Long-term satellite-derived observations unrevealed the spatio-temporal hydrological variations in the Congo River Basin

Despite being the second-largest watershed with significant impacts on the global water cycle, the Congo River Basin’s (CRB) hydroclimatology remains among the least studied worldwide due to the insufficient in situ observations. To better characterize CRB surface hydrology and the variability of its different components at large scale, we jointly used a trove of large records of in situ and satellite-derived observations, specifically, Surface Water Level (SWL) from radar altimetry (a total of ~2,300 virtual stations) and Surface Water Extent (SWE) from the Global Inundation Extent from Multi-Satellite (GIEMS) dataset. A good performance is found between SWL and in situ water height at different locations, with root mean square error varying from 10 cm for Sentinel-3A to 75 cm for European Remote Sensing-2.  The assessment of SWE also agreed relatively well over a ~25-year period with in situ discharge from sub-basin to basin scale. SWL annual amplitude exhibits large spatial variability across the basin, with Northern sub-basins varying more than 5 m while the central and the southern sub-basins vary in smaller proportions (1.5 to 4.5 m). Furthermore, SWL and SWE help capture the water travel time across the basin that varies from 0 to 3 months and the regional relative contribution to the flow at Brazzaville station characterized by a bimodal hydrological regime. Northern sub-basins and the cuvette centrale contribute much to the large peak in December-January while the southern sub-basins contribute to both peaks. We further combine these two datasets to estimate the quantity of the variability of Surface Water Storage (SWS) using two methods, one method used hypsometric curves approach combining topographic data and SWE, and the second method used SWL variation and SWE. SWS in rivers, lakes, floodplains, and wetlands of the CRB is estimated over the period 1992–2015. The CRB SWS shows an annual amplitude ranging between ~74 km³ and ~112 km³. Moreover, the combination of SWS and the annual variations of GRACE/GRACE-FO-derived terrestrial water storage permits us to estimate the long-term variation of groundwater storage. Our results provide a better understanding of the hydrological variables in the CRB and their link with climate.