Water Accounting of the Ogun River in the Tropical Environment of South-West Nigeria

The study examines the water resources potential of Ogun River Basin in Southwestern Nigeria using the rapid water accounting plus (WA+). This was achieved by using data from the Water Productivity (WaPOR) version 2.0 Level 1 with 250m resolution open access database of the Food and Agricultural Organization (FAO). Python and Jupyter notebook programming were used to download data from WaPOR database as well as for the implementation of the Water Pixel model. QGIS and ArcGIS were used to carry out raster calculation and mapping of water indices. Ground based rainfall data from Nigeria Meteorological Agency (NiMET) for three stations (Ikeja, Abeokuta and Ibadan) were used for rainfall validation while Ogun River discharge data at Abeokuta provided the hydrological flow data for the computation of the water balance. The results showed that the WaPOR 2.0 level 1 precipitation data showed close agreement with the in situ observations by the NiMET at the Ikeja, Abeokuta, and Ibadan stations with the coefficient of correlation values varying from 0.618 to 0.687. The basin was a net water generator as precipitation (16471 mm/year) was higher than total evapotranspiration (13888 mm/year), which implies that as a whole, the basin generates surplus water (150.85 mm/year) for the 2018 hydrological year. The exploitable water resources are 12.7 km³/year, while the renewable water resources are higher (14.14 km³/year), which is due to an increase of the basin storage by 1.4 km³/year. The gross inflow is 29.2 km³/year and the total outflow and consumed water is 27.8 km³/year, which resulted in 1.4 km³/year stored as total water storage change (Balance). Also, the average available water was depicted to be 12.1 km³/year, out of which approximately 2.9% was managed. The Managed Water, which is Incremental ET of Managed Water use, was on average 0.1 km³ /year. The study recommends provision of medium sized dams across the basin for proper harvest of the surplus water to increase urban water supply, reduce proliferation of boreholes and control flooding.