Abstract

A quantitative evaluation of spatial and temporal distribution of groundwater recharge is a pre-requisite for the management of ground water resources system in an optimal manner. The amount of groundwater recharge depends upon the rate and duration of rainfall, as rainfall is the principal means for replenishment of moisture in the soil water system and recharge to ground water. This paper investigated the relationship between rainfalls and groundwater recharge within Ona River basin, southwest Nigeria, using soil moisture balance and water table fluctuation. Analysis of rainfall trends within the Ona River basin suggests that there is considerable high annual rainfall occurrence, with a mean of 1623.48. It must be noted that the mean annual lost due to evapotranspiration of 1361.68 mm is very high when compared to the rainfall (83.9%). The results obtained from the soil moisture balance when considering the three dominant soil types within the basin, that is, sandy loam, clay and find sand, having water capacity of root zone value of 70, 70, and 50 respectively, suggests that groundwater recharge follows a positive trend as the corresponding rainfalls. However, empirical relationships of: y = 0.540x – 606.2, with a coefficient of determination (r²) value of 0.719, for sandy loam and clay; and y = 0.552x – 621, with a coefficient of determination (r²) value of 0.726 for fine sand was established for the basin area. On the other hand, recharge ranging from 220.25 to 40.50 mm was computed from the water table fluctuation method.

Key words: Rainfall–recharge relationship, soil types, soil moisture balance, Ona River basin, water table fluctuation.