Groundwater recharge using the chloride mass balance method in the Kanye area, in southeast Botswana

Abstract

This paper reports on groundwater recharge evaluation conducted in the area around Kanye village in southeast Botswana, using the chloride mass balance (CMB) technique. Thirty-seven (37) years of rainfall data were examined to define the seasonal distribution and the average rainfall in the study area. The rainfall pattern through the twelve months of each year was further analysed using rainfall coefficient techniques. Thirteen (13) groundwater samples from different well-fields were collected during the rainy and dry seasons, and analysed for chloride and bromide concentrations. Additionally, chloride concentrations of the rainfall during the wet and dry seasons were obtained from a previous groundwater recharge evaluation study conducted in the same region. Two rainy seasons and one dry season are recognised in the Kanye area. The average annual rainfall in Kanye is 466 mm. 57% of this occurs in the first rainy season (January to April); 37% occurs in the second rainy season (October to December), and 6% occurs in the dry season (May to September). The CMB method reveals that the groundwater recharge ranges from 4.8 to 26.6 mm/yr. The recharge in the first rainy season is estimated to be 5.70%, whereas the recharge in the second rainy season and the dry season are 4.09 and 1.03%, respectively, of the average rainfall. The results show that large amounts of water are lost from the area in different forms after the rains. Harvesting the water lost as surface runoff could enhance the groundwater recharge of the Kanye area.