GROUNDWATER RESPONSE TO WEATHER VARIABILITY IN A POOR AQUIFER UNIT: AN EXAMPLE FROM TROPICAL BASEMENT COMPLEX ROCK OF NIGERIA

More than 50 % of Nigeria is underlain by basement complex rock which is a poor aquifer unit and evidences abound that the climate of Nigeria is changing. The posing question is how this poor aquifer will respond to the vagaries of climate variability and change. However, understanding the response of groundwater to climate variability and change in Nigeria will be hampered by dearth of data, because the nature of change in groundwater is not monitored. On this basis, the study tried to understand how groundwater responds to weather variability in a poor aquifer unit of Ilara-mokin and its environs in the tropical area of Nigeria. Rainfall and temperature data for forty years (1973-2012) were collected from NIMET and groundwater level were monitored in the area for two years (2012-2014).The general trends in rainfall and temperature received in the last forty years were examined using regression analysis and moving average. The dry and wet episodes were also examined using Standard Rainfall Anomalies Index (SAI). Also, the percentage changes in the rainfall and temperature received were determined using reduction pattern analysis. The response of groundwater to weather variability was however established using Pearson Moment Correlation and multiple regression analysis. The results of the analyses revealed an average of six years dry episode every decade in the last 40 years. The temperature of the study area is increasing in the last 20 years. Groundwater responded negatively to temperature but positively to rainfall in the area. Rainfall and temperature accounted for 67 % of variability in monthly groundwater level. This study is a good starting point in understanding groundwater response to climate in poor aquifer units of Nigeria despite the dearth of data.