Groundwater Flows Modeling of the Upper Catchment, Abbay Basin, Eastwollega Zone, Oromia Regional State, and Western Ethiopia.

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Date

2019-05-05

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Addis Ababa University

Abstract

The problematic area selected is finding in the East Wollega Zone, Western Ethiopia called upper Anger River Catchment of Abay Basin. The total surface area of the catchment is 7159.21 Km2. The purpose of this study has to understand the Regional Groundwater flow and the response of groundwater system to different scenarios with the help of numerical groundwater flow modeling. One layer and unconfined Aquifer system simulated under steady state flow condition by using MODFLOW. The boundary of the study area delineated from ASTR DEM with the 30m resolution by using Arc SWAT (Soil Water Assessment tool) software. The input parameters for the model gathered from the well completion reports. SWAT software used to calculate the regional recharge rate of the upper Anger River Catchment. Annual recharge to the deep and shallow aquifer calculated by the model is 388.84 mm/year. The general Groundwater flow direction is from the north- east to south- west by following the topographical trend and structural orientation. The model calibrated by Trial and Error method using groundwater contours produced from different heads collected from 7 observation wells. The calibration shows that about 97.75% of the simulated heads with the calibration targets and the overall water of the simulated model result show 8889163.3130. Model sensitivity analyses conducted with changing of the hydraulic conductivity and recharge the model is more sensitive to them. Change in the hydraulic head by 20%, 30%, 40%, 50% result for the RMSE changes in the head from the calibrated value. In addition, the same change happens in the recharge like the difference in hydraulic head. The result of the numerical simulations indicates while increasing well withdrawals by 25%, 50%, 75% and 100% respectively it also resulted to the change of RMSE heads by 8%. Water Budget results of the model shows that the reviled groundwater recharge comprise by 0.5% and 97.75% by river leakage of the total water input for the total study area. The spatial distribution of groundwater decline is limited towards the river. However, there is hydraulic connection between the sediment bed and Anger River.

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Keywords

Anger Catchment, Agro Ecological Zone, Water Balance, Nekemte, Ethiopia

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