Numerical Groundwater Flow Modeling of Dijil River Catchment, Debre Markos Area, Ethiopia
Dijil River catchment is a sub-catchment of the Abay drainage basin and covers 138.28 km2. This paper presents numerical groundwater flow modeling at steady-state conditions, in a single-layer aquifer system under different stress or scenarios. A numerical groundwater flow models represent the simplification of complex natural systems, different parameters were assembled into a conceptual model to represent the complex natural system in a simplified form. The conceptual model was input into the numeric model to examine the system response. Based on geologic and hydrogeological information, confined subsurface flow condition was considered and simulated using MODFLOW 2000. The model calibration accounts matching of 24 observation points with the simulated head with a permissible residual head of ±10m. The sensitivity of the major parameters of the model was identified during the calibration process. According to the simulated water budget in the model, the simulated inflow is found to be 1.2791870E+05 m3/day which is nearly equal to the simulated outflow of 1.2791755E+05 m3/day with the difference being only 1.1484375E+00 m3/day. Water budget analysis reveals that outflow from river leakage accounts for 92.8 % of the total outflow and 14.1 % of the total inflow comes from the river leakage in the study area. Three scenarios of increased withdrawals and one scenario of altered recharge were used to study the system response. Accordingly, an increase in well withdrawal in scenario-I (existing wells pump simultaneously), scenario-II (existing drilled wells yield withdrawal increased by 30%), and scenario-III (additional eight wells having expected yield of 30 l/s drill and pump) resulted in an average decline of the steady-state water level by 1.06m, 1.68m, and 4.46m, respectively. They also caused the steady-state stream leakage to be reduced by about 2.93%, 4.58%, and 11.23%, and subsurface outflow by 9.41%, 14.67%, and 37.86%, respectively. A decrease in recharge by 25% and 50% results in a decrease of the head by 6.1m and 13.4m respectively, and a stream leakage decrease by 20.3%, and 40.3% respectively as compared to the simulated steady-state value. Therefore, adequate groundwater level monitoring wells should be placed in the catchment to control the total abstraction rates from the aquifer and fluctuations in groundwater levels.
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