Interannual Variations of Terrestrial Water Storage in the East African Rift Region

Abstract. The US-German GRACE (Gravity Recovery and Climate Experiment, 2002–2017) and GRACE-FO (GRACE-Follow-On, since 2018) satellite missions observe terrestrial water storage (TWS) variations. Over twenty years of data allow for investigating interannual variations beyond linear trends and seasonal signals. However, the origin of observed TWS changes, whether naturally caused or anthropogenic, cannot be determined solely with GRACE and GRACE-FO observations. This study focuses on the East African Rift region region around lakes Turkana, Victoria, and Tanganyika. It aims to characterise and analyse the interannual TWS variations together with surface water and meteorological observations and determine whether natural variability or human interventions caused these changes. To this end, we apply the STL method (Seasonal Trend decomposition based on Loess) to separate the TWS signals into a seasonal signal, an interannual trend signal, and residuals. By clustering these interannual TWS dynamics for the African continent, we define the exact outline of the study's region. In this area, a TWS decrease until 2006 was followed by a steady increase until around 2016, and Africa's most significant TWS increase occurred in 2019 and 2020. We found that besides precipitation and evaporation variability, surface water storage variations in the large lakes of the region explain large parts of the TWS variability. Storage dynamics of Lake Victoria regulated by the Nalubaale Dam alone contribute up to 50 % of the TWS changes. Satellite altimetry reveals the anthropogenically altered discharge downstream of the dam. It thus indicates that human intervention in the form of dam management at Lake Victoria substantially contributes to the TWS variability seen in the East African Rift region.