Numerical Ground-Water Flow Modelling Of the Awassa Catchment
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Date
2006-06
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Addis Ababa Universty
Abstract
Groundwater is of major importance to civilization, because it is the largest
reserve of drinkable water in regions where humans can live, so it is very
important to calculate the quantity and quality of this resource for better
protection and management. Groundwater modeling is a result of careful
understanding of hydrology, hydrogeology and dynamics of groundwater flow in
and around the study area.
The modeled aquifer is important water resource in the study area and is used
extensively for irrigation, municipal, and domestic water supplies. This thesis
describes a conceptual model of groundwater flow in the aquifer and documents
the development and calibration of a numerical model to simulate groundwater
flow. Data for a two year period (from 2004 to 2006) and other source without
specified time were analyzed for the conceptual model. Regional steady state
groundwater model was calibrated to average conditions from 2004 to 2006. A
three-dimensional groundwater flow model, with one unconfined layer, was used
to simulate groundwater flow in the Lake Awassa Basin. The study area was
divided into uniform grid size of 200m by 200m, with 230 rows and 250 columns.
Arial recharge to the Lake Awassa basin aquifer occurs from precipitation and
initial recharge rates were given in five different zones ranging from 1.195 × 10-4
m2/day to 2.787 × 10-4 m2/day. Discharge from the aquifer occurs through
discharge to perennial streams, well withdrawals and springs, and groundwater
outflow to the next Ziway-Shalla Basin. Discharge rates in million cubic meters
per year (MCM/year) for the steady state simulation were 111 for base flow
including Tikur Wuha River and 3.03 for outflow through wells and springs.
Outflow to the neighboring catchment is simulated using general-head boundary.
Estimated horizontal hydraulic conductivity used for the numerical model ranged
from 0.05 to 100 m/d, which were adjusted during model calibration. Model calibration was accomplished by varying parameters within plausible
ranges to produce the best fit between simulated and observed hydraulic heads.
The root mean square error for simulated hydraulic heads for all wells was 4.42
meter. Simulated hydraulic heads were within ±10 meter of observed values for
all observation wells.
A sensitivity analysis was used to examine the response of the calibrated steady
state model to changes in model parameters including horizontal hydraulic
conductivity, recharge, and pumpage. The model was most sensitive to recharge
and relatively less sensitive to horizontal hydraulic conductivity. Three different
scenarios were simulated to see the response of aquifer. Increased withdrawals
decrease the groundwater outflow through river leakage and general-head
boundary. Complete disappearance of Lake Shallo result in an increase in water
level particularly in wells found around this lake, while decreased recharge result
in more inflow from constant-head boundaries as well as lower streamflow and
groundwater discharge through the general-head boundary
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Keywords
Groundwater modeling