Effects of land use/cover change on hydrological flow of river Manafwa, Uganda

Abstract

Flooding is an increasing global problem attributed to many factors including climate change and land use/cover change particularly in mountain watersheds and floodplains. In Uganda, river Manafwa watershed has been greatly affected by persistent flooding and landslides. Despite the existence of an Early Warning System in the watershed, flood risk and associated impacts still occur. This research investigated the effects of land use/cover change on hydrological flow and downstream flooding risks of river Manafwa, by determining the trend in hydrological flow of river Manafwa from 1985 to 2015; establishing the association between land use/cover change and hydrological flow from 1985 to 2015; and analyzing the effects of change in hydrological flow on flooding events downstream in the same period. Land use/cover mapping of River Manafwa catchment for 1987, 1995, 2003 and 2015, was carried out in ArcGIS 10.1 using supervised classification. Flow data was obtained from National Water and Sewage Corporation and gap filled with ArcSWAT simulated flow of calibrated and validated Nash-Sutchliffe coefficient, NSE and p<0.05 of -72.24, 0.001 and -85.33, 0.004 respectively. Standardized Precipitation Index, SPI analysis was used to scan the potential flooding events from 1985 to 2014, for which floodplain areas were determined in Hydrologic Engineering Center River Analysis System, HEC-RAS and HEC GeoRAS. The results showed that the average river Manafwa flow is significantly increasing due to intense rainfall events p<0.001 that result into downstream flooding in the watershed. Flooding in the Manafwa watershed is as a result of converting forested and wetland land use/cover in the watershed to subsistence farmland and built up area, bare land. The most likely areas to be affected by flooding are the counties of Bubulo, Bungokho, and Manjiya within the districts of Bududa, Mbale and Manafwa. It was recommended that encouraging wetland regeneration, upstream reforestation and agroforestry within the watershed can help reduce flooding risk in the identified areas.