STUDY OF RESERVOIR DYNAMICS IN THE OPA DAM, OAU, ILE-IFE NIGERIA, USING GIS TECHNIQUES

Abstract

Small reservoirs are mainly utilized for domestic purposes, livestock watering, fishing and irrigation. In spite of their importance in water resource provision, planning and management, recent studies have shown that the major factor militating against reservoir study is high survey cost and lack of equipment especially in developing world. Two aspects of reservoir dynamics were studied: shoreline changes and bathymetric map production as baseline data for future studies. Landsat imageries covering the study area were acquired for the periods of 1986, and 2002 for the purpose of determining the extent of shoreline changes during these periods. The Normalized Difference Water Index (NDWI) map of the reservoir was generated in ArcGIS environment; then the shorelines of the reservoir for each of the periods under consideration was extracted from the NDWI using slicing and reclass operations in ArcGIS. Shoreline for each year was overlayed and shoreline change detection map of the reservoir was subsequently produced. A simple, less costly method with good enough accuracy was applied for rapid bathymetric survey and map generation (as developed by ITC, Enschede, The Netherlands). Transect lines and points were generated from IKONOS satellite imagery and uploaded on to handheld GPS for navigation, while observation was carried out at each of the transect points generated using dual-frequency GPS. The location coordinates as well as the depth at each of the transect points using handheld depth finder was determined and recorded. Interpolation analysis operation was performed to produce interpolated surface which was then used for modelling the reservoir bed. The outcome of the research shows a decrease within the 16 years period of investigation (1986-2002) as revealed, from 440,000m2 in 1986 to 410,000m2 in 2002. This shows a reduction of 30,000m2, with an annual decrease rate of 1,875m2. The bathymetric map generated revealed a maximum depth of 4.12m and a minimum of 1.12m Also landuse/Landcover change were observed which could be a factor to reservoir change especially on its shoreline.