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Abstract. In the context of changing climate and increasing waterdemand, large-scale hydrological models are helpful for understanding andprojecting future water resources across scales. Groundwater is a criticalfreshwater resource and strongly controls river flow throughout the year. Itis also essential for ecosystems and contributes to evapotranspiration,resulting in climate feedback. However, groundwater systems worldwide arequite diverse, including thick multilayer aquifers and thin heterogeneousaquifers. Recently, efforts have been made to improve the representation ofgroundwater systems in large-scale hydrological models. The evaluation ofthe accuracy of these model outputs is challenging because (1) they areapplied at much coarser resolutions than hillslope scale, (2) they simplifygeological structures generally known at local scale, and (3) they do notadequately include local water management practices (mainly groundwaterpumping). Here, we apply a large-scale hydrological model (CWatM), coupledwith the groundwater flow model MODFLOW, in two different climatic,geological, and socioeconomic regions: the Seewinkel area (Austria) and theBhima basin (India). The coupled model enables simulation of the impact ofthe water table on groundwater–soil and groundwater–river exchanges,groundwater recharge through leaking canals, and groundwater pumping. Thisregional-scale analysis enables assessment of the model's ability tosimulate water tables at fine spatial resolutions (1 km for CWatM, 100–250 m for MODFLOW) and when groundwater pumping is well estimated. Evaluatinglarge-scale models remains challenging, but the results show that thereproduction of (1) average water table fluctuations and (2) water tabledepths without bias can be a benchmark objective of such models. We foundthat grid resolution is the main factor that affects water table depth biasbecause it smooths river incision, while pumping affects time fluctuations.Finally, we use the model to assess the impact of groundwater-basedirrigation pumping on evapotranspiration, groundwater recharge, and watertable observations from boreholes. 

Abstract. We develop a new large-scale hydrological and water resources model, theCommunity Water Model (CWatM), which can simulate hydrology both globallyand regionally at different resolutions from 30 arcmin to 30 arcsec atdaily time steps. CWatM is open source in the Python programming environmentand has a modular structure. It uses global, freely available data in thenetCDF4 file format for reading, storage, and production of data in acompact way. CWatM includes general surface and groundwater hydrologicalprocesses but also takes into account human activities, such as water useand reservoir regulation, by calculating water demands, water use, andreturn flows. Reservoirs and lakes are included in the model scheme. CWatMis used in the framework of the Inter-Sectoral Impact Model IntercomparisonProject (ISIMIP), which compares global model outputs. The flexible modelstructure allows for dynamic interaction with hydro-economic and water qualitymodels for the assessment and evaluation of water management options.Furthermore, the novelty of CWatM is its combination of state-of-the-arthydrological modeling, modular programming, an online user manual andautomatic source code documentation, global and regional assessments atdifferent spatial resolutions, and a potential community to add to, change,and expand the open-source project. CWatM also strives to build a communitylearning environment which is able to freely use an open-source hydrologicalmodel and flexible coupling possibilities to other sectoral models, such asenergy and agriculture.