Impacts of climate variability and landuse/cover changes on streamflow: the case of Upper Rwizi Sub Catchment, Uganda

River Rwizi catchment experiences human threats due to expanding agricultural land and high urbanization rates with increasing demand for timber, fuel wood and charcoal that has resulted into deforestation and wetland degradation. But little is known about the impact of both climate variations and human factors on streamflow in data poor catchment like Rwizi. A systematic way to quantify the impact of climate variability and landuse/cover changes on streamflow is by integrating semi-distributed rainfall-runoff models together with GIS and remote sensing tools. In this case SWAT model with the ability to use semi-distributed model domain was applied to assess the impact of landuse/cover and climate variability on upper river Rwizi flows. Uganda meteorological Authority (UNMA) provided required climatic data and spatial data (90m-DEM, soil, landuse/cover) were obtained from other global sources. For trend analysis, Mann-Kendall and Sen's slope tests were used to detect trends in hydrometeorological data which indicated increasing trend but not significant while evapotranspiration showed decreasing trend in the catchment. In addition, for landuse and cover changes there was increase in agricultural area of 11.09%, increase in Urban/built up, 6.13% and bare land with 3.9%. On other side, decrease in Natural vegetation area by 17.3% was observed followed by 6.48% decreases in wetland area between 2003 and 2013. SWAT model simulated streamflow under specific landuse/cover and climate between 2000-2014 to assess the impact of both factors on Rwizi river flows after calibration and validation using observed flows. Simulated model flows show reduction in surface runoff by -3.24%, and -33% in ground water under landuse/cover change but increased by 27.2% (surface runoff) and 85% (ground water) hence increasing river average annual flows under climate change scenario. The study revealed that climate variability has bigger effect on streamflow hence recommending for more efforts to climate change adaptation in planning, development and management of future water resources.