Analyses of groundwater storage change using GRACE satellite data in the Usutu-Mhlatuze drainage region, north-eastern South Africa

The Usutu-Mhlatuze Water Management Area located in northern KwaZulu-Natal province, South Africa. Recent studies have indicated a steady decline in groundwater levels in the Usutu-Mhlatuze Water Management Area. However, the absence of representative aquifer storage parameter values including storativity and specific yield, lack of sufficient groundwater monitoring wells and inconsistent and erratic groundwater level observation data from existing limited networks make it difficult to understand groundwater storage changes in the region. Therefore, this study analyses groundwater storage change of the primary and secondary aquifers within the Usutu-Mhlatuze Water Management Area using GRACE satellite derived terrestrial water storage data, Global Land Data Assimilation System (GLDAS) soil moisture data and in-situ measured surface water storage information. The GRACE derived groundwater storage anomalies for the Usutu-Mhlatuze Water Management Area showed good agreement with in-situ groundwater storage anomalies observed from the limited groundwater level monitoring piezometers in the primary and secondary aquifers. The goodness of fit (R2) between the GRACE derived and the observed groundwater storage changes in the primary and secondary aquifers were 0.79 and 0.74, respectively. The GRACE derived groundwater storage change data for the entire study period (between 2002 and 2020) indicated that the primary and secondary aquifers experienced a groundwater storage loss of 925 × 106 m3 and 3614 × 106 m3, respectively. •GRACE satellite data analyses showed groundwater storage depletion.•Good correlation between GRACE derived and in situ-observed groundwater storage changes.•Both primary and secondary aquifers experienced large storage depletion.•Decline in groundwater levels impacted on the extent of groundwater dependent lakes and wetlands.•Groundwater pumping, land use and climate change reduced groundwater storage.