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This study develops a groundwater model to assess the flow system and interactions with surface water in the Little Akaki watershed, aiming to optimize water resources management.
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Groundwater Modeling for Integrated Water Resources Use in Little Akaki Watershed, Ethiopia Presented By: Mesfin Benti Tolera Mesfin Benti Tolera1,2, Il-Moon Chung1,2, Sun Woo Chang1,2 1Civil and Environmental Engineering, University of Science and Technology (UST), Daejeon 34113, South Korea 2Land and Water Resources Research, Korea Institute of Civil Engineering and Building Technology, South Korea Contributors:
Content • Introduction • Study area • Methods • Recharge Estimation using SWAT model • Developing groundwater conceptual model • MODFLOW Model calibration • Result and Discussion • Conclusion
Introduction • Rapid population growth, urbanization and increased demand of water in domestic and industrial production have led to fresh water shortage in many parts of Ethiopia and to an increasing dependence on groundwater. • Integrated use of surface and groundwater resources is essential to provide reliable water supply and to sustainably manage the water resources. • In the Akaki watershed, groundwater pumping contributes for more than 25% of the water supply to Addis Ababa City, the capital of Ethiopia. • With continuous increased withdrawals from groundwater reservoirs results in systematic or continuous lowering of water table. Hence, the need for better management of water resources is crucial. • Groundwater modelling is an essential tool to evaluate the groundwater flow, groundwater surface water interaction, and quantifying its potential.
Intro…. • In this study, a steady-state model is developed for the aquifer in the Little Akaki catchment. • The main objective of this study is to investigate the groundwater flow system in the Little Akaki catchment and its interaction with surface water. • To optimize conjunctive use of groundwater and surface water and to assess the risk of contaminations in the Little Akaki watershed, this study analyzed • the groundwater flow pattern in the area • the surface-groundwater interactions and • spatial locations where groundwater discharge to streams or stream loses to groundwater
Study Area • Little Akaki catchment is located in the central Ethiopia. • It is located between UTM grids 4513149 m to469476 m East and 991929 m to 1009619 m north. • The elevation of the catchment area varies from 2370 to 3368 m-asl. • The physiographic components in this area are rolling plains, valleys, steep river banks, hills, and mountains. • Annual rainfall of around 1,200 to 1,300 mm. • In addition to the river, Gefersa lake is the surface water reservoirs available.
Methods Part I: Recharge estimation using SWAT Model • Model Inputs • ASTER global DEM v 2.0 (30m x 30m) from USGS Global data Explorer website (freely available) • GlobeLand30 (30m x 30m) released freely by government of China • Harmonized World Soil Data Base (HWSD) v 1.2 (30 acr sec x 30 acr sec) (freely available) • Conventional weather data from National meteorological Agency of Ethiopia • Daily streamflow data from Ethiopia Ministry of Water, Irrigation, and Electricity department of Hydrology • A SWAT model having 21 sub-basins and 229 HRUs was built using ArcSWAT 2012 for Little Akaki watershed. • SWAT model calibration was made in SWAT-CUP, SUFI-2
Part II. Developing Groundwater Conceptual Model • The initial step in creating a conceptual MODFLOW model is to define the boundary of the model. • The areal boundary was developed from a shapefile made by using the ArcGIS and then imported into GMS as GIS layer. • The boundary of all the other layers was made by duplicating the boundary of the model and specifying the properties of each in the Coverage Setups. • The boundary conditions selected based on the actual condition of the site.
MODFLOW Model Discretization • A conceptual steady-state groundwater flow model for the Little Akaki watershed (134 km2) was built using MODFLOW-2005. • The model rows and columns size assigned automatically because of refinement tool. • The grid cells outside and inside the model domain were designated as “inactive” and as “active” respectively. • The model developed for one layer aquifer. • The top surface elevation was used as the initial head of the model.
Boundary conditions • The considered boundary conditions are: • A general head boundary package was used to simulate groundwater outflow through the outlet of the basin. • Specific head boundaries was defined for Lake Gefersa. • The catchment boundaries in the western, northern, and eastern part of the catchment are taken as a no-flow boundary. • The rivers are handled with river package. • The tributary rivers are considered with a drain package.
Boundary conditions (Continued) • Creating the wells • A GIS shape file containing the Well information was imported into GMS and converted to feature object format. A well refinement was assigned to each well. • River Package • River (RIV) Package in groundwater modeling is to simulate the effects of flow between surface-water features and ground-water systems. • River-aquifer seepage is simulated between each reach and the model cell that contains that reach. • Recharge zone • The recharge zone is obtained from the SWAT sub-basin classification, and the sub-basin average values estimated using SWAT was used as input for MODFLOW.
MODFLOW Model Calibration • The MODFLOW model was calibrated using automatic calibration tool, PEST for hydraulic conductivity using observed water table and estimated baseflow at the out late.
Results and Discussions Evaluation of Model Performance: The SWAT model monthly streamflow simulation model performance was evaluated based on basic statistics such as RSR, R2, NSE, and PBIAS.
Estimated average basin recharge (m/day) • The groundwater recharge estimated ranges for the 21 sub-basin ranges from 0.0112 to 0.11 mm. • The major recharge from the aquifer comes from the precipitation and river channel losses. • High recharge values were observed in forest dominated areas and low values obtained in urban dominated land use classes.
Parameter Estimation • Hydraulic conductivity values were estimated through calibration process for zones specified using PEST • The hydraulic conductivity result from calibration shows higher values below the Gefersa river which could be due to the presence of Geological faults (Ayenew et al. 2006).
Calibration performance evaluation RMSE =4.415 R2 = 0.9981 • A scatter plot of measured heads against simulated heads and observed head against residual heads showed the calibration performance. • The scatter plots are observably examined to see how much the points in a plot differed from the straight line. • Additionally, the calibrated model results were evaluated statistically by evaluating with the common methods of error quantifications, Root Mean Squared Error.
Groundwater Head, Flow Pattern and Interactions • The calibrated model-predicted groundwater contours. It shows that flow converges from all sides towards the rivers. • The model estimated groundwater head distribution slightly higher than the regional groundwater head made for the Akaki basin by Ayenew et al. (2008).
River-aquifer interactions • The interaction between rivers and groundwater indicates that: • flow occurs from the rivers into aquifers above the Gefersa Lake in river 1 (R1) and above an elevation of 2375 masl in the second river (R2) • Flow from the aquifer to the rivers in the downstream courses of the rivers. R1 R2
Conclusion • In the little Akaki watershed, the model simulations indicate that groundwater converges towards the outlet, near the Akaki well field. • The groundwater contours indicated that the water table of the groundwater varies with the topography. • The hydraulic conductivity value near the center of the watershed, below the Gefersa lake, shows high level of sensitivity to the water table indicating there could be significantly different geological feature in those areas. • The study clearly indicated areas of interactions between the river and groundwater.
