Spatial and temporal simulation of groundwater recharge and cross-validation with point estimations in volcanic aquifers with variable topography

Study region: This study is performed for the volcanic aquifers of semi-humid Lake Tana basin in northwest Ethiopia, the source region of Blue Nile basin. Study focus: estimating groundwater recharge at required spatial and temporal scale is a challenge in groundwater management, sustainability and pollution studies. In this study, the physicallybased WetSpass model is applied. The recharge by WetSpass is validated with estimations by water table fluctuation (WTF) and chloride mass balance (CMB) methods. Evaluating the groundwater recharge estimation mechanism for the volcanic aquifers lying at different topographical setting, that represents wide part of the world groundwater aquifers, and suggesting more appropriate methods will benefit different similar studies. New hydrological insights for the region: the mean annual rainfall, recharge, surface runoff, and evapotranspiration are estimated at 1431 mm, 315 mm, 416 mm, and 770 mm, respectively. The recharge varies from 0% to 57% of the rainfall. A high variation is also noted using WTF and CMB methods showing the strong heterogeneous nature of the hydro(meteoro)logical characteristics of the area. WetSpass is effective in aquifers where diffuse recharging mechanism is the predominant type and recharge is controlled by rainfall. Hence, it is found less effective in the storagecontrolled flat floodplain, alluvial and fractured rock aquifer areas. In these areas, the point estimates by the WTF and CMB are effective and can be considered as reliable values.