Assessing the effects of land use changes on flow regime of river Rwizi using swat model.

Abstract

The hydrology of most tropical catchments is largely influenced by variations in land use. Soil and Water Assessment Tool (SWAT) was used to assess the effect of land use changes on stream flow regime in the River Rwizi upper catchment covering a surface area of about 137900.5784 ha in western region of Uganda. River Rwizi has frequently featured in national news due to water resources problems especially low water levels as a result of prolonged dry seasons. Classified land use maps of 1983, 2003 and 2013 were evaluated to examine land use changes in the catchment. DWRM, DWD, NEMA and NARO provided hydro – meteorological data, soil data and historical aerial photographs for the study. The results of the study show that in the period between 2003 and 2013, there has been a 2.42% decrease in forest cover, 0.28% decrease in wetlands, 11.33% decrease in Bush land and Cropland Agriculture had increased by 9.14% while Grass/ Pasture land had increased by 19.14%. The SWAT Model results showed that there is a substantial relationship between the observed and simulated average monthly stream flows in the river Rwizi upper Catchment. The Nash-Sutcliffe Efficiency (NSE) and coefficient of determination (R) during calibration period (1983-1991) were 0.8 and 0.86, respectively, while for the validation period (1997-2004) they were 0.81 and 0.94, respectively. The average monthly stream flows increased by 22.56 m3/s during the wet season and decreased by 1.75 m3/s during the dry season. The changes in stream flow were ascribed to the land cover change and rainfall variation. The high water demand in the dry months can be met by constructing water storage reservoirs to harvest the high runoff during the wet months. More research about the effect of climate change on catchment hydrology should also be conducted.