Chloride mass balance for estimation of groundwater recharge in a semi-arid catchment of northern Ethiopia

Limited scientific information has been published on the application of tools in the geographic information system (GIS) environment to understand factors that influence groundwater recharge. The objectives of the research reported here were to understand the spatial variability of factors that influence groundwater recharge using GIS, and to estimate the amount of groundwater recharge and its spatial distribution in Illala catchment, northern Ethiopia. Reconnaissance surveys coupled with satellite imagery were used to collect data related to water, dry-deposition and hydrogeology from the study catchment. The data analysis involved geo-statistics. The chloride mass balance (CMB) method was applied to estimate mean groundwater recharge. The study catchment is distinguished by a semi-arid climate (average aridity index value of 0.35) and it is dominated by limestone-shale-marl intercalation. Mean chloride concentration in rainwater ranges from 0.4 to 1.28mg/L, while values in dry deposition vary from 1.78 to 1.82mg/m(2). Groundwater and runoff chloride concentration ranges are 1.4-31.96mg/L and 0.60-1.56mg/L, respectively. Mean annual groundwater recharge estimated by the CMB method varies from 6.1 to 288.3mm, and the mean groundwater recharge represents 11.7% of the 548mm mean annual rainfall. The CMB-derived groundwater recharge estimation showed a nearly comparable value with the recharge estimated by other approaches. More effort should be made to boost groundwater recharge using various recharge enhancing techniques such as constructing artificial recharge wells and water harvesting structures, targeting areas with the lowest recharge.