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

Study region: The Usutu-Mhlatuze Water Management Area located in northern KwaZulu-Natal province, South Africa. Study focus: Recent studies have indicated a steady decline in groundwater levels in the UsutuMhlatuze 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. New hydrological insights for the region: 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 x 10(6) m(3) and 3614 x 10(6) m(3), respectively.

Automated summary

This study analyzes the groundwater storage change in the primary and secondary aquifers of the Usutu-Mhlatuze Water Management Area in South Africa using GRACE satellite data, GLDAS soil moisture data, and in-situ measured surface water storage information. The results show good agreement between the GRACE derived groundwater storage anomalies and the observed results, indicating significant groundwater storage losses in both aquifers.