Quantitative mapping of groundwater depletion at the water management scale using a combined GRACE/InSAR approach

GRACE gravity variation recovery and InSAR-derived ground displacement data show promise in supporting and assessing groundwater management policies. However, GRACE system's resolution is too low, and the inversion of InSAR data into volume of groundwater storage loss requires extensive and often unavailable lithological data. Here we present how InSAR can be used to constrain and spatially focus GRACE-derived groundwater mass loss to depletion areas, reducing the gap between the GRACE scale and the typical water management scales. While we highlight the tremendous potential of a fully geodetic, quantitative, and high resolution mapping of groundwater storage loss, we also point out the crucial need for producing guidelines on the proper GRACE solution to use for any study area and/or application. In order to illustrate the GRACE/InSAR combination procedure, we present a case study in Central Mexico, where groundwater depletion of ~ 5000 Million Cubic Meters per year (MCM/yr) is reported by the water governance agencies and is well documented in the scientific literature. However, in this region, not all GRACE solutions provide reasonable groundwater depletion estimates. Using two of them, an inversion is performed to focus the groundwater-related GRACE signal over different mass distribution maps. Several mass distributions are tested, including two from InSAR-derived aquifer compaction mapping. The results show that the regions of Mexico City and Bajio, an agricultural and industrial corridor 250 km North of Mexico City, are the main contributors to the regional groundwater depletion. The mass distribution map produced directly from InSAR leads to results closer to official groundwater budgets than the others tested.