Estimating the spatio-temporal assessment of GRACE/GRACE-FO derived groundwater storage depletion and validation with in-situ water quality data (Yazd province, central Iran)

Due to the lack of surface water availability in Yazd province, groundwater has been overexploited as a key source of water supply, resulting in the overexploitation of groundwater resources for agricultural activities. This study employed the data of the Gravity Recovery and Climate Experiment (GRACE)/GRACE Follow-On (GFO) satellite to determine terrestrial water storage (TWS) and deriving groundwater storage (GWS) variations using global land data assimilation system (GLDAS) water storage components in Yazd province from 2003 to 2020. The time series results showed that the annual changes of GWS are positive from 2003 to 2007, and continue negative with a decrease in precipitation significantly causing depletion of GWS in the aquifers. The Pearson correlation coefficient is close to 0.58 between observed and GRACE-derived GWS on yearly basis and both show a decreasing GWS trend. The results of calculating the groundwater balance for aquifers showed that the renewable water resources have decreased from 786 to 250 million cubic meters (MCM) between the water year 2009-2010 and 2019-2020. This has caused the electrical conductivity (EC) to increase from about 8500 to 12,500 & mu;S/cm. The correlation coefficient between EC and observed and GRACE-derived GWS was around -0.50 and -0.79, respectively. GRACE-derived and analytically calculated groundwater storage deficit (GSD) is around 2491 and 2775 MCM showing a similar downward trend. These results, while confirming GRACE-derived GWS, point out the need to revise the analytical calculation of the groundwater balance based on the unmodified aquifer domains and the non-uniform distribution of observation wells in most aquifers to reduce the difference between observed and GRACE-derived GWS anomalies.