Numerical Groundwater Flow Modeling for Sustainable Groundwater Resource Development: The Case of Sibilu River Catchment Abay Basin
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Date
2021-12
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Addis Ababa University
Abstract
Groundwater plays an important role in Ethiopia as a significant source of water for domestic, bottling, industrial and /or agriculture uses. In rural areas, which cover more than 85% of the population of Ethiopia, development and utilization of groundwater is most common.
When the groundwater is used for irrigation, industry and domestic purposes, it is necessary to maintain the groundwater reservoir in state of dynamic equilibrium over a period of time and the water level fluctuations have to be kept within a particular range over the wet and dry seasons. To do so detail groundwater study, monitoring and quantification through different analysis methods like numerical modeling is a crucial and this study focuses on the numerical groundwater modeling of Sibilu River catchment under the steady state flow conditions. The catchment drains about 650 km2 land areas and covered by alluvial deposit and different types of volcanic geological formation.
The surface hydrologic dynamic was modeled by using ArcSWAT integrated with ArcGIS. The area was classified into 1215 HRUs, to simulate the groundwater recharge distribution, evapotranspiration and interactions between Stream network and Aquifers in each Hydrologic Response Units. Accordingly, the recharge and ET distribution of the area were 292.2 and 253.01 mm per annum. As estimated with Base flow separation and CMB method in the validation process the recharge were 318.8 and 304.9 mm/year which is indicating the good agreement between the models with slight differences and the source of the recharge in the catchment were found to occur from direct precipitation and riverbed infiltration.
Furthermore, Processing Modflow pro (version Pm 5) has been used to simulate the subsurface dynamism of groundwater of the area. The area discritized by 32,185 finite-difference Block Centered rectangular cells (205 rows, 157 column, 200m*200m cell size) and 350 m thick single upper layer and then in and out flow system of groundwater has been modeled.
Zoned horizontal hydraulic conductivity has been used through the trial-and-error method to calibrate the model using the observed and simulated hydraulic heads. The water budget of the catchment reached at equilibrium conditions and annually about 181 and 20.3 million cubic meters of water flowing into the Aquifer system of Sibilu catchment from Precipitation Recharge and River leakage respectively. Similarly, a total of 201.25 million cubic meters of water is flowing out of the Aquifer system of the study area through head dependent boundary (0.91 MCM), Wells abstraction (5.6MCM), Drains (5.47 MCM) ,ET (164 MCM) and leakage of groundwater to the river course (25.1 MCM) in a period of a year. Discharging of groundwater from the Aquifer system is occurred through unregulated Well abstraction, spring discharges, ET and groundwater outflow to the stream. Intensive well abstraction for Water bottling, different industrial uses, Agro processing, commercials, irrigation, hotels and domestic water supplies are the main groundwater abstraction means in the Sibilu Catchments due to an anthropogenic activities.
Under increasing abstraction and reducing recharge rates scenarios to see the impact of future potential stresses on the groundwater resources, the result shows that there is a significant groundwater level drop (on average by 31.3 meters for 50% abstraction rate increment and 19.31 meters for 10% recharge reduction) and there is a decrease in base flows to the Sibilu River, evapotranspiration flux from the area and Groundwater outflow through head dependent boundary. However, except under recharge reduction with 10% scenario the River leakage to the Aquifer system is increasing positively.
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Keywords
Abay, Sibilu catchment, Central Ethiopia, Groundwater, Numerical, Modeling