Groundwater recharge estimation from multiple independent methods in the fractured hard rock aquifers in the Densu River Basin, Ghana

Knowledge of recharge is essential for efficient and sustainable management of groundwater resources. The application of different groundwater recharge estimation methods, especially those that are based on independent parameters and/or datasets enhances confidence in the process. In this study, the Chloride Mass Balance (CMB), Water Table Fluctuation (WTF), and two Base Flow Filter techniques (BFF) have been used to quantify groundwater recharge in the Densu River Basin in southwestern Ghana. Based on the three methods this study finds that groundwater recharge in the basin is in the range of 27–338 mm/year, representing 4–24% of the total annual rainfall. The basin averages were estimated at 34, 150, and 80 mm/year representing 3%, 15%, and 7% for CMB, WTF, and BFF respectively. Overall, the average groundwater recharge for the basin from the three methods is approximately 144 mm/year, representing ~ 15% of the rainfall. The CMB and BFF methods largely agree on basin average recharge, whereas the WTF methods generate much higher average recharge estimates. The CMB and WTF do however agree on the upper bound of recharge estimated locally in wells (around 400 mm/year), which is not described by the BFF method which only operates on sub-basin resolution. The CMB method revealed weaknesses in the lower bound estimates (underestimated) compared to the other methods. For all methods, the highest recharge estimates were obtained in the northern and high-elevation areas, whereas the low estimates occurred in the south of the basin. Temporally, recharge in the basin begins around March/April, the beginning of the rainy season, and peaks around June and October. Although the major rainy season peaks around May/June in the basin, the highest groundwater recharge occurs in the peak of the minor rainy season in October, which is attributable to the 2.5 months lag time between rainfall and recharge. About 90% of the groundwater recharge occurs between May and November annually and may be the ideal period for large groundwater abstractions in the basin.