Spatial and temporal simulation of groundwater recharge amount and cross-validation with point measurement-based estimations on a tropical basin comprising volcanic aquifers: case study of Lake Tana basin, northwestern Ethiopia

A physically distributed water balance model called WetSpass is applied to estimate the recharge for the semi-humid Lake Tana basin in northwest Ethiopia. Lake Tana basin, one of the major sub-basins of the Upper Blue Nile River basin, covers 15,077 km² of which 3,156km² is the lake water body. The basin is regarded as one of the growth corridors of the country, where huge waterworks infrastructure is being developed. The basin has complex volcanic aquifer systems due to the multi-stage volcanism of the Cenozoic and Quaternary eras comprising many dikes, extended volcanic necks, and centers. Hence, estimating hydrological terms such as groundwater recharge considering the high basin physical heterogeneities is difficult, though highly important. In this study, the WetSpass model is developed, and recharge surface, surface runoff, and evapotranspiration at 90 m grid resolution have been developed. The spatial recharge map is cross-validated with water table fluctuation (WTF) and chloride mass balance (CMB) methods. The mean annual recharge, surface runoff, and evapotranspiration over the whole basin using WetSpass are estimated at 315 mm, 416 mm, and 770 mm, respectively. The mean annual recharge ranges from 0 mm to 1085 mm: 0 mm at water bodies and highest on highly fractured Quaternary basalt. Similarly, a high range of recharge is also noted using WTF and CMB methods showing the strongly heterogeneous nature of the hydro(meteoro)logical characteristics of the area. Generally, the recharge is found higher in the southern and eastern catchments and lower in the northern catchments, primarily due to higher rainfall amounts and highly permeable geological formations in the former parts. A fair general correlation between the recharge by WTF and WetSpass is found. However, WetSpass is more effective in the highland areas where the recharge is controlled by rainfall, while the WTF method is more effective in the storage controlled flat floodplain area. CMB is applied in a less spatially distributed way, and hence, the spatial performance of the method is not well evaluated. However, logged water infiltration in the floodplains, and transpiration from the groundwater in shallow water table areas have disturbed the estimated recharge by the CMB method. The land-use change from 1986-2014 brought relatively small hydrological change, although the land use has changed significantly.