Using the Soil and Water Assessment Tool (SWAT) to assess land use impact on water resources in an East African watershed

Abstract

Summary Land cover and land use changes in Kenya’s Rift Valley have altered the hydrologic response of the River Njoro watershed by changing the partitioning of excess rainfall into surface discharge and groundwater recharge. The watershed contributes a significant amount of water to Lake Nakuru National Park, an internationally recognized Ramsar site, as well as groundwater supplies for local communities and the city of Nakuru. Three land use maps representing a 17-year period when the region underwent significant transitions served as inputs for hydrologic modeling using the Automated Geospatial Watershed Assessment (AGWA) tool, a GIS-based hydrologic modeling system. AGWA was used to parameterize the Soil and Water Assessment Tool (SWAT), a hydrologic model suitable for assessing the relative impact of land cover change on hydrologic response. The SWAT model was calibrated using observation data taken during the 1990s with high annual concordance. Simulation results showed that land use changes have resulted in corresponding increases in surface runoff and decreases in groundwater recharge. Hydrologic changes were highly variable both spatially and temporally, and the uppermost reaches of the forested highlands were most significantly affected. These changes have negative implications for the ecological health of the river system as well as Lake Nakuru and local communities.