Contributions of Water Storage Compartments to TWS in the East African Rift Region

The northern part of the East African Rift has exhibited a distinct temporal TWS pattern in the last twenty years. Before 2006, we observed a decrease of TWS. It then recovered constantly until 2017. In 2019 and 2020, TWS peaked with the most significant gain in Africa in the GRACE and GRACE-FO period.

The study region is dominated by some of the largest lakes in the world: Victoria, Tanganyika, and Turkana. The climate ranges from arid conditions in the North to tropical climates in most parts of the region.

This presentation aims to analyse and characterise these interannual TWS variations compared to meteorological and satellite-based observations of the water storage compartments (surface water, soil moisture, and groundwater). Surface water storage (SWS) variations of the lakes in the region are observed with a combination of altimetric water levels and optical surface water extent. Soil moisture variability (root-zone soil moisture storage – RZSM) is monitored with microwave remote sensing and is available from the Copernicus Climate Change Service (C3S). Groundwater storage (GWS) changes cannot be observed directly with satellites but as the difference between TWS and SWS, RZSM, and snow and glaciers. The latter are neglected for the study region. To this end, the different data sets need to be harmonised in the spatial domain by smoothing SWS and RZSM with a Gaussian filter with a half-width of 250km.

The main drivers of the changes observed in TWS are the meteorological variabilities and surface water storage. During the meteorological drought before 2006, the loss of SWS of Lake Victoria alone contributed up to 50% of the TWS variability, while GWS variations are comparable small. On the other hand, the significant TWS increase around 2020 can be attributed to nearly equal gains in groundwater and surface water storage, which coincide with a substantial precipitation surplus. In these years, the lakes in the region have suffered severe flooding. Soil moisture explains most of the seasonal TWS variability but does not influence the interannual variations.