Modelling the impact of land use and climate change on sediment and stream flow variability in the Upper Mpanga catchment

Abstract

Land use and climate change, coupled with population growth and industrialization have been reported to have immense effects on water resources worldwide resulting in increased risks to their sustainability and resilience. Various studies world-wide, however, still show differing effects of these on stream flow and sediment transport. Mpanga catchment is one of the heavily degraded areas in western Uganda and is characterized by high and low flows attributed to climate change. This has affected water usage in the catchment, among which is drinking water treatment by National Water and Sewerage Corporation, a public water company. High flows are heavily sedimented while low flows are below the minimum treatment capacity, disrupting quality water production. This study used the SWAT hydrological model to evaluate the combined and individual impacts of land-use and climate change on stream flow variability and sediment transport in the upper Mpanga catchment. Land use classification was carried out for the period 2000 to 2020 using Landsat Imagery from https://glovis.usgs.gov/, and predicted for the year 2040. The results showed that land degradation with small scale farming increased by 18.5%, built-up area by 5% while bushland, grassland, woodland, wetland and forests decreased by a combined 27% between 2000-2040 with an increased sediment load (22%) and limited change in stream flow. Climate change assessment indicated increased average annual temperature by 2050 for both RCP 8.5 (+0.74oC) and RCP4.5 (+0.70oC) scenarios while annual rainfall decreased by 5.0% and 3.5% for RCP 4.5 and RCP 8.5 scenarios respectively. This resulted in a decrease in annual stream flow (8.4% and 4.2%) and an increase in sediment load (0.4% and 2.2%) for RCP4.5 and RCP8.5 respectively. The sediment load increase was attributed to peaks in seasonal rainfall. The combined effect of both land use and climate change showed a decrease in flow and an increase in sediment load. The model showed that targeted Soil Land Water Management (SLM) practices in vulnerable sub-catchments including tie bunds, river bank stablisation and contour bunds reduced sediment loads by 50% in 2040 but with less impact on stream flow. Tie bunds showed more efficiency in reducing sediments. Therefore, implementing SLMs locally can have significant reduction in sediment loads in the catchment in addition to mitigating climate change. However, to ensure their sustainability, the costs and benefits associated with adopting them should be evaluated.