Terrestrial water storage (TWS) strongly modulates the hydrological cycle and is a key determinant of water availability and an indicator of drought. While historical TWS variations have been studied, future changes in TWS and the linkages to droughts remain unexamined. Here, using ensemble hydrological simulations, we show that climate change could reduce TWS in many regions, especially in the southern hemisphere. A strong inter-ensemble agreement indicates high confidence in the projected changes that are driven primarily by climate forcing, rather than land-water management activities. Declines in TWS translate to increase in future droughts. By the late-21 st century global land area and population in extreme-to-exceptional TWS drought could more than double, each increasing from 3% during 1976-2005 to 7% and 8%, respectively. Our findings highlight the importance of climate change mitigation to avoid adverse impacts on TWS and related droughts, and the need for adaptation to improve water resource management. TWS-the sum of continental water stored in canopies, snow and ice, rivers, lakes and 51 reservoirs, wetlands, soil, and groundwater-is a critical component of the global water and energy budget. It plays key roles in determining water resource availability 1 and modulating water flux interactions among various Earth system components 2 . Further, observed changes in TWS are inherently linked to droughts 2-6 , floods 7 , and global sea level change [8][9][10][11] . Despite such importance, global TWS remains less studied relative to hydrological fluxes (e.g., river discharge, evapotranspiration, and groundwater flow) owing to the lack of large-scale observations and challenges in explicitly resolving all TWS components in hydrological modeling 12 . This generally holds true for historical analyses; crucially, no study has to date examined the potential impacts of future climate change on global TWS. Recent modeling advancements 13 have improved the representation of TWS in global hydrological models 14,15 (GHMs) and land surface models 12 (LSMs). The Gravity Recovery and Climate Experiment (GRACE) satellite mission provided added opportunities to improve and validate TWS simulations in these models. GRACE TWS data and model simulations, often in combination, have been used for wide ranging applications including the assessment of water resources and impacts of human activities on the water cycle 14,16 , quantifying aquifer depletion 12,14,[17][18][19] , monitoring drought [3][4][5][6]20 , and assessing flood potential 7 . These studies have advanced the understanding of global TWS systems that are continually changing under natural hydro-climatic variability and accelerating human land-water management activities, but the 70 focus has been on historical variabilities in TWS. Further, future projections from general 71 circulation models (GCMs) have been used to quantify climate change impacts on hydrological 72 fluxes [21][22][23] and storages, but the projections of storages are limited to a subset of T...

Anticipation and preparedness for large-scale flood events have a key role in mitigating their impact and optimizing the strategic planning of water resources. Although several developed countries have well-established systems for river monitoring and flood early warning, figures of population affected every year by floods in developing countries are unsettling. This paper presents the Global Flood Awareness System, which has been set up to provide an overview on upcoming floods in large world river basins. The Global Flood Awareness System is based on distributed hydrological simulation of numerical ensemble weather predictions with global coverage. Streamflow forecasts are compared statistically to climatological simulations to detect probabilistic exceedance of warning thresholds. In this article, the system setup is described, together with an evaluation of its performance over a two-year test period and a qualitative analysis of a case study for the Pakistan flood, in summer 2010. It is shown that hazardous events in large river basins can be skilfully detected with a forecast horizon of up to 1 month. In addition, results suggest that an accurate simulation of initial model conditions and an improved parameterization of the hydrological model are key components to reproduce accurately the streamflow variability in the many different runoff regimes of the Earth

Groundwater (GW) overexploitation is a critical issue in North China with large GW level declines resulting in urban water scarcity, unsustainable agricultural production, and adverse ecological impacts. One approach to addressing GW depletion was to transport water from the humid south. However, impacts of water diversion on GW remained largely unknown. Here, we show impacts of the central South-to-North Water Diversion on GW storage recovery in Beijing within the context of climate variability and other policies. Water diverted to Beijing reduces cumulative GW depletion by~3.6 km 3 , accounting for 40% of total GW storage recovery during 2006-2018. Increased precipitation contributes similar volumes to GW storage recovery of~2.7 km 3 (30%) along with policies on reduced irrigation (~2.8 km 3 , 30%). This recovery is projected to continue in the coming decade. Engineering approaches, such as water diversions, will increasingly be required to move towards sustainable water management.

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.

Abstract. EURO-CORDEX, a new generation of downscaled climate projections, has become available for climate change impact studies in Europe. New opportunities arise in the investigation of potential effects of a warmer world on meteorological and hydrological extremes at regional scales. In this work, an ensemble of EURO-CORDEX RCP 8.5 scenarios is used to drive a distributed hydrological model and assess the projected changes in flood hazard in Europe through the current century. Changes in magnitude and frequency of extreme streamflow events are investigated by statistical distribution fitting and peak over threshold analysis. A consistent method is proposed to evaluate the agreement of ensemble projections. Results indicate that the change in frequency of discharge extremes is likely to have a larger impact on the overall flood hazard as compared to the change in their magnitude. On average in Europe, flood peaks with return period above 100 years are projected to double in frequency within three decades.