Groundwater Recharge Estimation And Water Resources Assessment In A Tropical Crystalline Basement Aquifer

While most groundwater recharge estimation methods give reasonable long-term annual average estimates very few if any methods offer guidance on monthly recharge. In crystalline basement aquifers (CBAs) the problem is compounded by the high seasonal, intra-annual and inter-annual variability. The chloride mass balance (CMB), the daily catchment water balance (WB) and the water table fluctuation (WTF) groundwater recharge estimation methods have been used to estimate groundwater recharge in a small catchment (180 Mm2). The study concluded that though all methods yielded the same range of recharge, between 8% and 15% of annual rainfall, they show a high spatial variability with coefficients of variation of up to 65% indicating that no single point measurements of recharge is a good indicator of regional recharge. Through water balance modelling, the study also showed that renewable groundwater resources in the Nyundo catchment have a storage period in the order of 100-120 days (3-4 months) whilst daily potential interception can be as high as 5 mm/d. A combination of multiple regression and interception models suggest that it is feasible to obtain a first order monthly recharge estimate as a function of number of rain days in the month, average monthly rainfall, daily interception threshold, average root depth, effective porosity and average depth to water table