Ground Referencing Grace Satellite Estimates of Groundwater Storage Changes in the 1

13 There is increasing interest in using GRACE (Gravity Recovery and Climate Experiment) 14 satellite data to remotely monitor groundwater storage variations; however, comparisons with 15 ground-based well data are limited but necessary to validate satellite data processing, 16 especially when the study area is close to or below the GRACE footprint. The Central Valley is a 17 heavily irrigated region with large-scale groundwater depletion during droughts. Here we 18 compare updated estimates of groundwater storage changes in the California Central Valley 19 using GRACE satellites with storage changes from groundwater level data. A new processing 20 approach was applied that optimally uses available GRACE and water balance component data 21 to extract changes in groundwater storage. GRACE satellites show that groundwater depletion 22 totaled ~31.0±3.0 km 3 for GRGS (Groupe de Recherche de Geodesie Spatiale) satellite data 23 (27 km 3 for both). General correspondence between GRACE and groundwater level data 26 validates the methodology and increases confidence in use of GRACE satellites to monitor 27 groundwater storage changes. 28