Numerical Ground-water Flow Modelling Of The Awassa Catchment

Groundwater is of major importance to civilization, because it is the largestrnreserve of drinkable water in regions where humans can live, so it is veryrnimportant to calculate the quantity and quality of this resource for betterrnprotection and management. Groundwater modeling is a result of carefulrnunderstanding of hydrology, hydrogeology and dynamics of groundwater flow inrnand around the study area.rnThe modeled aquifer is important water resource in the study area and is usedrnextensively for irrigation, municipal, and domestic water supplies. This thesisrndescribes a conceptual model of groundwater flow in the aquifer and documentsrnthe development and calibration of a numerical model to simulate groundwaterrnflow. Data for a two year period (from 2004 to 2006) and other source withoutrnspecified time were analyzed for the conceptual model. Regional steady staterngroundwater model was calibrated to average conditions from 2004 to 2006. Arnthree-dimensional groundwater flow model, with one unconfined layer, was usedrnto simulate groundwater flow in the Lake Awassa Basin. The study area wasrndivided into uniform grid size of 200m by 200m, with 230 rows and 250 columns.rnArial recharge to the Lake Awassa basin aquifer occurs from precipitation andrninitial recharge rates were given in five different zones ranging from 1.195 × 10-4rnm2/day to 2.787 × 10-4 m2/day. Discharge from the aquifer occurs throughrndischarge to perennial streams, well withdrawals and springs, and groundwaterrnoutflow to the next Ziway-Shalla Basin. Discharge rates in million cubic metersrnper year (MCM/year) for the steady state simulation were 111 for base flowrnincluding Tikur Wuha River and 3.03 for outflow through wells and springs.rnOutflow to the neighboring catchment is simulated using general-head boundary.rnEstimated horizontal hydraulic conductivity used for the numerical model rangedrnfrom 0.05 to 100 m/d, which were adjusted during model calibration. Model calibration was accomplished by varying parameters within plausiblernranges to produce the best fit between simulated and observed hydraulic heads.rnThe root mean square error for simulated hydraulic heads for all wells was 4.42rnmeter. Simulated hydraulic heads were within ±10 meter of observed values forrnall observation wells.rnA sensitivity analysis was used to examine the response of the calibrated steadyrnstate model to changes in model parameters including horizontal hydraulicrnconductivity, recharge, and pumpage. The model was most sensitive to rechargernand relatively less sensitive to horizontal hydraulic conductivity. Three differentrnscenarios were simulated to see the response of aquifer. Increased withdrawalsrndecrease the groundwater outflow through river leakage and general-headrnboundary. Complete disappearance of Lake Shallo result in an increase in waterrnlevel particularly in wells found around this lake, while decreased recharge resultrnin more inflow from constant-head boundaries as well as lower streamflow andrngroundwater discharge through the general-head boundary

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