Abstract. We develop a new large-scale hydrological and water resources model, the Community Water Model (CWatM), which can simulate hydrology both globally and regionally at different resolutions from 30 arcmin to 30 arcsec at daily time steps. CWatM is open source in the Python programming environment and has a modular structure. It uses global, freely available data in the netCDF4 file format for reading, storage, and production of data in a compact way. CWatM includes general surface and groundwater hydrological processes but also takes into account human activities, such as water use and reservoir regulation, by calculating water demands, water use, and return flows. Reservoirs and lakes are included in the model scheme. CWatM is used in the framework of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP), which compares global model outputs. The flexible model structure allows for dynamic interaction with hydro-economic and water quality models for the assessment and evaluation of water management options. Furthermore, the novelty of CWatM is its combination of state-of-the-art hydrological modeling, modular programming, an online user manual and automatic source code documentation, global and regional assessments at different spatial resolutions, and a potential community to add to, change, and expand the open-source project. CWatM also strives to build a community learning environment which is able to freely use an open-source hydrological model and flexible coupling possibilities to other sectoral models, such as energy and agriculture.

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We develop a new large-scale hydrological and water resources model, the Community Water Model (CWatM), which can simulate hydrology both globally and regionally at different resolutions from 30 arc min to 30 arc sec at daily time steps.CWatM is open-source in the Python programming environment and has a modular structure. It uses global, freely available data in the netCDF4 file format for reading, storage, and production of data in a compact way. CWatM includes general surface 15 and groundwater hydrological processes, but also takes into account human activities, such as water use and reservoir regulation, by calculating water demands, water use, and return flows. Reservoirs and lakes are included in the model scheme.CWatM is used in the framework of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP), which compares global model outputs. The flexible model structure allows dynamic interaction with hydro-economic and water quality models for the assessment and evaluation of water management options. Furthermore, the novelty of CWatM is its combination of state-of 20 the-art hydrological modeling, modular programming, an online user manual and automatic source code documentation, global and regional assessments at different spatial resolutions, and a potential community to add to, change, and expand the opensource project. CWatM also strives to build a community learning environment which is able to freely use an open-source hydrological model and flexible coupling possibilities to other sectoral models, such as energy and agriculture.PCR-GLOBWB Wada et al., 2014, Sutanudjaja et al., 2018, }, SAFRAN-ISBA-MODCOU (Habets et 30

Growing populations and dietary shifts to include higher proportions of meat are projected to double global food demand by 2050. Previous global studies have proposed and evaluated possible solutions by closing agricultural yield gaps, defined as the difference between current and potential crop yields. We compliment previous studies by developing a method for more accurately calculating potential changes in cereal grain production under different irrigation scenarios, explicitly incorporating yield differences associated with different sources of irrigation. Irrigating with groundwater often leads to higher crop yields than irrigating with surface water because of the greater facility to tailor both the volumes of water and the timing of application. Two possible scenarios for increasing production in India are examined, the first where all non-irrigated fields are irrigated proportionally to the State-specific distribution of irrigation sources, and the second where all non-irrigated fields are irrigated with groundwater: Rice production increases by 14 and 25 % in scenarios 1 and 2 respectively, but wheat production increases by only 3 % in both scenarios. Increased irrigation water consumption from irrigating fields that are currently non-irrigated is estimated at 31 % for rice and 3 % for wheat using the Global Crop Water Model. A third scenario estimates the potential loss in production without the use of irrigation: rice would be 75 % and wheat 51 % of current production. Our methodology and results can help policy makers estimate the current and potential contribution of irrigation sources to agricultural production and food security in India and can with facility be applied elsewhere.

Abstract. The Global Runoff Data Centre provides time series of observed discharges that are valuable for calibrating and validating the results of hydrological models. We address a common issue in large-scale hydrology that has not been satisfactorily solved, though investigated several times. To compare simulated and observed discharge, grid-based hydrological models must fit reported station locations to the resolution-dependent gridded river network. We introduce an Intersection over Union ratio approach to selected station locations on a coarser grid-scale, reducing the errors in assigning stations to the correct upstream basin. We update the 10-year-old database of watershed boundaries with additional stations based on a high-resolution (3 arc seconds) river network and provide source codes and high- and low-resolution watershed boundaries. The dataset is stored on Zenodo with the associated DOI https://doi.org/10.5281/zenodo.6906577.