Investigating the sensitivity of tropical catchments to changes in precipitation and evapotranspiration under different climatic conditions

Abstract

Globally, effective management of water resources is needed to provide sustainable water sources for human and ecosystem consumption. For the sustainability of water resources, careful planning is required for the management of catchments. One way to yield relevant information to support actionable policy is scenario analysis using hydrological modeling as a tool. This study investigated the sensitivity of Tropical catchments to changes in precipitation and evapotranspiration under different climatic conditions. The idea was to determine the extent to which flows would change given some incremental changes in rainfall and Potential evapotranspiration (PET). Six sub-catchments were considered and data from 1999-2016 (Malaba), 1980-2018 (Mpanga), 1990-2019 (Kabalega), 1980-2000 (Blue Nile), and 1980-2002 (El-diem & Ribb) were utilized. The AWBM and Hydrological Model focusing on Sub-Flow Variation (HMSV) were applied to data from each selected sub-catchment. Performance of AWBM in terms of Nash-Sutcliffe Efficiency (NSE) considering the full data period was 0.838 (Malaba), 0.686 (Kabalega), 0.676 (Mpanga), 0.812 (Blue Nile), 0.745 (El-Diem) and 0.56 (Ribb). Corresponding values for HMSV were 0.806 (Malaba), 0.748 (Kabalega), 0.620 (Mpanga), 0.806 (Blue Nile), 0.800 (El-Diem) and 0.520 (Ribb). Mean annual flows exhibited increasing trends in all sub-catchments except Kabalega. A positive trend ranged from 0.005-2.294m3/s/year in magnitude. A negative trend in the flow of Kabalega was at a rate of -0.015m3/s/year. All trends in flows were not significant since p-values were all above 0.05. The sensitivity of sub-catchments to increasing rainfall under constant PET ranged from 0.968m3/s/year (Malaba) to 2.700m3/s/year (Blue Nile) for every percentage increase in rainfall. The sensitivity of sub-catchments to increasing PET under constant rainfall ranged from -0.3603 m3/s/year (Kabalega) to -1.288 m3/s/year (Blue Nile) for every percentage increase in PET. Under simultaneous changes in rainfall totals and PET, the sensitivity of sub-catchments varied, ranging from 0.00821 m3/s/year (Malaba) to 1.2218 m3/s/year (Blue Nile) for every percentage increase in rainfall. Climate variability is characterized by both increases and decreases in flows depending on underlying factors. These results underscore the importance of careful planning of watershed management given factors like changes in climatic conditions that influence spatial and temporal variation in rainfall and PET.