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

Abstract

Groundwater (GW) represents nearly a quarter of total human water consumption globally and is a key component of many natural cycles and processes. Despite its crucial importance as a source of drinking water within the Congo River basin (CRB), its temporal and spatial variability is still barely known, mainly due to a lack of GW monitoring networks. Here, we provide an unprecedented quantification of the temporal dynamics and, for the first time, the spatial distribution 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. The relative contributions of each hydrological reservoir, including surface water storage, root-zone soil moisture, and GW, represent, respectively, ~18%, ~59%, and ~23% of the seasonal amplitude of TWSA (~365 ± 105 km3), in line with previous model-based estimates. Moreover, the mean annual amplitude of GWSA is found to be ~121 ± 30 km3. Additionally, at sub-basin scale, our study reveals that GWSA ranges between 42% (for Ubangui and Kasai) and 16% (for Lower-Congo) of TWSA. The spatial distribution of GWSA also 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 = 0.58) and consolidated (BFI = 0.60) sedimentary aquifers in contrast with lower GWSA variability and BFI in fracture flow (BFI = 0.28) and basement (BFI = 0.22) aquifers.