Numerical Groundwater Flow Modeling Of the Ziway Lake Basin
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Date
2013-07-05
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Addis Ababa University
Abstract
Ziway lake Basin is a large, heterogeneous, lithological unit that contains groundwater aquifers
of domestic, agricultural, industry and environmental significance. Population growth has
resulted in expanding residential developments and consequently increased demand for water.
It is crucial that this resource has to be managed in a sustainable manner to safeguard it both for
current users and for future generations. Eight aquifer systems having distinct groundwater
potential and hydraulic properties. A shallow aquifer occurs in the lacustrine and alluvial
deposits in the rift and highlands of weathered and fractured regions. Water levels in the
modeled aquifers follow the topography and groundwater occurs under unconfined conditions
in the system.
The aquifer system was modeled numerically using Processing Modflow Pro (Version 8.0.15)
under a steady-state condition with one layer of 500 meters constant thickness. The model area
which is about seven thousand four hundred fourteen square kilometer was divided into grid
blocks of 250 by 250 meters. A number of data were collected from different sources for the
development of this model. These data include: DEM, aquifer thickness, recharge,
hydrogeological conditions of the study area, flow data, and pumping and observation wells.
These data were prepared and preprocessed using ArcGIS, surfer10 and Globalmapper11.The
calibration of the groundwater model was performed using trial and error calibration. The
calibration was implemented by changing several parameters such as recharge, hydraulic
conductivity, and river conductance. The steady-state groundwater model was calibrated
successfully and the simulated heads were in a good agreement with the observed head. The
calibrated model gave ME of -0.070613 m and RMS of 4.497221m.
A sensitivity analysis was conducted to examine the response of the calibrated steady State
model to changes in model parameters including horizontal hydraulic conductivity, recharge,
and well withdrawal. The model was most sensitive to recharge and relatively less sensitive to
well withdrawal. Three different scenarios were simulated to observe the response of aquifer by
considering both increased and decreased 25%, 50% and 75% in the parameters (recharge,
hydraulic conductivity and pumpage). The scenario and sensitivity analysis provided important
information for future transient model development and on the data gaps. This model has
pertained for developing detailed predictive groundwater model, which can be readily used for
groundwater management practices.
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Keywords
Numerical Groundwater, Modeling, Ziway Lake Basin