Synergetic application of GRACE gravity data, global hydrological model, and in-situ observations to quantify water storage dynamics over Peninsular India during 2002-2017

Long-term basin-scale assessment of water storage is imperative for ensuring timely and equitable water share, which is critical for India, where prevailing water resources govern food security and economic developments. Here, we holistically quantify the surface- and subsurface- water resources dynamics over three moderate-to-severely water-stressed river basins in Peninsular India, namely, the Godavari, Krishna, and Mahanadi river basins. For the first time, we jointly assimilate GRACE data, PCR-GLOBWB simulation, and in-situ groundwater observations. A comprehensive comparison of decadal trends of terrestrial water storage (TWS) and its constituent components in three basins reveal a stable annual cycle and minimal trends during the period from July 2002 to June 2010, and declining trends of TWS, soil moisture storage (SMS) and groundwater storage (GWS) during the period from July 2010 to June 2017. In particular, Krishna River Basin experienced the largest decrease in TWS and GWS with -30.95 mm yr(-1) and -20.18 mm yr(-1), respectively. The analysis of water storage deficits well captures the influence of meteorological droughts in the region. It shows that the groundwater extractions primarily govern the water availability during these dry phases. GRACE-derived GWS anomalies and those obtained from the in-situ data are in good agreement (r >= 0.86, RMSE <= 65 mm, rho >= 0.85), depicting the profound applicability of satellite observation in quantifying basin-wide water resources. We further isolate the impact of climatic variability and human interventions on TWS anomaly trends by comparing output derived from GRACE and PCR-GLOBWB-no human intervention (NHI) model runs. The results depict that the human interventions account for 61.29%, 76.12%, and 44.76% of TWS trends in three basins. The observation-based assessment of the various water cycle components in response to the changing climate and human impacts puts forward a blueprint for managing the basin-scale water resources and food security more timely and efficiently.