Numerical modeling of the groundwater flow system of the Gumera sub-basin in Lake Tana basin, Ethiopia

The groundwater flow contribution into Lake Tana was simulated using Gumera River sub-basin as a case study. Draining into Lake Tana, the Gumera sub-basin boundary was delineated from a 90-m Shuttle Radar Terrain Mapping (SRTM) digital elevation model (DEM) using ArcHydro tools. The sub-basin boundary served as the divide line of the groundwater flow while stream networks were used as internal drainage lines. Input parameters such as transmissivity and hydraulic conductivity were obtained from past studies and experts' knowledge. Based on the geological information of the sub-basin, unconfined subsurface flow condition was considered and simulated using MODular 3D finite difference ground-water FLOW model (MODFLOW). Analytical solutions were also used for comparison purposes. The result indicates that head contours are aligned to the streams showing their relationship as a subdued form of the surface water flow, which are dictated by the morphology of the basin. This suggested the need to account groundwater contribution of the sub-basins to Lake Tana as baseflow, and to estimate groundwater inflow from the floodplain separately. The contribution from the floodplain was estimated using a finite differences method which showed an inflow of 0·15 billion cubic meter (BCM) from rivers into the floodplain out of which 0·09 BCM is flowing out to the Lake Tana and 0·06 BCM stored in the aquifers of the floodplain. The stored quantity could meet irrigation demands as well as contribute to the preservation of the ecological structure and function of the area. The study suggests further research on validation of parameters through inverse modeling approaches and optimization of water allocations. Copyright © 2009 John Wiley & Sons, Ltd.