STUDY OF REGIONAL HYDROLOGICAL IMPACTS OF CLIMATE CHANGE ON WATER RESOURCES OF NIGERIAN SECTOR OF NIGER RIVER BASIN

Abstract

The study explored and assessed the potential impacts of climate change on water resources of Nigerian sector of Niger River basin of Sub-Saharan West Africa, using parametric and non parametric approaches for trend detection, and simulation models-Thornthwaite water balance accounting scheme and Artificial Neural Networks (ANNs) for climate change impacts assessment. In this regard, hydrometeorological data and landuse satellite data has offered excellent inputs to monitor the trend and dynamic changes in the hydrological condition vis-a-vis water surplus ( that indicates the available water resources ) of the Niger basin due to varying climatic conditions. The water balance model uses mean monthly climate variables of precipitation, temperature, net radiation, as well as soil water holding capacity and potential evapotranspiration, for estimating water surpluses of five Niger sub-basins and linking these water surpluses of these sub-basins with the corresponding observed monthly average discharges using the ANNs model. Ninety six monthly data have been used for calibrating the ANNs and twenty four months data were used for verification. Areally averaged temperature and precipitation changes from formulated climate change synthetic scenarios were imposed on each subbasin for assessing climate change impacts on runoff. GIS offered a vital means for the hydrological modeling, especially the building of grid containing the water holding capacity parameter of the soil, creating climatic surfaces and merging various spatial themes (data layers) that was useful in interpretation, analysis( soil water budgeting) and change detection of spatial structures and objects. Data was examined for quality control and studies. reveal consistency in the temperature of 19 meteorological stations within the basin following the homogeneity test, while 18 stations showed consistency in the rainfall. The correlation coefficients (r2) range from 0.9997 to 1, with an average of 0.9999 for temperature and 0.999 to 0.9999. with an average of 0.9995 for rainfall.- Further results show that temperature significantly increased by a value < 3.49%, and temporal changes in areal temperature, with an increasing trend at the rate of 0.001°C/month and 0.2°C/year; and the coefficient of variability of 1.43% on long-term periods. The magnitude of variability is higher within 1973 — 2001 periods, the post — vi economic growth era, relative to 1943 — 1972 periods. Rainfall significantly decreased by a value < 25.19%, and temporal changes in areal rainfall, with a decreasing trend at the rate of 2.45mm/year and coefficient of variability of 8.64% on long-term periods, with the magnitude of variability being more during the 1943 — 1972 reference periods; and discharge decreased significantly by a value < 40.8 %. Spatial changes analyses show that variability in climatic variables is higher and stronger over the Upper Niger and Lower Niger sub-basins. Both the hydroclimatic and trends are significant at 95% confidence level. Effects of annual fluctuations of rainfall on the hydrological regime show that only the Niger-South sub-basin (coastal area) had the most stable hydrological regime relative to the other sub-basins in the hinterland. The groundwater recharge estimated using stable chlorine isotope technique shows a linear relationship with rainfall of the basin, a downward trend. Since Niger River system is hydraulically connected with aquifer of the region, this may invariably results in decreasing trends of the groundwater level. The dependable rainfall estimated over the region, necessary for both irrigation and hydropower reservoir operations at both 75% and 90% dependability is expected to decrease to 1113.55mm and 1060.62mm, giving a yield of 16200929.71M.m3 and 15431342.71Mm3 respectively in the future, if temperature increases by 2°C and rainfall decreases by -20 %. Further results show the sensitivity of the Niger basin to climate fluctuations, annual water surplus is highest over the Niger south( 1241.2mm), followed by Lower Benue (973.6mm), Lower Niger (729.4mm) Upper Benue (495.3mm) sub-basins and the least value is observed over Upper Niger Sub-basin(360.7mm). It is further observed that the water surplus is much more sensitive to the accuracy of potential evaporation estimate (that depends on temperature) in the humid climate than the arid climate. In addition, when temperature increases by 2°C, the mean monthly runoff on the average is expected to change by -10 to -50%, -5 to -40% and 15 to 60% respectively for precipitation changes of -20%, 0% and 20%