Delineation of River Watershed and Stream Network Using ILWIS 3.7.1 Academic

Abstract

The delineation of watersheds can be carried out manually using topographic maps but these maps are mostly outdated, scarce and incomplete for most parts of Nigeria and may be of different scales for large watersheds that require several sheets of the map. Watershed boundaries need to be delineated accurately because the areas and perimeters of watersheds are used in many hydrological analyses. This study is aimed at delineating the River Dep watershed and stream network using 90m SRTM Digital Elevation Model and Geographic Information Systems technique and comparing the shape and pattern with the one previously delineated manually using topographic maps and areal photographs. The 90 m SRTM Digital Elevation Model used for the study was obtained from the Global Land Facility Cover site and processed in ERDAS Imagine 9.2 before exporting to ILWIS 3.7.1 ACADEMIC. The watershed area obtained from the GIS delineation processes was 10640 km2 while it was 9600 km2 from the report of the one delineated by manual methods in 1980. The same 1980 manually delineated watershed when scanned, imported, georeferenced and digitised in ILWIS 3.7.1 gave a watershed area of 10244 km2. The differences in the watershed area could be due to the effect of input data or errors from the estimation using traditional delineation. The study showed similarity in shape and pattern for both the watersheds and stream networks delineated using the two methods. The study shows the capability of the ILWIS DEM hydro-processing tools used for watershed and stream network delineation using the 90m SRTM Digital Elevation Model and it is recommended that the both can be used with a reasonable level of accuracy.