Uncertainties Associated with Climate Change Projections on Future Water Resources in Kilombero River Basin, Tanzania

While there have been efforts in improving climate change impact uncertainty using ensembles of climate models, there has been less focus on the combined effect of climate models and hydrological model uncertainty. In this study, the impact of climate change on the hydrological regime of the Kilombero River Basin was evaluated. The hydrological model, Soil and Water Assessment Tool (SWAT) was forced with identical climate data and calibrated against river discharge using automatic calibration routine within the SWAT calibration and uncertainty program (SWAT-CUP). Satisfactory model performance result, with daily NSE value of 0.82 was obtained. The results from the hydrological models were utilized to project the effect of climate change on river discharge and other water balance components as given by ensemble mean from Regional Climate Models (RCMs) given by CORDEX-Africa for the two Representative Concentration Pathways (RCPs), namely RCP4.5 and RCP8.5 emission scenarios. The RCM projections of change in monthly rainfall, temperature, potential evapotranspiration, actual evapotranspiration, and total water yield were estimated by comparing the two future time slices (near future [2031-2060] far future [2071-2100]) climate conditions to the reference (historical) period (1957-2005). Results indicated a pronounced wetter future in combination with hotter future. Rainfall and water yield are expected to increase by 17% and 25% for the RCP4.5 and RCP8.5 respectively. The potential evapotranspiration on the other hand is expected to increase between 146 mm and 284 mm across all scenarios. In general, the Kilombero River basin is projected to experience a drier future in some parts of the dry season and moderate to severe flood events during the rainy season. Therefore, wise water management is needed within the basin to properly use and save the potentially occurring water benefit during the rainy season to make it available for consumption during the more pronounced dry periods. Such water could be used to promote the dryland agriculture thereby improving the livelihood of the people living in the study area.