APPLICATION OF NUMERICAL MODEL FOR GROUNDWATER FLOW SYSTEM ASSESSMENT IN UPPER AWASH RIVER BASIN (CASE STUDY MOJO RIVER CATCHMEN

Abstract

The project area, Mojo Catchment, is located 40k.m southeast of Addis Ababa in Oromiya Regional State. Its geographical location is between 38°49' and 39°17' longitude and 8°25'and 9°05' latitude. It covers an area of about 2104.5 km 2. The study targeted numerical simulation of the groundwater flow system of the Mojo River Catchment there by to evaluate the response of the hydrologic system to different scenarios so that the resulting consequence on the system might be projected. The modeling process was accomplished using groundwater flow modeling software, MODFLOW 1996 (McDonald and Harbaugh, 1988) The water budget of the whole model domain was calculated with a percent discrepancy of 0.12. We found the total inflow is 1051.83MCM per year (2.88X10 6 m 3 /day) and the total outflow is 1050.53 MCM per year (2.88x10 6 m 3 /day) This study sheds light, on the nature of the recharge water, on the subsequent ground water movement through the storage aquifer; it also produced a calibrated ground water-flow model that can be used in predicting the effects of future recharge and ground water extraction operations in the Mojo River Catchment aquifer. The results of sensitivity analysis show that small errors in the values of the aquifer properties to which the model is most sensitive which in this case recharge and hydraulic conductivity can have significant effect on model simulation. However, other properties such as pumpage and river bed conductance can be varied in magnitude with little effect on the result. The system respond in such a way that the water level and base flow decrease upon the increment of water withdrawal beyond 25 percent.