Comparison of groundwater recharge estimation methods for the semi-arid Nyamandhlovu area, Zimbabwe

The Nyamandhlovu aquifer is the main water resource in the semi-arid Umguza district in Matebeleland North Province in Zimbabwe. The rapid increase in water demand in the city of Bulawayo has prompted the need to quantify the available groundwater resources for sustainable utilization. Groundwater recharge estimation methods and results were compared: chloride mass balance method (19-62 mm/year); water-table fluctuation method (2-50 mm/year); Darcian flownet computations (16-28 mm/year); C-14 age dating (22-25 mm/year); and groundwater modeling (11-26 mm/year). The flownet computational and modeling methods provided better estimates for aerial recharge than the other methods. Based on groundwater modeling, a final estimate for recharge (from precipitation) on the order of 15-20 mm/year is believed to be realistic, assuming that part of the recharge water transpires from the water table by deep-rooted vegetation. This recharge estimate (2.7-3.6% of the annual precipitation of 555 mm/year) compares well with the results of other researchers. The advantages/disadvantages of each recharge method in terms of ease of application, accuracy, and costs are discussed. The groundwater model was also used to quantify the total recharge of the Nyamandhlovu aquifer system (20 x 10(6)-25 x 10(6) m(3)/year). Groundwater abstractions exceeding 17 x 10(6) m(3)/year could cause ecological damage, affecting, for instance, the deep-rooted vegetation in the area.