Assessment and estimation of groundwater recharge for a catchment located in highland tropical climate in central Ethiopia using catchment soil–water balance (SWB) and chloride mass balance (CMB) techniques

Two commonly applied groundwater recharge estimation techniques, namely soil–water balance (SWB) and the chloride mass balance (CMB) methods, were applied and compared to quantify and analyse groundwater recharge in the Akaki catchment, located in central Ethiopia. The semi-distributed SWB method estimated natural groundwater recharge at 105 mm/a (10 % of the mean annual areal precipitation, MAAP). The chloride mass balance method applied to the same catchment estimated mean annual groundwater recharge at 273 mm/a (25 % of the MAAP). The SWB recharge value is much less than the recharge estimated by CMB method, highlighting the importance of preferential flow path recharge mechanism which is not captured by the SWB method. In situ permeability measurements undertaken as part of this research and an earlier investigation based on environmental isotopes have demonstrated that the catchment gets recharged both from direct and preferential flow recharge mechanisms which explains the discrepancy between recharge estimated through the two methods. Therefore, SWB model result which only takes into consideration the piston-type flow recharge mechanism should be taken as the minimum possible groundwater recharge for the catchment characterized by fractured aquifers. The mean recharge estimate from the two methods represents the maximum possible recharge estimate for the catchment investigated. Although calibration of the recharge estimates was not possible, the SWB recharge estimate is found to be highly sensitive to changes in precipitation input. The findings of this research underline the fact that multiple recharge estimation methods are important to understand and capture possible recharge mechanisms and to reach to an acceptable recharge estimate.