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High-resolution large-scale predictions of hydrologic states and fluxes are important for many multi-scale applications including water resource management. However, many of the existing global to continental scale hydrological models are applied at coarse resolution and or neglect lateral surface and groundwater flow, thereby not capturing smaller scale hydrologic processes. Applications of high-resolution and more complex models are often limited to watershed scales, neglecting the mesoscale climate effects on the water cycle. We implemented an integrated, physically-based coupled land surface groundwater model; Parflow-CLM version 3.6.0, over a pan-European model domain at 0.0275o (~ 3 km) resolution. The model simulates three-dimensional variably saturated groundwater flow solving Richards equation and overland flow with a twodimensional kinematic wave approximation, which is fully integrated with land surface exchange processes. A comprehensive evaluation of hydrologic states and fluxes, resulting from a 10 year (1997-2006) model simulation, was performed using in10 situ and remote sensing observations including discharge, surface soil moisture (SM), evapotranspiration (ET), snow water equivalent and water table depth. Overall, the uncalibrated PF-CLM-EU3km model shows good agreement in simulating river discharge for 176 gauging stations across Europe. Comparison with satellite-based datasets of SM shows that PF-CLM-EU3km performs well in semi-arid and arid regions, but simulates overall higher SM in humid and cold regions. Comparisons with reanalysis and remotely sensed ET datasets (GLEAM and GLASS) show no significant differences, both, across the European domain (on average the difference is -0.09 and 0.30 mm d-1 for GLEAM and GLASS products, respectively), and within regions (R > 0.9). The large-scale high-resolution setup forms a basis for future studies, demonstrating the added value of capturing heterogeneities for improved water and energy flux simulations in physically-based fully distributed hydrologic models over very large model domains. This study also provides an evaluation reference for climate change impact projections and a climatology for hydrological forecasting, considering the effects of lateral surface and groundwater flows. [ABSTRACT FROM AUTHOR]

Accurate and reliable hydrologic simulations are important for many applications such as water resources management, future water availability projections and predictions of extreme events. However, the accuracy of water balance estimates is limited by the lack of observations at large scales and the uncertainties of model simulations due to errors in model structure and inputs (e.g. hydrologic parameters and atmospheric forcings). In this study, we assimilated ESA CCI soil moisture (SM) information to improve the estimation of continental-scale soil moisture and runoff. The assimilation experiment was conducted over a time period from 2000 to 2006 with the Community Land Model, version 3.5 (CLM3.5) integrated with the Parallel Data Assimilation Framework (PDAF) at spatial resolution of 0.0275° (∼ 3 km) over Europe. The model was forced with the high-resolution reanalysis COSMO-REA6 from the Hans-Ertel Centre for Weather Research (HErZ). Our results show that estimates of soil moisture have improved, particularly in the summer and autumn seasons when cross-validated with independent CCI-SM observations. On average, the mean bias in soil moisture was reduced from 0.1 mm3/mm3 in open-loop simulations to 0.004 mm3/mm3 with SM assimilation. The assimilation experiment also shows overall improvements in runoff, particularly during peak runoff. The results demonstrate the potential of assimilating satellite soil moisture observations to improve high-resolution soil moisture and runoff simulations at the continental scale, which is useful for water resources assessment and monitoring. [ABSTRACT FROM AUTHOR]

In many partially glacierized watersheds glacier recession driven by a warming climate could lead to complex patterns of streamflow response over time, often marked with rapid increases followed by sharp declines, depending on initial glacier ice cover and rate of climate change. Capturing such ''phases'' of hydrologic response is critical in regions where communities rely on glacier meltwater, particularly during low flows. In this paper, we investigate glacio-hydrologic response in the headwaters of the Zongo River, Bolivia, under climate change using a distributed glacio-hydrological model over the period of 1987-2100. Model predictions are evaluated through comparisons with satellite-derived glacier extent estimates, glacier surface velocity, in situ glacier mass balance, surface energy flux, and stream discharge measurements. Historically (1987-2010) modeled glacier melt accounts for 27% of annual runoff, and 61% of dry season (JJA) runoff on average. During this period the relative glacier cover was observed to decline from 35 to 21% of the watershed. In the future, annual and dry season discharge is projected to decrease by 4% and 27% by midcentury and 25% and 57% by the end of the century, respectively, following the loss of 81% of the ice in the watershed. Modeled runoff patterns evolve through the interplay of positive and negative trends in glacier melt and increased evapotranspiration as the climate warms. Sensitivity analyses demonstrate that the selection of model surface energy balance parameters greatly influences the trajectory of hydrological change projected during the first half of the 21st century. These model results underscore the importance of coupled glacio-hydrology modeling. [ABSTRACT FROM AUTHOR]

Large-scale Hydrological Models (LHMs) are increasingly used in studies concerning the water cycle at synoptic, continent, and global scales. These models are not constrained to reproduce hydrological processes and streamflow at the local scale, i.e. the scale at which water resources are managed.In the present study, we evaluated suitable modifications of the Community Land Model v3.5 (CLM3.5), which improve streamflow reproduction in a relatively large Alpine catchment: the Adige river basin, south-eastern Alps with a drainage area of about 10500 km2 at the Trento gauging station. In particular, we added to CLM3.5 a simple module dealing with deep infiltration to aquifers and return flow, and replaced the grid based routing scheme with the multi-scale grid independent scheme embedded in the HYPERstreamHS hydrological model. The performance of a calibrated run of HYPERstreamHS stand-alone model in reproducing observed streamflow at the Trento gauging station was also evaluated to provide a benchmark for assessing the effectiveness of the introduced parameterizations of deep infiltration, return flow and streamflow routing. Note, all simulation runs were informed by the same meteorological forcing.Runoff computed by CLM3.5 with the HYPERstream routing scheme improved the performances of the Nash Sutcliffe Efficiency (NSE) from -0.05 (uncalibrated CLM3.5 run) to 0.15. Furthermore, results indicate that, at least in the case of the Alpine watershed considered in this study, low NSE values are due to an underestimation of the water recharging deep aquifers and not re-entering the river network in CLM3.5. In addition, simulations using HYPERstreamHS informed by actual evapotranspiration outputs as provided by CLM3.5 showed a good agreement with observations (NSE = 0.61), which is very close to the performance obtained with the stand-alone run of HYPERstreamHS (NSE=0.65), thus suggesting that the land surface energy and mass balance is correctly simulated in CLM3.5 model also at the local scale.Results of this study suggest that a significant improvement of the performance of CLM3.5 in reproducing observed streamflow time series at the local scale may be achieved by replacing the grid based routing scheme with a more accurate scale-independent routing scheme and adding a simple parametrization of deep infiltration, with the latter producing the most significant improvement. [ABSTRACT FROM AUTHOR]