HYDROLOGICAL RESPONSE TO CLIMATE CHANGE AND WATER RESOURCES VULNERABILITY IN NIGER CENTRAL HYDROLOGICAL AREA, NIGERIA

Abstract

For an improved assessment of water resource, Nigeria is delineated into eight hydrological areas, among which is Niger Central Hydrological Area (NCHA). This study aims at evaluating hydrological variability in a changing climate over the NCHA with a view to study its impact on water resources. Statistical, trend and drought analyses were performed on selected hydro-climatic variables (rainfall, temperature and streamflow) to determine the direction of change in climate as well as the existence of drought. Thereafter, water quantification was done using Soil and Water Assessment Tool (SWAT). The method includes calibration (1986–1995) and validation (1996-2000) of the SWAT model based on observed river discharges. The ensemble mean from CORDEX_RCMs (Coordinated Regional Climate Downscaling Experiment _ Regional Climate Models) was then used to force SWAT model at four specific global warming levels (GWLs, i.e. 1.5 °C, 2.0 °C, 2.5 °C, and 3.0 °C) and at the baseline (1971-2000) to assess potential impact of climate change on the hydrological response of the area. The results indicated that the NCHA had experienced 61% moderate rainfall and 39% high concentration rainfall. There has been a decreasing trend in rainfall and 1969 was the probable change point for the whole area. However, some of the sub-basins had experienced a change in climate earlier (1934). That suggested the heterogeneous nature of the area. The rainfall over the area in the post-change point has been on the decrease for the rainy seasons and annual rainfall indicating probable drier period than wet in the future. A warming trend with an upward shift of 0.43 o C after the shift change in 1980 had been witnessed and the change in the annual temperature is characterized by a SW-NE orientation. Wet conditions alternated by pockets of droughts which were severe in 1973 and 1983 had been experienced. The results of the calibration (R 2 = 0.83; NSE = 0.68) and validation (R 2 = 0.83; NSE = 0.74) for the discharged data were quite satisfactory. The spatial distribution of the water resources varied greatly across the area. However, the water components (blue and green water resources) are more available at the downstream. The patterns of the variability suggest the influence of precipitation, soil properties and land use type. The CORDEX_RCMs effectively simulated climate variables. The future projections (1950-2100) show increase in annual streamflow at GWLs over the baseline, though early rainy season decreases. Similarly, all other water balance components (except ET) also increase. The implication is that while there may be lack of water in the early rainy season, the flood event presently being witnessed in the late rainy season may become aggravated. This study provides a guide for an effective rain-fed agricultural planning and optimal water use within the basin.