Numerical Groundwater flow Modeling 0f Walga Darge Catchments
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Date
2018-06-01
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Addis Ababa University
Abstract
the Area Concerned In this Study Is Situated In the Northern Parts of Omo-Gibe River Basin. the Total Surface Area of the Catchments Is About 2915km2. In this Study, The Numerical Groundwater Flow Model Was Used to Study Regional Groundwater Flow and the Response of the Groundwater System to Different Scenarios. The Aquifer System Was Considered as a Single Layer and Simulated Under Steady State Condition Using MODFLOW. The Study Area Boundary Was Delineated From 30m Resolution of Digital Elevation Model (DEM) Using arc SWAT Software With Arcgis Interface. the Recharge Rate Was Estimated Using the Semi-Distributed Hydrologic Model Soil Water Assessment Tool (SWAT) Which Account For the Spatial Heterogeneity In Climate, Topography, Land Use And Soil Of The Catchments. the Long-Term Annual Recharge to Shallow Aquifer Estimated By the Model Is 315mm/Year. the General Trend of Groundwater Flow Direction Was Determined As It Is From the North-East To South-West Following Topography, With Some Local Variations. Model Calibration Was Carried Out By Manual Trial and Error Calibration Method Using Head Observations From 121 Wells. the Model Was Calibrated to Mean Error -2.43m, Absolute Mean Error 10.82m And Root Mean Squared Error 13.47m. the Simulated Water Budget of The Entire Model Domain Is 86532382m3/Year Inflows and 86530619m3/Year Out Flow With Difference the Of 1762.27 M3/Year. Sensitivity Analysis Was Made By Taking Hydraulic Conductivity and Recharge to Understand the Uncertainty In The Calibrated Model Caused By Limitations In The Estimates Of Aquifer Parameters and Stresses. Decreasing and Increasing the Hydraulic Conductivity By 20% And 40% Resulted in MAE Head Changes From the Calibrated Value By 24.90, 15.54, 14.26 And 23.86 Respectively. Similar Changes In Recharge Resulted In MAE Head Changes From the Calibrated Value By 39.66, 30.32, 19.37and 21.24.the Model Is More Sensitive To Recharge Compared to Hydraulic Conductivity. Scenario Modeling Was Made With Increased Groundwater Abstraction Rates And Decreased And Increased Aquifer Recharges As Part of the Forecast. the System Responses to Scenarios Were Evaluated By Resulting Changes In Water Level And Groundwater Outflow From the Model Domain. Increasing the Steady-State Well Discharges Up To Fourfold Resulted In Insignificant Declines In Groundwater Levels In Most of the Model Domain. Decreasing and Increasing of Aquifer Recharges Has Significant Change In Water Level And Fluxes.
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Keywords
Gibe Basin, Modflow, Numerical Model, Scenarios Model, Water Budget, Swat