Model Based Groundwater System AnalysisfFor Hayk -Ardibo Catchment
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Date
2005-06
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Addis Ababa Universty
Abstract
A research was undertaken to understand better the interaction between the surface and
groundwater of the Hayk-Ardibo Catchment. A two-dimensional aerial finite-difference
groundwater flow model was used to simulate the groundwater flow and establish a hydro
geologic budget of the region .
The Hayk-Ardibo Catchment lie on the western Afar rift escarpment, an area with a complex
geologic history. The geologic history includes volcanic rocks with the major NNE-SSW fault
trend in the area. Faulting have modified the distribution and thickness of volcanic rocks, their
secondary porosity & permeability and the degree of weathering and fracturing which intern
influence the occurrence and movement of ground-water in the region. The permeability of
the aquifer which are highly weathered and fractured are associated with high hydraulic
conductivity value which in most case is the typical nature of the Tertiary volcanic located in
the center of the Catchment along lake area that control the gradient of the potentiometric
surface and ground water flow in that region. The geology, hydrogeology and water balance
of the study area has been discussed and it helped to understand the groundwater flow system
in the study area . These are the basics, which helped to conceptualize and determine the type
of aquifer, its aerial extent and boundaries.
A Conceptual model was first developed to decide in order to determine the modeling
approach .. The model boundaries determined to represent natural hydrological boundaries
incorporate (1) no-flow boundary along the westem, Eastern and Southern boundaries, (2)
flux boundary along Northwestern boarder of the study area (3) general head boundary on the
two lakes and internal boundary along Ankerka River draining the central catchment.
Several simplifying assumptions were made for the conceptualization and simulation of flow
in the Catchment: 1) Recharge to the groundwater is assumed only from precipitation with in
the catchment and there is no additional subsurface inflow across boundary in any part of the
study area. Recharge varies spatially, 2) single layer has been used in the model 3) Darcy’s
law is applicable to flow through the fractures 4) unconfined aquifer is assumed in the model,
and 5) evaporation was not explicitly simulated in the model. Recharge rates were considered
as ‘net recharge,’ and 6) discharge occurs from the aquifer along the Northern boundary
where aquifers are hydraulically connected to down Catchment.
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Hayk -Ardibo Catchment