Estimating Groundwater Storage Variations from GRACE by Incorporating a Global Reservoir Storage Dataset

NASA's Gravity Recovery and Climate Experiment (GRACE) satellites provide an unprecedented tool to evaluate the variations of terrestrial water storage at a global scale. Despite the considerable influence of surface water changes on groundwater estimations, our understanding of surface water storage variations (mainly contributed by reservoirs) remains only partially explored. This hinders estimations of groundwater variations, and leads to an increase in uncertainty. Here, we incorporate a remotely sensed Global Reservoir Storage (GRS) dataset to effectively estimate the groundwater variations of 140 large river basins. The terrestrial water storage variations collected by GRACE (from April 2002 to June 2017) were provided by CSR RL06 Mascon Solutions (version 02), and the monthly soil moisture and snow water equivalent values were obtained from the Global Land Data Assimilation System (GLDAS) Noah Land Surface Model L4 V2.1. We considered 6,737 reservoirs that were constructed before 1999 (with a total storage capacity of 5837.67 km3) from GRSwhich can provide monthly storage values over the GRACE observation periodto represent the surface water dynamics. Our results show that about 75 and 115 river basins exhibit significant trends (p < 0.05) for surface water and groundwater storage changes. In particular, 18 river basins show significant declines in both surface and ground water storage. Moreover, 25 river basins show contrasting variations in surface water and groundwater storage, which is likely caused by the decoupling of the two systems in regional water resources management. Overall, the trend analysis can help to assess the long-term vulnerability of freshwater accessible to humans at the river basin scale. In the future, we will compare the results with the water use data generated from water management models to evaluate the influence of anthropogenic activities on regional freshwater availability.