Hydrogeological control of groundwater variations in the Congo River Basin revealed by GRACE water storage change decomposition

The Congo River Basin (CRB) holds significant groundwater (GW) resources that play a crucial role as the primary source of drinking water for CRB’s human population. Here, we provide the quantification of the temporal dynamics and, for the first time, the spatial distribution of monthly Groundwater Storage Anomaly (GWSA) in CRB from 2002 to 2015, through the decomposition of the Total Water Storage Anomaly (TWSA) measured by the Gravity Recovery and Climate Experiment (GRACE) mission. Observation based-surface water storage anomaly (SWSA) developed from multi-satellite missions was a game changer in providing reasonable GWSA seasonal dynamics. The contributions of each hydrological reservoir, including SWSA, root-zone soil moisture, and GWSA, represent, respectively, ∼18 %, ∼59 %, and ∼23 % of the seasonal amplitude of TWSA (∼365 ± 105 km 3 ), in line with previous model-based estimates. Additionally, at sub-basin scale, our study reveals that GWSA ranges between 42 % (for Ubangui and Kasaï) and 16 % (for Lower-Congo) of TWSA. The spatial distribution of GWSA confirms the expected hydrogeological behavior for each type of aquifer related to the four main hydrogeological formations encountered in CRB with high GWSA variability and high Base-Flow Index (BFI) in unconsolidated (BFI = 0.68) and consolidated (BFI = 0.67) sedimentary aquifers in contrast with lower GWSA variability and BFI in fracture flow (BFI = 0.14) and basement (BFI = 0.12) aquifers. • Observed surface water storage enhances groundwater storage temporal dynamics. • Groundwater storage changes reflect the expected behavior of the main aquifer types. • Kasaï and Ubangui sub-basins exhibit large contribution of groundwater storage. • Groundwater storage variations follow baseflow index fluctuations.