Rainfall and potential evapotranspiration patterns and their effects on climatic water balance in the Western Lithoral Hydrological Zone of Nigeria

Abstract

This study examined the climatic water balance and aridity in the Western Lithoral hydrological zone of Nigeria. Monthly climatic data from 1976-2015 collected from six Nigeria Metrological Agency (NIMET) stations located in Ikeja, Ibadan, Akure, Abeokuta, Osogbo and Ilorin were used. The climatic variables are rainfall amount, temperature, sunshine hour, relative humidity and wind speed. The FAO Penman-Monteith method was adopted in estimating the potential evapotranspiration (PET). Water balance was determined from PET and rainfall amount. The spatial and temporal pattern of rainfall amount and PET were evaluated using universal kriging interpolation method, Mann-Kendall and regression trend analysis. The results of the Mann-Kendall and regression trend analysis revealed a statistically significant (Z= 2.74, P= 0.005) upward trend in rainfall amount between 1976 and 2015. Similarly, a statistically significant (Z= 4.40, p= 0.001) increasing trend was observed in PET during the same period. Six months of water surplus (May-October) and six months of water deficit (November-April) were identified. Overall, 27 years of water deficit was discovered over the 40 years period. Water balance pattern revealed the highest annual mean water surplus (324.51 mm) in the hinterland of Idanre, Akure, and Owo rather than the coastal city of Ikeja which can be attributed to orographic effect in the hinterland. The study area with a mean Aridity index (AI) of 0.94 can still be regarded as humid environment, although the spatial variability of AI indicates that the northern part is tending towards a dry sub-humid condition (AI= 0.76). This study recommends conservation of water surplus in the wet months for the augmentation of deficit in the dry months; this will not only boost agricultural production, but also alleviate water supply problem in the study area. Keywords. Aridity index, potential evapotranspiration, water balance.