Numerical Groundwater Flow Modeling of the Lake Tana Basin, Upper Nile, Ethiopia

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Addis Ababa University


The Lake Tana basin embraces surface area of some 15,151km2 with a prominent hydrological feature of Lake Tana that covers 3042 Sq km about 20% of the basin. The basin also embarks a number of small streams feeding the lake, Gilgel-Abay, Rib, Gumera and Megech Rivers. Lake Tana Basin is modeled with a help numerical ground water flow Modflow 2000 to simulate hydraulic head distribution of the aquifer system. It is conceptualized with single unconfined homogeneous aquifer with the water budget of 1,756,791,794.90 M3 /Y contributed from the precipitation with recharge rate ranges 0.00000520 M/D to 0.00083730 M/D. The major hydrological feature perennial stream, Lake and ground water withdrawal considered as river, GHB and Well boundary respectively. The hydraulic parameter and recharge is spatially distributed with the available data base. the model is calibrated with more of matching of simulated and estimated hydraulic head contour of 147 well located in and out of the region under study. And residual of not greater than 0r less than 20m of 37 well constructed 2003 after wards. The model is calibrated with absolute residual mean of 13.42 M and residual standard deviation of 12.5M. The simulated water budget is surplus the estimated one, 1,756,791,794.90 M3/Y, by 52,212,238.60 M 3/D with 2.9% deviation not too far to approximate the field situation. The base flow discharge of simulated model is 1,563,6171,42.54 which holds 86.43% of the out flow and the major rivers recharge the aquifer with 7, 043, 7495.6 M3/Y 3.89% of the simulated inflow. The river and aquifer interaction implies huge amount discharge of ground water to the dominantly gaining streams. The hydraulic profile simulation head, under steady state, has resulted ground water flow toward major stream and finally to converge to the Lake tana. The lake Tana is conceptualized GHB with a simulated 243,086,633.77 M3 discharge of Ground Water Lake almost 2.78% of the inflow of lake as SMEC 2007 water balance. It is a consistent result preliminary isotope studies suggested less than 7% of the total inflow is ground water (Kebede et. Al 2005). the model is found to be with relatively more sensitive to the hydraulic conductivity than recharge. The model is simulated for increasing of pumping of ground water with 25% 50%, 100%, 300%, 500%,700% 100%, 1200% and 1500%.The model response has been recognized for the average water level , ground water discharge to the river and GHB. The average water level of the system shows a drawdown starting from 0.08m with 25% increment of pumping to 7.26m with 1500% increment of ground water withdrawal. the aquifer system, with a desiccated lake, responses with a relative rise of water level by 18.17m and increase of the ground water discharge to river by 11.93 %