Numerical Groundwater flow Modeling of Beresa and Gado Rivers Catchment, Debre-Birhan Area, North Shoa Zone, Amhara Regional State
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Date
2013-02-02
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Addis Ababa University
Abstract
The study area is found in Amhara regional state at the Beresa and Gado rivers catchment. Its total surface area coverage is 1062.2km2. This research is conducted with the objective of to simulate the groundwater flow system of the area and to evaluate the response of the hydrogeologic system to different stress, so that the resulting consequence on the system can be estimated. Input parameters such as hydraulic conductivity, recharge, and discharges were obtained from previous works. Based on geologic information of the study area, unconfined subsurface flow condition was considered and simulated using MODFLOW-2000. The area is characterized by Tertiary volcanic rocks and Quaternary superficial deposits. Tertiary volcanic rocks are mainly exposed on the plateau and along the NE-SW trending rift margin. Quaternary superficial deposits are mostly found on the plateau covering flat lying and gentle topography. The simulation is made in a one layer, unconfined aquifer with spatially variable recharge and hydraulic conductivity under well-defined boundary condition in a model grid design of 200 rows and 220 columns. The model calibration accounts the matching of the 19 observation point with simulated head of a permissible residual head of ±10m and 100% of the difference between the observed and calculated water level head of the study area is below 10m. The model was calibrated with variance of 14.95, mean error -0.88, absolute mean error 3.25, and root mean square value of 3.93, which is an indicator of good calibration. The calibrated model is used to forecast the groundwater flow pattern, and evaluate the behavior of the groundwater system under possible future utilization scenarios. The sensitivity of the major parameters of the model was identified during calibration process. Based on calibration process, the model is very sensitive in both changes in recharge and hydraulic conductivity and less sensitive to river bed conductance. The simulated water budget has been computed for the study area. The simulated inflow of the model is 1.3466171E+06m3/day which is nearly equal to simulated outflow 1.3465134E+06m3/day with difference 1.0364551E+02m3/day and 0.01% of error. The river leakage simulated discharge holds 94.3% of the out flow. It also contributed as recharge in to the aquifer that accounts to 54.98% of the inflow. This share of river leakage implies the recharge of the groundwater dominantly from the losing streams. The simulation result indicates that the groundwater flows from the recharge area to discharge area following the morphology of the area
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Numerical Groundwater, Modeling, Beresa and Gado Rivers Catchment, Debre-Birhan Area, North Shoa Zone, Amhara Regional State