTs changed between regions and between events within each region. High-pressure systems centred over the respective region were most frequently associated with droughts as well as WTs with a northern (N, NE or NW) or a southern (S, SE or SW) airflow over the Danish and British regions. Five of the six WTs associated with drought for all regions represented a northern high-pressure system (i.e. over Great Britain, Fennoscandia or the Norwegian Sea). This article demonstrates (1) hydrological response time to be fundamental in moderating drought response to mesoscale climatic drivers and (2) severe hydrological droughts may be caused by a complex set of hydroclimatological processes that vary between regions and events.

A central objective of the LOIS Community Research Programme is to provide estimates of the land–ocean flux of suspended sediment from the LOIS study area. While high frequency (15-minute interval) suspended sediment concentration data are recorded within the programme at six tidal limit sites, infrequent (weekly–monthly) sampling undertaken by the Harmonized Monitoring (HM) Programme provides additional concentration data for 23 tidal limit sites in the LOIS study area. In order that these infrequent data can be used to generate sediment flux estimates, the accuracy and precision of 22 load estimation procedures are assessed in the context of sampling frequency (weekly, fortnightly and monthly) and basin scales representative of the LOIS study area. Sampling frequency is demonstrated to exert a significant influence on the precision of the individual procedures, with precision being inversely related to sampling frequency. Accuracy is less clearly influenced by sampling frequency, ...

Hedgerow is one of the most important rural landscapes in the world, especially in Europe. Knowledge about the hydrological role of hedgerows is useful in many fields of study, such as hydrological modelling and rural landscape management. The aim of this study was to investigate the impact of a hedgerow on rainfall distribution, soil-water potential gradient, lateral water transfer and water balance. A hillslope with a hedgerow perpendicular to the slope was monitored. To evaluate hedgerow rainfall interception, rainfall was measured (hourly, daily, and by rainfall event) both next to and up to 16 m upslope and 12 m downslope perpendicularly away from the hedgerow. The strongest correlation between rainfall next to the hedgerow and rainfall at more distant points was obtained using data measured per rainfall event. The average percentage of rainfall intercepted equalled 28% for the leafed period and 12% for the leafless period. The impact of the hedgerow on spatial rainfall distribution was related to distance from the hedgerow and rainfall amount. Annual distribution of soil-water potential showed that the hedgerow influenced it up to 9 m upslope and 6 m downslope, the area in which most of the hedgerow's roots were observed. The soil was driest at the end of summer, which delayed soil rewetting during autumn in areas surrounding the hedgerow. Annual groundwater dynamics exhibited three distinct periods due to temporal rainfall distribution and, especially at the end of summer, root-water uptake. In addition, the total potential gradient showed that unsaturated flow was directed towards the hedgerow in summer and autumn. These results indicate that at the local scale hedgerows influences (1) spatial rainfall distribution, (2) soil rewetting, and (3) groundwater recharge, often at distances well beyond the hedgerow's drip line. Consequently, the processes involved in soil-water dynamics around hedgerows should be integrated into relevant hydrological models, especially for catchments with a dense hedgerow network.

This study focuses on the determination of the Forchheimer equation coefficients a and b for non-Darcian flow in porous media. Original theoretical equations are evaluated and empirical relations are proposed based on an investigation of available data in the literature. The validity of these equations is checked using existing experimental data, and their accuracy versus existing approaches is studied. On the basis of this analysis, some insight into the physical background of the phenomenon is also provided. The dependence of the coefficients a and b on the Reynolds number is also detected, and potential future research areas, e.g. investigation of inertial effects for consolidated porous media, are pointed out.

In this study, an approach is presented for handling hydraulic uncertainties in the prediction of floodplain. Different factors affect river flood characteristics. Furthermore, the high changeability of flooding conditions leads to high variability of the inundation. River morphology is one ...

The main objective of this study was to identify the most vulnerable areas to soil erosion in the Lake Tana Basin, Blue Nile, Ethiopia using the Soil and Water Assessment Tool (SWAT), a physically based distributed hydrological model, and a Geographic Information System based decision support system that uses multi-criteria evaluation (MCE). The SWAT model was used to estimate the sediment yield within each sub-basin and identify the most sediment contributing areas in the basin. Using the MCE analysis, an attempt was made to combine a set of factors (land use, soil, slope and river layers) to make a decision according to the stated objective. On the basis of simulated SWAT, sediment yields greater than 30 tons/ha for each of the sub-basin area, 18Ð4% of the watershed was determined to be high erosion potential area. The MCE results indicated that 12-30Ð5% of the watershed is high erosion potential area. Both approaches show comparable watershed area with high soil erosion susceptibility. The output of this research can aid policy and decision makers in determining the soil erosion 'hot spots' and the relevant soil and water conservation measures.

Lake Tana's flow system is governed by four main components: the inflow from surrounding river catchments into the lake, the outflow at Bahir Dar through the Blue Nile, the direct rainfall on the lake and the direct evaporation from the lake. While recent studies applied simple pragmatic approaches to estimate runoff from ungauged catchments, here emphasis is placed on more advanced approaches based on regionalization and spatial proximity principles. In the regionalization approach, model parameters of the conceptual HBV (Hydrologiska Byråns Vattenbalansavdelning) rainfall-runoff modelling of gauged catchments are transferred to ungauged catchments to allow runoff simulation. Parameter transfer was attempted through regression, proximity procedures and catchment size. This yielded 42, 47 and 46%, respectively, of the total river inflow for the three procedures. Lake areal rainfall is estimated by interpolation of the rain gauges around the lake, open water evaporation is estimated by the Penman-combination equation while observed inflows and outflow data are directly used in the lake water balance. The water balance closure term was established by comparing the measured lake levels with the calculated levels. Results show that runoff from ungauged catchments is around 880 mm per year for the simulation period 1995-2001 with a water balance closure error of 5%. In addition, use is made of river and lake water chemistry to arrive at an estimate of the unknown inflow and outflow components through the mixing cell approach. The results obtained with this method also provide independent information with regard to the errors in the individual water balance components.

Crop coefficients were developed to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions. Starting in the 1980s, crop coefficients developed through lysimeter studies or set by expert opinion began to be supplemented by remotely sensed vegetation indices (VI) that measured the actual status of the crop on a field-by-field basis. VIs measure the density of green foliage based on the reflectance of visible and near infrared (NIR) light from the canopy, and are highly correlated with plant physiological processes that depend on light absorption by a canopy such as ET and photosynthesis. Reflectance-based crop coefficients have now been developed for numerous individual crops, including corn, wheat, alfalfa, cotton, potato, sugar beet, vegetables, grapes and orchard crops. Other research has shown that VIs can be used to predict ET over fields of mixed crops, allowing them to be used to monitor ET over entire irrigation districts. VI-based crop coefficients can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows instead of to a modeled crop growing under optimal conditions. Recently, the concept has been applied to natural ecosystems at the local, regional and continental scales of measurement, using time-series satellite data from the MODIS sensors on the Terra satellite. VIs or other visible-NIR band algorithms are combined with meteorological data to predict ET in numerous biome types, from deserts, to arctic tundra, to tropical rainforests. These methods often closely match ET measured on the ground at the global FluxNet array of eddy covariance moisture and carbon flux towers. The primary advantage of VI methods for estimating ET is that transpiration is closely related to radiation absorbed by the plant canopy, which is closely related to VIs. The primary disadvantage is that they cannot capture stress effects or soil evaporation.

Global synthesis of the findings from ¾140 recharge study areas in semiarid and arid regions provides important information on recharge rates, controls, and processes, which are critical for sustainable water development. Water resource evaluation, dryland salinity assessment (Australia), and radioactive waste disposal (US) are among the primary goals of many of these recharge studies. The chloride mass balance (CMB) technique is widely used to estimate recharge. Average recharge rates estimated over large areas (40-374 000 km 2 ) range from 0Ð2 to 35 mm year 1 , representing 0Ð1-5% of long-term average annual precipitation. Extreme local variability in recharge, with rates up to ¾720 m year 1 , results from focussed recharge beneath ephemeral streams and lakes and preferential flow mostly in fractured systems. System response to climate variability and land use/land cover (LU/LC) changes is archived in unsaturated zone tracer profiles and in groundwater level fluctuations. Inter-annual climate variability related to El Niño Southern Oscillation (ENSO) results in up to three times higher recharge in regions within the SW US during periods of frequent El Niños relative to periods dominated by La Niñas (1941)(1942)(1943)(1944)(1945)(1946)(1947)(1948)(1949)(1950)(1951)(1952)(1953)(1954)(1955)(1956)(1957). Enhanced recharge related to ENSO is also documented in Argentina. Climate variability at decadal to century scales recorded in chloride profiles in Africa results in recharge rates of 30 mm year 1 during the Sahel drought (1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986) to 150 mm year 1 during non-drought periods. Variations in climate at millennial scales in the SW US changed systems from recharge during the Pleistocene glacial period (½10 000 years ago) to discharge during the Holocene semiarid period. LU/LC changes such as deforestation in Australia increased recharge up to about 2 orders of magnitude. Changes from natural grassland and shrublands to dryland (rain-fed) agriculture altered systems from discharge (evapotranspiration, ET) to recharge in the SW US. The impact of LU change was much greater than climate variability in Niger (Africa), where replacement of savanna by crops increased recharge by about an order of magnitude even during severe droughts. Sensitivity of recharge to LU/LC changes suggests that recharge may be controlled through management of LU. In irrigated areas, recharge varies from 10 to 485 mm year 1 , representing 1-25% of irrigation plus precipitation. However, irrigation pumpage in groundwater-fed irrigated areas greatly exceeds recharge rates, resulting in groundwater mining. Increased recharge related to cultivation has mobilized salts that accumulated in the unsaturated zone over millennia, resulting in widespread groundwater and surface water contamination, particularly in Australia. The synthesis of recharge rates provided in this study contains valuable information for developing sustainable groundwater resource programmes within the context of climate variability and LU/LC change.

Every year the Ganges and Brahmaputra rivers in Bangladesh transport 316 and 721 million tonnes of sediment, respectively. These high loads of suspended sediment re¯ect the very high rate of denudation in their drainage basins. The average mechanical denudation rate for the Ganges and Brahmaputra basins together is 365 mm 10 3 yr À1 . However, the rate is higher in the Brahmaputra Basin than that in the Ganges Basin. Several factors, including mean trunk channel gradient, relief ratio, runo, basin lithology and recurring earthquakes are responsible for these high denudation rates. Of the total suspended sediment load (i.e. 1037 million tonnes) transported by these rivers, only 525 million tonnes (c. 51% of the total load) are delivered to the coastal area of Bangladesh and the remaining 512 million tonnes are deposited within the lower basin, osetting the subsidence. Of the deposited load, about 289 million tonnes (about 28% of the total load) are deposited on thē oodplains of these rivers. The remaining 223 million tonnes (about 21% of the total load) are deposited within the river channels, resulting in aggradation of the channel bed at an average rate of about 3 . 9 cm yr À1 . Although the Brahmaputra transports a higher sediment load than the Ganges, the channel bed aggradation rate is much higher for the Ganges. This study also documents a wide range of interannual, seasonal and daily variation in suspended sediment transport and water discharge. Interannual variation in sediment deposition within the basin is also suggested.

Water samples from the Yamuna and its tributaries, one of the major river systems draining the Himalaya, have been analysed for their stable oxygen and hydrogen isotopes during three seasons (summer, monsoon and post-monsoon). The data show clear seasonal and altitudinal variations; waters from higher altitudes and those collected during monsoon season are characterized by relatively depleted isotopic composition. Regression analysis of δD–δ18O data of samples collected during summer and monsoon seasons shows that the slope of the best-fit lines are nearly identical to those of precipitation at New Delhi for the same period. The similarity in their slopes suggests that the isotopic composition of precipitation contributing water to these rivers are reasonably well preserved in both monsoon and non-monsoon seasons, however, during the non-monsoon period both rainfall and river waters carry signatures of evaporation. The ‘deuterium excess’ in river waters during the three seasons though overlap with each other, the values during October are higher. This can be understood in terms of recycled moisture contributions to precipitation. The ‘altitude effect’ for δ18O in these waters is determined to be 0·11‰ per 100 m, a factor of about two less than that reported for the Ganga source waters from similar altitudinal range. The variability in altitude effects in rivers draining the Himalaya seems to be controlled by the ‘amount effect’ associated with the monsoon. The significant spatial variability in altitude effect in these river basins, which are a few hundred kilometers apart, suggests that reconstruction of palaeoelevation in the Himalaya, based on δ18O-altitude gradients, would depend critically on its proper assessment in the region. This study has established a relationship between total cation abundance and δ18O in waters of the Yamuna mainstream; total cations (corrected for cyclic components) double for a 1·4 km decrease in altitude as the Yamuna flows downstream. Copyright © 2002 John Wiley & Sons, Ltd.

Watershed scale hydrological and biogeochemical models rely on the correct spatial-temporal prediction of processes than SWAT-CN (Nash-Sutcliffe efficiencies (NSE) of 0.76 and 0.67, respectively). In the temperate Catskills, SWAT-WB and SWAT-CN predictions were approximately equivalent (NSE>0.5). Interestingly, and perhaps more importantly, the spatial distribution of runoff generating areas differed greatly between the two models, with SWAT-WB providing a more realistic distribution of saturated and thus runoff source areas. These results suggest that the addition of a water balance in SWAT significantly improves model predictions in monsoonal climates, and provides equally acceptable levels of accuracy in stream flow prediction under temperate northeastern USA conditions. Spatially distributed watershed areas are predicted realisticly with SWAT-WB

Research on stream and river temperatures is reviewed with particular attention being given to advances in understanding gained since 1990 and on investigations of fundamental controls on thermal behaviour, thermal heterogeneity at different spatial scales, the influence of human impacts and the nature of past and future trends. Copyright © 2008 John Wiley & Sons, Ltd.

Crop coefficients were developed to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions. Starting in the 1980s, crop coefficients developed through lysimeter studies or set by expert opinion began to be supplemented by remotely sensed vegetation indices (VI) that measured the actual status of the crop on a field-by-field basis. VIs measure the density of green foliage based on the reflectance of visible and near infrared (NIR) light from the canopy, and are highly correlated with plant physiological processes that depend on light absorption by a canopy such as ET and photosynthesis. Reflectance-based crop coefficients have now been developed for numerous individual crops, including corn, wheat, alfalfa, cotton, potato, sugar beet, vegetables, grapes and orchard crops. Other research has shown that VIs can be used to predict ET over fields of mixed crops, allowing them to be used to monitor ET over entire irrigation districts. VI-based crop coefficients can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows instead of to a modeled crop growing under optimal conditions. Recently, the concept has been applied to natural ecosystems at the local, regional and continental scales of measurement, using time-series satellite data from the MODIS sensors on the Terra satellite. VIs or other visible-NIR band algorithms are combined with meteorological data to predict ET in numerous biome types, from deserts, to arctic tundra, to tropical rainforests. These methods often closely match ET measured on the ground at the global FluxNet array of eddy covariance moisture and carbon flux towers. The primary advantage of VI methods for estimating ET is that transpiration is closely related to radiation absorbed by the plant canopy, which is closely related to VIs. The primary disadvantage is that they cannot capture stress effects or soil evaporation.

Lake Tana Basin is of significant importance to Ethiopia concerning water resources aspects and the ecological balance of the area. Many years of mismanagement, wetland losses due to urban encroachment and population growth, and droughts are causing its rapid deterioration. The main objective of this study was to assess the performance and applicability of the soil water assessment tool (SWAT) model for prediction of streamflow in the Lake Tana Basin, so that the influence of topography, land use, soil and climatic condition on the hydrology of Lake Tana Basin can be well examined. The physically based SWAT model was calibrated and validated for four tributaries of Lake Tana. Sequential uncertainty fitting (SUFI-2), parameter solution (ParaSol) and generalized likelihood uncertainty estimation (GLUE) calibration and uncertainty analysis methods were compared and used for the set-up of the SWAT model. The model evaluation statistics for streamflows prediction shows that there is a good agreement between the measured and simulated flows that was verified by coefficients of determination and Nash Sutcliffe efficiency greater than 0Ð5. The hydrological water balance analysis of the basin indicated that baseflow is an important component of the total discharge within the study area that contributes more than the surface runoff. More than 60% of losses in the watershed are through evapotranspiration.

The upstream-downstream sediment budget along the Napo River (100,520 km 2 , 6,300 m 3 s -1 ) was studied in the Andean Foothill of Ecuador, at the west of the Amazon basin. A comparative study was made, during 4 hydrological cycles (2001)(2002)(2003)(2004)(2005) for 3 hydrological stations located upstream; and during 1 hydrological cycle (2004)(2005) for the fourth one located near the mouth of the Napo River (region of Iquitos in Peru). This analysis showed an unusual increase in the concentration of suspended sediment recorded for the western part of the Amazon plain. These relatively high erosion rates for a floodplain basin (1,160 t km -2

Growing interest in the use of artificial neural networks (ANNs) in rainfall-runoff modelling has suggested certain issues that are still not addressed properly. One such concern is the use of network type, as theoretical studies on a multi-layer perceptron (MLP) with a sigmoid transfer function enlightens certain limitations for its use. Alternatively, there is a strong belief in the general ANN user community that a radial basis function (RBF) network performs better than an MLP, as the former bases its nonlinearities on the training data set. This argument is not yet substantiated by applications in hydrology. This paper presents a comprehensive evaluation of the performance of MLP-and RBF-type neural network models developed for rainfall-runoff modelling of two Indian river basins. The performance of both the MLP and RBF network models were comprehensively evaluated in terms of their generalization properties, predicted hydrograph characteristics, and predictive uncertainty. The results of the study indicate that the choice of the network type certainly has an impact on the model prediction accuracy. The study suggests that both the networks have merits and limitations. For instance, the MLP requires a long trial-and-error procedure to fix the optimal number of hidden nodes, whereas for an RBF the structure of the network can be fixed using an appropriate training algorithm. However, a judgment on which is superior is not clearly possible from this study.

The freshwater flux from the Greenland Ice Sheet (GrIS) to the ocean is of considerable importance to the global eustatic sea level rise. A physical modelling approach using SnowModel, a state-of-the-art snow-evolution modelling system that includes four submodels (MicroMet, EnBal, SnowPack,, was used to quantify the 1995-2007 GrIS surface massbalance (SMB), including freshwater flux. Meteorological observations from 26 meteorological stations located on the GrIS (Greenland Climate Network; GC-Net stations) and in coastal Greenland (Danish Meteorological Institute (DMI) WMOstations) were used as model inputs. The GrIS minimum surface melt extent of 29% occurred in 1996, while the greatest extent of 51% was present in 2007. The 2007 melt extent was 20% greater than the average for 1995-2006. The year 2007 had the highest GrIS surface runoff (523 km 3 y 1 ) and the lowest SMB ( 3 km 3 y 1 ); the only year with a negative GrIS SMB. Runoff in 2007 was approximately 35% greater than average for 1995-2006. From 1995 through 2007 overall,

Understanding the impacts of land-use changes on hydrology at the watershed scale can facilitate development of sustainable water resource strategies. This paper investigates the hydrological effects of land-use change in Zanjanrood basin, Iran. The water balance was simulated using the Soil and Water Assessment Tool (AVSWAT2000). Model calibration and uncertainty analysis were performed with sequential uncertainty fitting (SUFI-2). Simulation results from January 1998 to December 2002 were used for parameter calibration, and then the model was validated for the period of January 2003 to December 2004. The predicted monthly streamflow matched the observed values: during calibration the correlation coefficient was 0·86 and the Nash–Sutcliffe coefficient 0·79, compared with 0·80 and 0·79, respectively, during validation. The model was used to simulate the main components of the hydrological cycle, in order to study the effects of land-use changes in 1967, 1994 and 2007. The study reveals that during 1967 a 34·5% decrease of grassland with concurrent increases of shrubland (13·9%), rain-fed agriculture (12·1%), bare ground (5·5%) irrigated agriculture (2·2%), and urban area (0·7%) led to a 33% increase in the amount of surface runoff and a 22% decrease in the groundwater recharge. Furthermore, the area of sub-basins that was influenced by high runoff (14–28 mm) increased. The results indicate that the hydrological response to overgrazing and the replacing of rangelands (grassland and shrubland) with rain-fed agriculture and bare ground (badlands) is nonlinear and exhibits a threshold effect. The runoff rises dramatically when more than 60% of the rangeland is removed. For groundwater this threshold lies at an 80% decrease in rangeland. Copyright © 2009 John Wiley & Sons, Ltd.

The observed retreat of several Himalayan glaciers and snow packs is a cause of concern for the huge population in southern Asia that is dependent on the glacial-fed rivers emanating from Himalayas. There is considerable uncertainty about how cryospheric recession in the Himalayan region will respond to climate change, and how the water resource availability will be affected. As a first step towards quantifying the contribution of glacier-melt water, hydrograph separation of River Ganga at Rishikesh into its constituent components, namely (i) surface runoff, (ii) glacial ice-melt and (iii) groundwater discharge has been done in this paper. A three-component mixing model has been employed using the values of υ 18 O and electrical conductivity (EC) of the river water, and its constituents, to estimate the time-varying relative fraction of each component. The relative fraction of the surface runoff peaks (70-90%) during winter, due to the near-zero contribution of glacial ice-melt, essentially represents the melting of surface snow from the catchment. The contribution of glacial ice-melt to the stream discharge peaks during summer and monsoon reaches a maximum value of ¾40% with an average of 32%. The fraction of groundwater discharge varies within a narrow range (15 š 5%) throughout the year. On the basis of the variation in the d-excess values of river water, it is also suggested that the snow-melt and ice-melt component has a significant fraction derived from winter precipitation with moisture source from mid-latitude westerlies (also known as western disturbances).

The Soil Conservation Service curve number (CN) method is widely used for predicting direct runoff from rainfall. However, despite the extent of cultivation on hillslope areas, very few attempts have been made to incorporate a slope factor into the CN method. The objectives of this study were (1) to evaluate existing approaches integrating slope in the CN method, and (2) to develop an equation incorporating a slope factor into the CN method for application in the steep slope areas of the Loess Plateau of China. The dataset consisted of 11 years of rainfall and runoff measurements from two experimental sites with slopes ranging from 14 to 140%. The results indicated that the standard CN method underestimated large runoff events and overestimated small events. For our experimental conditions, the optimized and non-optimized forms of the slope-modified CN method of the Erosion Productivity Impact Calculator model improved runoff prediction for steep slopes, but large runoff events were still underestimated and small ones overpredicted. Based on relationships between slope and the observed and theoretical CN values, an equation was developed that better predicted runoff depths with an R2 of 0.822 and a linear regression slope of 0.807. This slope-adjusted CN equation appears to be the most appropriate for runoff prediction in the steep areas of the Loess Plateau of China.

Uncertainty in discharge data must be critically assessed before data can be used in, e.g. water resources estimation or hydrological modelling. In the alluvial Choluteca River in Honduras, the river-bed characteristics change over time as fill, scour and other processes occur in the channel, leading to a non-stationary stage-discharge relationship and difficulties in deriving consistent rating curves. Few studies have investigated the uncertainties related to non-stationarity in the stagedischarge relationship. We calculated discharge and the associated uncertainty with a weighted fuzzy regression of rating curves applied within a moving time window, based on estimated uncertainties in the observed rating data. An 18-yearlong dataset with unusually frequent ratings (1268 in total) was the basis of this study. A large temporal variability in the stage-discharge relationship was found especially for low flows. The time-variable rating curve resulted in discharge estimate differences of 60 to C90% for low flows and š20% for medium to high flows when compared to a constant rating curve. The final estimated uncertainty in discharge was substantial and the uncertainty limits varied between 43 to C73% of the best discharge estimate.

During the previous decade, the south-western United States has faced declining water resources and escalating forest fires due to long-term regional drought. Competing demands for water resources require a careful accounting of the basin water budget. Water lost to the atmosphere through riparian evapotranspiration (ET) is believed to rank in the top third of water budget depletions. To better manage depletions in a large river system, patterns of riparian ET must be better understood. This paper provides a general overview of the ecological, hydrological, and atmospheric issues surrounding riparian ET in the Middle Rio Grande (MRG) of New Mexico. Long-term measurements of ET, water table depth, and micro-meteorological conditions have been made at sites dominated by native cottonwood (Populus deltoides) forests and non-native saltcedar (Tamarix chinensis) thickets along the MRG. Over periods longer than one week, groundwater and leaf area index (LAI) dynamics relate well with ET rates. Evapotranspiration from P. deltoides forests was unaffected by annual drought conditions in much of the MRG where the water table is maintained within 3 m of the surface. Evapotranspiration from a dense Tamarix chinensis thicket did not decline with increasing groundwater depth; instead, ET increased by 50%, from 6 mm/day to 9 mm/day, as the water table receded at nearly 7 cm/day. Leaf area index of the T. chinensis thicket, likewise, increased during groundwater decline. Leaf area index can be manipulated as well following removal of non-native species. When T. chinensis and non-native Russian olive (Elaeagnus angustifolia) were removed from a P. deltoides understory, water salvaged through reduced ET was 26 cm/yr in relation to ET measured at reference sites. To investigate correlates to short-term variations in ET, stepwise multiple linear regression was used to evaluate atmospheric conditions under which ET is elevated or depressed. At the P. deltoides-dominated sites, ET anomalies were positively correlated to net radiation (R n ) and negatively correlated to sensible heat flux (H), cross-corridor wind speed (v), and along-corridor wind speed (u) (r 2 D 0Ð54). At the T. chinensis-dominated sites, ET anomalies were positively correlated with R n , u, the friction coefficient (u Ł ), and vapour pressure deficit (VPD) and were negatively correlated to surface humidity scale q Ł , daily high and low temperature, H, and precipitation r 2 D 0Ð66 . Both Tamarix and Populus can transpire prodigious quantities of water when conditions are favourable. In the MRG, T. chinensis is preferentially found where summer flooding and cold air drainage occurs, and P. deltoides is preferentially located in areas with shallow groundwater within 2 m of the surface.

Many water quality models use some form of the curve number (CN) equation developed by the Soil Conservation Service (SCS; U.S. Depart of Agriculture) to predict storm runoff from watersheds based on an infiltration-excess response to rainfall. However, in humid, well-vegetated areas with shallow soils, such as in the northeastern USA, the predominant runoff generating mechanism is saturation-excess on variable source areas (VSAs). We reconceptualized the SCS-CN equation for VSAs, and incorporated it into the General Watershed Loading Function (GWLF) model. The new version of GWLF, named the Variable Source Loading Function (VSLF) model, simulates the watershed runoff response to rainfall using the standard SCS-CN equation, but spatially distributes the runoff response according to a soil wetness index. We spatially validated VSLF runoff predictions and compared VSLF to GWLF for a subwatershed of the New York City Water Supply System. The spatial distribution of runoff from VSLF is more physically realistic than the estimates from GWLF. This has important consequences for water quality modeling, and for the use of models to evaluate and guide watershed management, because correctly predicting the coincidence of runoff generation and pollutant sources is critical to simulating non-point source (NPS) pollution transported by runoff.

Lake Tana Basin is of significant importance to Ethiopia concerning water resources aspects and the ecological balance of the area. Many years of mismanagement, wetland losses due to urban encroachment and population growth, and droughts are causing its rapid deterioration. The main objective of this study was to assess the performance and applicability of the soil water assessment tool (SWAT) model for prediction of streamflow in the Lake Tana Basin, so that the influence of topography, land use, soil and climatic condition on the hydrology of Lake Tana Basin can be well examined. The physically based SWAT model was calibrated and validated for four tributaries of Lake Tana. Sequential uncertainty fitting (SUFI-2), parameter solution (ParaSol) and generalized likelihood uncertainty estimation (GLUE) calibration and uncertainty analysis methods were compared and used for the set-up of the SWAT model. The model evaluation statistics for streamflows prediction shows that there is a good agreement between the measured and simulated flows that was verified by coefficients of determination and Nash Sutcliffe efficiency greater than 0Ð5. The hydrological water balance analysis of the basin indicated that baseflow is an important component of the total discharge within the study area that contributes more than the surface runoff. More than 60% of losses in the watershed are through evapotranspiration.

A method for estimation of mean baseflow residence time in watersheds from hydrograph runoff recession characteristics was developed. Runoff recession characteristics were computed for the period 1993-96 in the 2 km 2 Winnisook watershed, Catskill Mountains, southeastern New York, and were used to derive mean values of subsurface hydraulic conductivity and the storage coefficient. These values were then used to estimate the mean baseflow residence time from an expression of the soil contact time, based on watershed soil and topographic characteristics. For comparison, mean baseflow residence times were calculated for the same period of time through the traditional convolution integral approach, which relates rainfall d 18 O to d 18 O values in streamflow. Our computed mean baseflow residence time was 9 months by both methods. These results indicate that baseflow residence time can be calculated accurately using recession analysis, and the method is less expensive than using environmental and/or artificial tracers. Published in

Rainfall and its consequences are usually considered as the major factor triggering slope instabilities within clay deposits. The link between rainfall, water infiltration and landslide activity may be complex and comprehensive sets of data are still necessary to understand how water infiltrates in cohesive material. On unstable slopes made of fine-grained sediments, gravitational deformation generates superficial shear ruptures and tension fissures whose continuous opening could be amplified by shrinkage during dry periods. This fissure network at the surface can be very dense and strongly controls the water infiltration process. However, it remains difficult to assess the fissure evolution at depth and its eventual relationship with shallow slip surfaces. This work presents the results of a geophysical study which aimed to characterize the fissures that are observed along the unstable clay slopes of the Trièves area (French Alps). The site was first investigated combining geomorphological analysis, drilling, borehole logging, geotechnical tests and geophysical experiments. This investigation evidenced the presence of at least three rupture surfaces (at 5 m, 10 to 15 m and 42 m) with a water flow at the second one. In a second step, permanent instruments were installed (piezometers and soil water content (SWC) probes). Results indicate high water infiltration velocities, down to the water table located at 2.5 m depth, that are likely to be linked with sub-vertical preferential paths, such as fissures. Geophysical parameters (electrical resistivity, S-wave velocities, Rayleigh waves attenuation) were monitored. Geophysical results suggest that fissures are permanently open, that they serve as preferential infiltration paths, and that they can reach, at least, a depth of 2 m. The methods employed in this research put forward that the system of imbricated fissures drains water from the surface down to the shallow slip surfaces at 5 m and 10-5 m. This work highlights the role of fissures network in water infiltration in the numerous clay landslides of the Trièves area.

In order to quantify total error affecting hydrological models and predictions, we must explicitly recognize errors in input data, model structure, model parameters and validation data. This paper tackles the last of these: errors in discharge measurements used to calibrate a rainfall-runoff model, caused by stage-discharge rating-curve uncertainty. This uncertainty may be due to several combined sources, including errors in stage and velocity measurements during individual gaugings, assumptions regarding a particular form of stage-discharge relationship, extrapolation of the stage-discharge relationship beyond the maximum gauging, and cross-section change due to vegetation growth and/or bed movement. A methodology is presented to systematically assess and quantify the uncertainty in discharge measurements due to all of these sources. For a given stage measurement, a complete PDF of true discharge is estimated. Consequently, new model calibration techniques can be introduced to explicitly account for the discharge error distribution. The method is demonstrated for a gravel-bed river in New Zealand, where all the above uncertainty sources can be identified, including significant uncertainty in cross-section form due to scour and re-deposition of sediment. Results show that rigorous consideration of uncertainty in flow data results in significant improvement of the model's ability to predict the observed flow.

In situ soil moisture data from the Bibeschbach experimental catchment in Luxembourg are used to evaluate relative surface soil moisture observed with the MetOp-A Advanced Scatterometer (ASCAT). Filtered and bias-corrected surface soil wetness indices (SWIs) derived from coarse-resolution (25 km) C-band scatterometer observations are shown to be highly correlated (r = 0.86) with catchment-averaged soil moisture measured in the field. The combination of ASCAT and ENVISAT Advanced Synthetic Aperture Radar (ASAR) data sets yields high-resolution (1 km) relative surface soil moisture that is equally well correlated with in situ measurements. It is concluded that for soil moisture monitoring applications at a catchment scale, the two soil moisture products are equivalent. The best correlation between the SWI derived from ASCAT and ASCAT-ASAR with in situ soil moisture observations at ca. 5 cm depth is obtained with a characteristic time length parameter T equal to 288 h. These results suggest that satellite-derived surface soil wetness may serve as proxy for soil storage that enables the monitoring of abrupt switches in river system dynamics to appear when an effective field capacity is exceeded and rapid subsurface stormflow is initiated. In catchments where soil moisture is the main controlling factor of rapid subsurface flow, MetOp ASCAT-derived SWI has the potential to monitor how a river system approaches a critical threshold.

Wetlands form 14% of the Canadian landscape and, consequently, have considerable interaction with the hydrological resources, including water quantity and quality, both within and downstream of them. Most of these are peatlands, particularly in boreal and northern environments, and these have been well researched recently. New data also exist for mineral wetlands (e.g. prairie sloughs). Relatively little attention has been given to coastal wetlands, or the complicated systems in the Western Cordillera. This paper reviews the current studies in Canada.

The aim of this paper is to improve the communication of the probabilistic flood forecasts generated by Hydrological Ensemble Prediction Systems (HEPS) by understanding perceptions of different methods of visualising probabilistic forecast information. This study focuses on inter-expert communication, and accounts for differences in visualisation requirements based on the information content necessary for individual users. The perceptions of the expert group addressed in this study are important because they are the designers and primary users of existing HEPS. Nevertheless, they have sometimes resisted the release of uncertainty information to the general public because of doubts about whether it can be successfully communicated in ways that would be readily understood to nonexperts. In this paper we explore the strengths and weaknesses of existing HEPS visualisation methods and thereby formulate some wider recommendations about best practice for HEPS visualisation and communication. We suggest that specific training on probabilistic forecasting would foster use of probabilistic forecasts with a wider range of applications. The result of a case study exercise showed that there is no overarching agreement between experts on how to display probabilistic forecasts and what they consider the essential information that should accompany plots and diagrams. In this paper we propose a list of minimum properties that, if consistently displayed with probabilistic forecasts, would make the products more easily understandable.

Methods for estimating runoff that have been developed for temperate climates may not be suitable for use in the monsoonal climates of Africa, where there is a distinct dry season in which soils dry out to a considerable depth. This has a distinct effect on runoff generation that is not captured by "the temperate climate" models.

A geomorphological instantaneous unit hydrograph (GIUH) is derived from the geomorphological characteristics of a catchment and it is related to the parameters of the Clark instantaneous unit hydrograph (IUH) model as well as the Nash IUH model for deriving its complete shape. The developed GIUH based Clark and Nash models are applied for simulation of the direct surface run-off (DSRO) hydrographs for ten rainfall-runoff events of the Ajay catchment up to the Sarath gauging site of eastern India. The geomorphological characteristics of the Ajay catchment are evaluated using the GIS package, Integrated Land and Water Information System (ILWIS). The performances of the GIUH based Clark and Nash models in simulating the DSRO hydrographs are compared with the Clark IUH model option of HEC-1 package and the Nash IUH model, using some commonly used objective functions. The DSRO hydrographs are computed with reasonable accuracy by the GIUH based Clark and Nash models, which simulate the DSRO hydrographs of the catchment considering it to be ungauged. Inter comparison of the performances of the GIUH based Clark and Nash models shows that the DSRO hydrographs are estimated with comparable accuracy by both the models. Copyright © 2007 John Wiley & Sons, Ltd.