Analysing Climate Change Projection on Water Availability for Rainfed Agriculture in Awun Basin, Kwara State, Nigeria

Abstract

This study investigates the impact of climate change projections on water availability for rainfed agriculture in Awun basin, Kwara State, Nigeria using high resolution (25 km spatial resolution) RegCM4 simulations. The study was guided by four (4) specific objectives which are (i) assessment of crop types, cropping patterns and farmers’ perception of water availability, (ii) downscaling/bias correction of climate scenarios, (iii) evaluation of rainfall characteristics (onset, seasonality index, and hydrologic ratio), and (iv) assessment of the crops water requirements. RegCM4 runs for the control period and for two scenarios (RCP4.5 and RCP8.5) driven by two GCMs (MPI and GFDL) were collected at WASCAL competence centre, Burkina Faso. The simulations provided are rainfall, minimum and maximum temperature and relative humidity for the control period (1985-2004) and the scenario period (2080-2099). The observations (1985-2014) for the same parameters from the synoptic station of Ilorin were collected at NIMET, Abuja. A focus was made on major food crops (maize, sorghum, cassava, and yam) in the area. Onset of growing season was determined using Benoit method tested with HS and BMN ET models. BMN model was also used to compute ET0. The study showed that mean rainfall depth for the realistic scenario RCP4.5 will decrease by 9.6% and 13.1 % for MPI and GFDL driven runs while under the pessimistic scenario RCP8.5 the expected decreases in the mean rainfall depth are 15.2 % and 17.7 % for MPI and GFDL driven runs respectively. Minimum and maximum temperatures will increase from 1.5 0C to 2 0C for the realistic scenarios RCP4.5 and from 3.10C to 4.00C for the more pessimistic scenario RCP8.5 respectively. However the mean relative humidity will decrease by 10% by 2100s. The start of the growing season, independently of the ET model used, is projected to be late in the future. In fact the onset date is 5th May for baseline (1995-2014) while under RCP4.5 the dates are 23rd May and 2nd June for MPI and GFDL driven runs respectively and under the RCP8.5 the dates are 14th June and 9th June for MPI and GFDL respectively. A seasonality index (SI) included between 0.80 and 0.99, and a hydrologic ratio (HR) < 0.74 for all scenarios were found, meaning respectively that (i) the rainy season will get shorter and (ii) the area will get drier in the future. Results showed that the crop water need for the growing season of maize and sorghum will be satisfied while that of cassava will not. It showed that cassava could not be planted conveniently within its growing season without irrigation in Awun Basin. The present study might be helpful in explaining the plausible effects of present and future climate on crop water needs and better planning of agricultural water resources in Awun Basin.