Intra-Seasonal Rainfall Variability and its Implications on Streamflow in the Oti Basin, West Africa

Abstract

This study assessed the intraseasonal rainfall variability and its implications on streamflow in the Oti River Basin in West Africa. The specific objectives were to: (i) determine the intra-seasonal rainfall variability and trends for the historical period (1981-2010) and future period (2021-2050) in the Oti basin. (ii) determine the projected changes in intra-seasonal rainfall variability indicators in the Oti basin for the future period (2021-2050) and (iii) assess the impact of intra-seasonal rainfall variability on streamflow using the Hydrologiska Byråns Vattenbalansavdelning (HBV) model. The analysis was performed using high-resolution and quality-controlled climate data from the Ghana Meteorological Agency, National Meteorological Service of Togo, Meteorological Department of Benin, Climate Hazards Infrared and Precipitation Stations (CHIRPS), and NASA-POWER. The analysis for the future period was performed under RCP4.5 and RCP8.5 scenarios using a multi-model mean ensemble of eight bias-corrected regional climate models based on the quantile-quantile mapping method and from the Coordinated Regional Climate Downscaling Experiment (CORDEX-Africa). The study also used discharge data from the Water Resources Commission, Ghana, Water Resources Directorate (Togo), National Hydrological Service (DGIRH) of Burkina Faso and the Direction Generale de L’eau, Benin. The study revealed a likely decline in mean rainfall in the future by about 103.6 mm/yr (RCP4.5) and 45.9 mm/yr (RCP8.5). It also projected a decline in mean rainfall during the rainy season by about 90.8 mm/yr (RCP4.5) and 34.6 mm/yr (RCP8.5). The study further found a late onset of rains in the basin by about +16 days (RCP4.5) and +15 days (RCP8.5) which would possibly have a rippling effect on rainfall cessation in the basin causing it to occur earlier by a decrease of about 21 days under both RCP4.5 and RCP8.5 scenarios, with regards to the historical period. This subsequently resulted in shortening the length of rainy season by about 37 days (RCP4.5) and 36 days (RCP8.5). The number of wet and dry days are projected to increase and decrease respectively. Additionally, the streamflow of the basin was evaluated and projected for the future using the HBV hydrologic model. The mean multi-model ensemble predicted an increase in mean monthly streamflow at all sub-basins in the future (2021-2050) relative to the historical period (1981-2010) except Arly which is anticipated to have a decrease in its streamflow by about 2.79% (RCP4.5) while it will increase by about 31.17% (RCP8.5). The anticipated variations in the rainfall could affect the scheduling of agriculture, its sustainability and could place the basin a water-related stress.