Projecting drought characteristics over East African basins at specific global warming levels

Abstract

Drought poses a threat to socio-economic activities across most East African river basins. While there are indications that global warming may continue to enhance evaporation from basins and intensify droughts, most drought projections over East Africa are based on rainfall alone. The present study combines rainfall and Potential Evapotranspiration (PET) to examine the impacts of global warming on droughts in East Africa, with a focus on four major river basins. We characterised the drought by means of two drought indices: the Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI). While the SPEI is based on the Climate Water Balance (CWB) (precipitation minus evapotranspiration), SPI is based on only rainfall. Multi-ensemble simulations from the CORDEX (the Coordinated Regional Climate Downscaling Experiment) datasets were analysed for the study. The capability of the simulations to reproduce the relevant climate variables (temperature, precipitation, PET and water balance, SPEI and SPI) over the East Africa region and specifically across four river basins (Rufiji, Tana, Juba and Upper-Nile) was evaluated. The impacts of climate change on the drought indices were considered at four specific Global Warming Levels (GWLs) (i.e. 1.5 °C, 2.0 °C, 2.5 °C, and 3.0 °C) under the Representative Concentration Pathway (RCP) 8.5 scenario and a Self-Organizing Map (SOM) was used to classify the SPEI and SPI projections into 12 groups, based on their similarities.Our results show that the CORDEX models give a realistic simulation of the relevant climate variables (including the drought indices) over East Africa and the river basins. For most of the variables, the spatial correlations between the observed and the simulated variables are high (0.6 < r > 0.9). In the annual cycle over these basins, the observations fall within the simulation spreads, and the simulation ensemble mean closely follows the observations. However, the ensemble mean does not accurately reproduce the spatial distribution of the drought frequency in the region. The CORDEX simulations project an increase in the intensity and frequency of SPEI droughts over East Africa. The magnitude of the projection, which varies over the region, increases with increasing GWLs. In contrast to the SPEI projection, however, the SPI projects only a weak change for intensity and frequency of droughts, and the magnitude does not vary with the GWLs. The SOM classification shows that 90% of the simulations project more severe and more frequent droughts with SPEI than they do with SPI. Over the river basins, the SPEI projections are more robust than the SPI. Hence, using only rainfall for drought projections may underestimate the severity of future droughts, while incorporating PET may enhance the robustness of the projections. The results of the study have application in mitigating the impacts of droughts on socio-economic activities across the East African river basins.