Abstract
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 is one of the growth corridors of the country, where huge waterworks infrastructure is developing. Estimating groundwater recharge at required spatial and temporal scales is a challenge in groundwater management, sustainability and pollution studies. In this study, the WetSpass model is developed at 90 m grid resolution. The spatial recharge map by WetSpass 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 of rainfall, respectively. The mean annual recharge ranges from 0 mm to 1085 mm (0 % to 57 % of the rainfall): 0 mm at water bodies and highest on flat, sandy loam soil and bush land cover. Similarly, a high range of recharge is also noted using WTF and CMB methods showing the strong heterogeneous nature of the hydro(meteoro)logical characteristics of the area. Generally, the recharge is found higher in southern and eastern catchments and lower in the northern catchments, primarily due to higher rainfall amounts in the former parts. A fair general correlation between the recharge by WTF and WetSpass is found. WetSpass is effective in aquifers where diffuse recharging mechanism is the predominant type and recharge is controlled by rainfall. It is less effective in the storage-controlled flat floodplain alluvial and fractured rock aquifer areas. In these areas, the point estimates by WTF and CMB are effective and can be considered as reliable values. The land use change from 1986 to 2014 brought a relatively small hydrological change in recharge although the land use has changed significantly.
Highlights
Groundwater recharge is all water that reaches the groundwater aquifer from any direction (Scanlon et al, 2002)
No recharge at water body is the general assumption of the WetSpass model
The WetSpass model applied is a quasi-steady state that used long-term average seasonal rainfall amounts, though the rainfall characteristics vary within fine time scales
Summary
Groundwater recharge is all water that reaches the groundwater aquifer from any direction (Scanlon et al, 2002). Understanding recharge processes and its quantification is vital for proper management and protection of the limited groundwater resources (Ferede et al, 2020; Healy and Cook, 2002; Uugulu and Wanke, 2020). It is one of the most difficult water balance components to be evaluated with acceptable accuracy (Hornero et al, 2016). Important considerations in choosing a technique include space/time scales, range, and reliability of recharge estimates which in turn depend on the goal of the study (Scanlon et al, 2002). Groundwater resources assessment of an area may be achieved at a small space/time scale but flow and contaminant modeling need quantification of recharge amounts at large spatial as well as temporal scales
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