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1. Global groundwater droughts are more severe than they appear in hydrological models: An investigation through a Bayesian merging of GRACE and GRACE-FO data with a water balance model

   https://doi.org/10.1016/j.scitotenv.2023.169476  

   Forootan, Ehsan; Mehrnegar, Nooshin; Schumacher, Maike; Schiettekatte, Leire Anne; Jagdhuber, Thomas; Farzaneh, Saeed; van Dijk, Albert I.J.M.; Shamsudduha, Mohammad; Shum, C.K. (February 2024, Science of The Total Environment)

   Zhang, Kai (Ed.)

   Realistic representation of hydrological drought events is increasingly important in world facing decreased freshwater availability. Index-based drought monitoring systems are often adopted to represent the evolution and distribution of hydrological droughts, which mainly rely on hydrological model simulations to compute these indices. Recent studies, however, indicate that model derived water storage estimates might have difficulties in adequately representing reality. Here, a novel Markov Chain Monte Carlo - Data Assimilation (MCMC-DA) approach is implemented to merge global Terrestrial Water Storage (TWS) changes from the Gravity Recovery And Climate Experiment (GRACE) and its Follow On mission (GRACE-FO) with the water storage estimations derived from the W3RA water balance model. The modified MCMC-DA derived summation of deep rooted soil and groundwater storage estimates is then used to compute 0.5∘ standardized groundwater drought indices globally to show the impact of GRACE/GRACE-FO DA on a global index-based hydrological drought monitoring system. Our numerical assessment covers the period of 2003–2021, and shows that integrating GRACE/GRACE-FO data modifies the seasonality and inter-annual trends of water storage estimations. Considerable increases in the length and severity of extreme droughts are found in basins that exhibited multiyear water storage fluctuations and those affected by climate teleconnections. 

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2. Global patterns of water storage in the rooting zones of vegetation

   https://doi.org/10.1038/s41561-023-01125-2  

   Stocker, Benjamin_D; Tumber-Dávila, Shersingh_Joseph; Konings, Alexandra_G; Anderson, Martha_C; Hain, Christopher; Jackson, Robert_B (February 2023, Nature Geoscience)

   Abstract The rooting-zone water-storage capacity—the amount of water accessible to plants—controls the sensitivity of land–atmosphere exchange of water and carbon during dry periods. How the rooting-zone water-storage capacity varies spatially is largely unknown and not directly observable. Here we estimate rooting-zone water-storage capacity globally from the relationship between remotely sensed vegetation activity, measured by combining evapotranspiration, sun-induced fluorescence and radiation estimates, and the cumulative water deficit calculated from daily time series of precipitation and evapotranspiration. Our findings indicate plant-available water stores that exceed the storage capacity of 2-m-deep soils across 37% of Earth’s vegetated surface. We find that biome-level variations of rooting-zone water-storage capacities correlate with observed rooting-zone depth distributions and reflect the influence of hydroclimate, as measured by the magnitude of annual cumulative water-deficit extremes. Smaller-scale variations are linked to topography and land use. Our findings document large spatial variations in the effective root-zone water-storage capacity and illustrate a tight link among the climatology of water deficits, rooting depth of vegetation and its sensitivity to water stress. 

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3. Leading Satellite‐Based Evapotranspiration Products Insufficiently Capture Interannual Variability: Evidence From GRACE/FO and In Situ Observations

   https://doi.org/10.1029/2025GL116784  

   Zhao, Yanni; Hoeltgebaum, Lucas_Emilio B; Kukal, Meetpal S; Zhao, Meng (October 2025, Geophysical Research Letters)

   Abstract Satellite‐based evapotranspiration (ET) products such as OpenET and GLEAM are widely used for drought monitoring and ecosystem‐climate studies. However, their ability to accurately capture interannual variability (IAV), a key requirement for such applications, remains under‐evaluated. Here, we assessed IAV in OpenET and GLEAM using an independent water balance approach that combined precipitation, discharge, and GRACE/FO total water storage anomalies across nine river basins in the western United States. Even after accounting for observational uncertainty through a Monte Carlo approach, both products systematically underestimate IAV relative to water balance‐based ET, by more than 60% on average. This result is further supported by long‐term tower measurements from AmeriFlux. We also demonstrated that ET sensitivity to climate and vegetation drivers in OpenET and GLEAM differ substantially from water balance‐based estimates. These findings reveal important limitations in satellite‐based ET products and highlight the need for improved IAV representation to support ecosystem and climate applications. 

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4. Terrestrial water storage changes in the Bug river transboundary catchment observed by GRACE and water balance analysis [ЗМІНИ ВОДНИХ РЕСУРСІВ ТРАНСКОРДОННОГО БАСЕЙНУ РІЧКИ ЗАХІДНИЙ БУГ, ВИЯВЛЕНІ НА ПІДСТАВІ СУПУТНИКОВИХ СПОСТЕРЕЖЕНЬ GRACE I ВОДНО-БАЛАНСОВИХ РОЗРАХУНКІВ]

   https://doi.org/10.15407/Meteorology2024.06.004  

   Solovey, T; Śliwińska-Bronowicz, J; Janica, R; Brzezińska, A (February 2025, Meteorology. Hydrology. Environmental monitoring)

   Central and Southern Europe is undergoing a drying trend driven by increased evapotranspiration and rising air temperatures, even though precipitation levels remain stable. In the Bug River Basin, GRACE observations indicate that total water storage (TWS) declined at a rate of 8.8 ± 5.2 mm/year between 2012 and 2023. To validate this trend, we analysed spatial and temporal discrepancies between TWS-GRACE and water budget-based estimates (TWS-WB). Using ensemble data assimilation techniques, we integrated hydrometeorological data with TWS-GRACE. Regression models developed for TWS simulation were employed to adjust TWS-GRACE estimates. The results demonstrate that TWS fusion effectively mitigates uncertainties in TWS-GRACE caused by its low spatial and temporal resolution. Correlation analysis between TWS-fusion and TWS-GRACE identified errors in GRACE solutions and commonly used autoregressive methods for filling data gaps. Our findings show that model developed in this study significantly improved alignment between TWS-GRACE and TWS-WB, reducing RMSE from 34.7 to 14.9 mm/month. The proposed data fusion approach based on combining GRACE observations with precipitation, evapotranspiration, and runoff data, offers a viable alternative for extending TWS-GRACE time series beyond the GRACE observational period. Additionally, our research provides valuable insights for downscaling GRACE data and addressing challenges in spatial and temporal interpolation, which remain critical in water resource studies. 

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5. Quantifying the effects of land use and model scale on water partitioning and water ages using tracer-aided ecohydrological models

   https://doi.org/10.5194/hess-25-2239-2021  

   Smith, Aaron; Tetzlaff, Doerthe; Kleine, Lukas; Maneta, Marco; Soulsby, Chris (January 2021, Hydrology and Earth System Sciences)

   null (Ed.)

   Abstract. Quantifying how vegetation mediates water partitioning at different spatialand temporal scales in complex, managed catchments is fundamental forlong-term sustainable land and water management. Estimations fromecohydrological models conceptualising how vegetation regulates theinterrelationships between evapotranspiration losses, catchment water storage dynamics, and recharge and runoff fluxes are needed to assess water availability for a range of ecosystem services and evaluate how these might change under increasing extreme events, such as droughts. Currently, the feedback mechanisms between water and mosaics of different vegetation and land cover are not well understood across spatial scales, and the effects of different scaleson the skill of ecohydrological models needs to be clarified. We used thetracer-aided ecohydrological model EcH2O-iso in an intensively monitored 66 km2 mixed land use catchment in northeastern Germany to quantify water flux–storage–age interactions at four model grid resolutions (250, 500, 750, and 1000 m). This used a fusion of field (including precipitation, soil water, groundwater, and stream isotopes) and remote sensing data in the calibration. Multicriteria calibration across the catchment at each resolution revealed some differences in the estimation of fluxes, storages, and water ages. In general, model sensitivity decreased and uncertainty increased with coarser model resolutions. Larger grids were unable to replicate observed streamflow and distributed isotope dynamics in the way smaller pixels could. However, using isotope data in the calibration still helped constrain the estimation of fluxes, storage, and water ages at coarserresolutions. Despite using the same data and parameterisation for calibration at different grid resolutions, the modelled proportion of fluxes differed slightly at each resolution, with coarse models simulating higher evapotranspiration, lower relative transpiration, increased overland flow, and slower groundwater movement. Although the coarser resolutions also revealed higher uncertainty and lower overall model performance, the overall results were broadly similar. The study shows that tracers provide effective calibration constraints on larger resolution ecohydrological modelling and help us understand the influence of grid resolution on the simulation of vegetation–soil interactions. This is essential in interpreting associated uncertainty in estimating land use influence on large-scale “blue” (ground and surface water) and “green” (vegetation and evaporated water) fluxes, particularly for future environmental change. 

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