Estimating Groundwater Recharge Through Multiple Methods: Southern Sections of the Western Afar Rift Margin and Associated Rift Floor

Abstract

This study quantifies groundwater recharge in the southern sections of the western Afar Rift margin and associated rift floor by employing multiple methods, including Water balance, Chloride Mass Balance, WetSpass modeling, and Baseflow separation controlled by radon measurement. The mean annual groundwater recharge rates obtained from the first two methods are 114.39 mm/year, and 92.37 mm/year respectively. The spatially distributed recharge determined from the WetSpass model shows the rift margin has higher values than the rift floor, and results in a mean recharge of 100.88 mm/year. The baseflow separation method calculates 136.7 mm/year and 17.86 mm/year at the outlets of the marginal grabens and inside the rift floor respectively. The higher value suggests additional groundwater flows from the adjacent plateau to the rift margin, driven by the extended columnar basalt and transverse structures. In contrast, the presence of highly faulted Dalha basalt along the Arcuate Accommodation Zones (AAZ) promotes groundwater seepage, leading to lower values in the rift floor. However, from the previous hydrochemical and isotope analysis, groundwater flows from the rift margin to the rift floor, due to the presence of transverse structures create preferential pathways that connect these areas. Hence, the rift floor can have moderate recharge. Therefore, the WetSpass model and Base-flow separation methods, supported by other techniques, provide reliable results in tectonically active areas like the Afar Triangle and arid to semi-arid regions. Last but not least, the study underscores the importance of using multiple techniques for accurate assessments of recharge rates and mechanisms in similar areas.