Geospatial inventory and characterization of surface water resources in Ife and Ilesa areas, Southwest Nigeria between 1972 and 2007
The study investigated the types, number and sizes of surface water bodies in Ile-Ife and Ilesa areas of Osun State between 1972 and 2007. It also identified the anthropogenic activities around each water body with a view to determining the spatio-temporal patterns of water resource variability in the study area. The study was carried out in four Local Government Areas (LGAs) of Osun State, namely Ife Central, Ife East, Ilesa West and Ilesa East. The surface water resources in each LGA between 1972 and 2007 were identified using Land-use map and satellite imageries. The map was scanned and digitally processed in ArcGIS version 9.3 software using the processing techniques of georeferencing, clipping, mosaicing, sub-mapping, digitization and map presentation. The satellite imageries used included Landsat MSS (1972) of 80 m spatial resolution, Landsat TM (1986, 1991) and Landsat ETM+ (2002, 2007) each of 30 m spatial resolution. These were downloaded via the computer-based Window Explorer through the Geogateway of ILWIS 3.2 Academic and processed using the colour composite, image resampling, georeferencing, sub-mapping, image classification and segmentation techniques. The thematic maps were cartographically produced in ArcGIS version 9.3 software. Field survey (ground truthing) was carried out in each LGA during the dry and wet seasons to validate information on water bodies from satellite imageries. The coordinates of the surface water bodies were acquired with the Global Positioning System and observations made of anthropogenic activities within 100 m radius of each water body. Differences in number and sizes of water bodies between groups were determined using the Student T-test or OneWay ANOVA, as appropriate. The relationship between identified anthropogenic activities and changes in the number and sizes of water bodies were determined using the Spearman’s Correlation analysis. Fifty surface water bodies were identified (35 streams/rivers, 9 pools, 5 reservoirs and 1 pond) in the four LGAs. Satellite image analysis identified all the streams/rivers and reservoirs but did not identify any of the ponds and pools seen by ground truthing. The water bodies were significantly (p < 0.05) distributed among the four LGAs (26 in Ife East, 9 in Ife Central, 4 in Ilesa West and 11 in Ilesa East). Analysis of the satellite imageries revealed that the total area of surface water resources in the two LGAs in Ile-Ife increased from 196.65 Ha in 1972 to 1112.94 Ha in 1986 but declined steadily thereafter to 605.52 Ha by 2007. Similarly in Ilesa, the total surface area of water resources increased steadily from 142.29 Ha in 1972 to 1668.51 Ha in 2002 before it declined sharply to 605.52 Ha by 2007. The human activities observed within 100 m radius of the water bodies were farming, house and road construction, car-wash, abattoirs, saw-milling, cement block making and petrol stations. GIS analysis showed that the surface area of the water bodies decreased as the area under farming and construction increased over the study period. Thus, land use/land cover fluctuated significantly over the period of 35 years in the study areas. The study concluded that the remote sensing and GIS technologies are effective tools for surface water detailed inventory. Also, anthropogenic activities such as farming, human settlement, and deforestation are the major factors driving the surface water resources’ size and number dynamic in Ile-Ife and Ilesa.