Hydrological Response of Watershed Systems to Land Use/Cover Change. A Case of Wami River Basin

ABSTRACT

Wami river basin experiences a lot of human disturbances due to agricultural expansion, and increasing urban demand for charcoal, fuel wood and timber; resulting in forest and land degradation. Comparatively little is known about factors that affect runoff behaviour and their relation to landuse in data poor catchments like Wami. This study was conducted to assess the hydrological response of land use/cover change on Wami River flows. In data poor catchments, a promising way to include landuse change is by integrating Remote Sensing and semi-distributed rainfall-runoff models. Therefore in this study SWAT model was selected because it applies semi-distributed model domain. Spatial data (landuse, soil and DEM-90m) and Climatic data used were obtained from Water Resources Engineering Department, government offices and from the global data set. SWAT model was used to simulate streamflow for landuse/landcover for the year 1987 and 2000 to determine the impact of land use/cover change on Wami streamflow after calibrating and validating with the observed flows. Land use maps of 1987 and 2000 were derived from satellite images using ERDAS Imagine 9.1 software and verified by using 1995 land use which was obtained from Institute of Resource Assessment (IRA).

Findings show that there is decrease of Forest area by 1.4%, a 3.2% increase in Agricultural area, 2.2% increase in Urban and 0.48% decreases in Waterbody area between 1987 and 2000. The results from SWAT model simulation showed that the average river flows has decreased from 166.3 mm in 1987 to 165.3 mm in 2000. The surface runoff has increased from 59.4mm (35.7%) in 1987 to 65.9mm (39.9%) in 2000 and the base flow decreased from 106.8mm (64.3%) to 99.4mm (60.1%) in 1987 and 2000 respectively. This entails that the increase of surface runoff and decrease of base flows are associated with the land use change.

Keywords: