Assessment of groundwater storage variation at aquifer scale with ready-to-use GRACE Satellite Data: Spanish study cases

The satellite initiative Gravity Recovery and Climate Experiment (GRACE) has been thoroughly investigated in recent years to monitor groundwater storage (GWS). Since 2002, GRACE has provided distinctive perspectives on fluctuations in Earth's gravity field. Changes in gravity over time serve as valuable indicators for deducing alterations in total terrestrial water storage (TWS), encompassing soil moisture, surface water, snow and ice, canopy interception, wet biomass, and groundwater.

The importance of GRACE data in determining GWS holds crucial implications, particularly in regions with limited hydrogeological information. Numerous analyses published so far have consistently demonstrated a robust correlation between GWS derived from GRACE and measurements obtained directly from wells in extensive aquifers.

The Global Land Data Assimilation System (GLDAS) combines satellite and in situ data with sophisticated land surface modeling and data assimilation techniques. In the NASA GLDAS System Version 2 (GLDAS-2), GRACE data, initially on a 1° global grid, is downscaled to a higher resolution of 0.25°. This extension of data covers a daily scale from 1948, effectively interpolating temporal gaps present in the GRACE dataset. Following the isolation of contributions to temporal mass changes, GLDAS furnishes daily time series for GWS, making the GWS data readily available for utilization.

The present study adds to the existing body of knowledge by showcasing that GRACE is skilled at capturing regionally averaged seasonal variations in observed GWS at two Spanish detritic aquifers: Almonte- Marismas and Alto Guadalentín. Even in these cases, where the study area is relatively small compared to the broader GRACE track, there are good correlations between in situ and satellite information. On the other hand, this work aims to assess the efficacy of readily accessible GWS data obtained from the GLDAS web services. This will be accomplished through the validation of GWS ready-to-use product from GLDAS involving (i) an examination of its correlation with piezometric information in both confined and unconfined aquifers, and (ii) an evaluation of net groundwater recharge rates computed by using GWS data.