Impact of land use/cover change on storm runoff hydrograph: a scenario study at Ngomberi catchment

This study was carried out to assess the impact of land use/cover change on the storm runoff hydrograph in the Ngomberi catchment. three sets of land use data were developed in raster format from satellite images for 1976, 1987 and 2000 by using ilwis 3.0 and review gis 3.2 software. The simulations were done using the geographical information system-based hydrological model for Ngomberi catchment developed by Kimaro and Mbando 2006. The modeling is based on division of the catchment into a grid mesh of 200m spatial resolution. Each cell has a unique response function independent of the functioning of other cells. summation of the flow responses from the cells results in the storm runoff contribution from the catchment. the results of this study revealed that the land use/cover change between 1976 and 2000 are 15.81% and 21.72% increase for dense bush land and mixed cropping respectively and 42.07%, 16.17%, 0.86% decrease of ice cap, natural forest and cultivation with tree crops respectively. The change in the land use/covers resulted into a significant impact to the storm runoff hydrographs. the simulation results from the year 1976 to 2000 and 1987 to 2000 revealed that the peak flow was reduced by 4.221 % and 14.959% respectively while between the years 1976 and 1987 the flow was increased by 12.627%. Also the scenario of possible changes for 2000 land use were simulated and showed significant changes in the hydrograph of shortening of the response time from the catchment and raising the early peaks of the hydrograph. The use of remote sensing, gis and the Ngomberi model as tools for water resources management is recommended in the catchment and the further areas of study were suggested.