Assessing the impact of land use/cover change on the stream flow of River Rwizi using a physically based model

Abstract

Global water use has been increasing due to increasing water demands for domestic, industrial and agricultural purposes. This is because of demands related to land use and human development especially in developing countries. Land use /cover change effects collectively add up to global environmental change and as a result, there is growing concern about negative impacts of land use/cover changes. The stream flow of River Rwizi has increasingly become a concern at community, district and national levels, serving a wider catchment in the south western part of Uganda. With the increasing human population in the catchment, there has been an increase in land use activities such as agriculture, wetland degradation and water abstraction for industrial activities, posing a risk of water reduction. This study assessed land use / land cover change around the upper catchment of River Rwizi and their impact on the streamflow of the river using a physically based model, HEC HMS. Assessment of the reliability of river flow data was done through trend and randomness tests. This was followed by the set-up of HEC-HMS model consisting of a basin model, meteorological model, control specifications, and input data (time series data). Basin analysis using elevation data was carried out to obtain watershed characteristics with the help of ArcHydro tool and HEC-GeoHMS tool, as add-ons of ArcGIS. The model calibration was carried out using daily observed streamflow data as well as the meteorological (rainfall and temperature) data for the period (2001-2003). The model validation was carried out for the period (2006-2008). Assessing model performance was based on the Nash Sutcliffe Efficiency (NSE) and the Coefficient of correlation R2. These were found to be 0.8 and 0.95 respectively during calibration, and 0.76 and 0.92 respectively during validation. The validated model was then used to simulate future flows for Rwizi catchment. Model results indicate that increasing agriculture and loss in vegetation have had adverse effects on streamflow. The response of streamflow to the postulated changes in land use was also evaluated for the year 2025. Existing policies and laws on the management of water resources need to be strengthened. Furthermore, collaborative management of this river should be emphasized.