Groundwater Flow Modeling of Upper Fafan Sub Basin for Managed Groundwater System
No Thumbnail Available
Date
2017-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
Upper Fafan River valley is a sub catchment of Fafan drainage basin with an approximate surface area of around 1380km2, boundary length of 255.58km and it lies at the northwester edge of the Ogaden basin. The Fafan town and other smaller village are found in this catchment. The catchment is covered with Mesozoic and Precambrian rocks of various ages that correspond to different stratigraphic units. The rocks were subjected to rift tectonics that is manifested by a number of fault systems having a general trend of the rift system (NW – SE). In this study, local groundwater flow model was established. The major purpose of this study was to determine the groundwater elevations decline of the sub basin using a numerical model under different groundwater abstraction. The Model was run as three dimensional (3D) and steady-state conditions by considering unconfined aquifer. The model inputs were obtained from field work results and from the previous works done in the area. The horizontal hydraulic conductivity from 0.5 m/day to 40 m/day and groundwater recharge 27mm/year, which constitute the key model parameters were considered as calibration parameters. The SWAT model was used to calculate the recharge rate for the zone representing the Upper Fafan River sub basin. The measured water table elevations in average 5m from the surface were used as targets in the calibration and verification of the model. The model was run for 2 pumping scenarios and 12 sensitivity analysis conditions; the model is found to be highly sensitive for hydraulic conductivity parameter. Groundwater elevation and flow direction maps were produced based on modeling results. Model calibration was carried out by trial and error calibration method using groundwater contours constructed from heads collected in 10 observation points. The calibration showed that about 98.9% of simulated heads were within the calibration target and the overall RMSE for simulated hydraulic heads is about 8.42m. Model sensitivity analysis was conducted by taking hydraulic conductivity and river leakage as the model is most sensitive to them. A change in hydraulic conductivity -50%, -75%, 50% and 75% resulted in RMS head changes from the calibrated value by 380.68%, 40.55%, 21.78% and 26.13% respectively. Identical changes in recharge and river leakage (in the order mentioned for hydraulic conductivity recharge) resulted in RMS head changes from calibrated value by 0.34%, 0.51%, 0.18% and 0.36%, for the river package 15.61%, 20.77%, 86.21% and 35.51% respectively. The results of the numerical simulations showed that increased well withdrawals by 50% and 100% resulted in RMS head changes of 6cm, and 10cm, respectively. Water budget results of the model revealed that groundwater recharge comprised 0.2% and 99.8% by river leakage of the total water input for the entire study area. The spatial distribution of the groundwater decline was limited towards the river. Furthermore, it can be noted that there is a hydraulic connection between the alluvial valley aquifer and the Fafan river.
Key Words: Calibration; Ethiopia; Modeling; Unconfined Alluvial Aquifer; Upper Fafan (Ethiopian-Somali)
Description
Keywords
Calibration; Ethiopia; Modeling; Unconfined Alluvial Aquifer; Upper Fafan (Ethiopian-Somali)