Groundwater storage in response to severe anthropogenic and drought events using GRACE data assimilation

Groundwater (GW) storage in many parts of the world are depleting at an alarming rate owing to its unscientific exploitation. In the recent decade, GW has rapidly emerged as a dominant source of freshwater in India accounting for 80 % of the domestic water supplies and 63 % of the total irrigation water. This has motivated us to evaluate the current trend in groundwater storage of India using observations from monitoring wells and GRACE mascon products. The results from our analysis indicate that unsustainable anthropogenic activities along with frequent drought have led to dwindling GW storage in majority of Indian states. This decreasing GW trend is particularly observed over northwestern India, the southern plateau and the southeastern coastal region. Further assessment and sustainable management of groundwater balance over the transboundary region in north India is critical due to the high population density and intensive agriculture activities in the region. However, the GRACE measurements are influenced by the surrounding regions such as the loss in glacier mass balance in the Himalayas. Further, they are constrained by coarse spatial resolution. To overcome this, we implement data assimilation (DA) technique to dynamically downscale GRACE observations by integrating it with the Catchment land surface model (CLSM). This is achieved by correcting the model state estimates with GRACE based Terrestrial Water Storage (TWS) observations using Ensemble Kalman Smoother (EnKS) approach. To the best of the authors' knowledge, this is the first study to implement assimilation of GRACE mascon product over our study region. The results from our assimilation study indicate an improved agreement between updated model results and monitoring well observations. One of the crucial advantages of GRACE assimilation in our study region indicates the improvement in the simulation of the high sensitivity of GW resource with the precipitation in the alluvial aquifer region.