Numerical Groundwater Flow Modeling of the Adelle – Haromaya Dry Lakes Catchment (East Hararghe, Oromia Regional State)
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Date
2006-06
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Addis Ababa Universty
Abstract
Groundwater of the Adelle – Haromaya dry lakes catchment, an area of around
240km2 in Eastern Oromia, is the primary source of drinking water for Harar,
Haromaya and Awaday towns, and used also for irrigation and industrial purposes.
The demand for this water source is increasing from time to time since the last few
years as the surface water source became scarce and rapid population growth.
The groundwater flow system of the area was conceptualized as it consists two
aquifers and one confining units, based on the geo-hydrologic properties of the
sediments and rocks: the upper low permeability aquifer which includes poorly sorted
alluvial sediments, weathered basement complex and weathered and fractured
sedimentary formations; the lower high permeability aquifer which consists the well
sorted alluvial sediment, and the confining bedrock. The well sorted sandy gravel
sediment in the basin is the high yield but localized in the central part of the three
watersheds. The poorly sorted alluvial, weathered basement and weathered and
fractured Mesozoic formations, found at the outer part of the study area, are low yield
aquifers and overlies in most parts the high yield alluvial aquifer.
Groundwater inflow to the area is mostly from infiltration of precipitation surplus left
from evapotranspiration, surface runoff and soil moisture deficit to reach the water
table. Groundwater outflow of the study area is mostly through withdrawals from wells
for water supply, irrigation and industries. Natural discharges by two perennial springs,
and seasonal outflows along the streams to the surface water bodies (seasonal lakes)
also contribute to the groundwater outflow of the area. The monitored groundwater
level and the information obtained from local people showed that groundwater level in
Haromaya sub-basin declines starting from the development of wells in the area. The two-layer steady-state and transient groundwater flow models were developed
using the numerical modeling code MODFLOW 96 + interface with advective transport
to help better understand the aquifer system, assess the long-term availability of
groundwater and evaluate groundwater conditions owing to current pumping and to
plan for future water needs. Boundary conditions, hydraulic conductivity, altitude of the
bottom of the layers, vertical conductance, storage coefficient, recharge, and
discharge were determined from the existing data and estimated from literatures. The
models were calibrated to water levels of the Feb, 2006 and drilling report static water
levels for steady-state simulation and 13 months water level monitoring data from Feb,
2005 to Feb, 2006 at Harar water supply observation boreholes for transient
simulation.
Model calibration was accomplished by varying parameters within plausible range to
produce the best fit between simulated and observed hydraulic heads. For steadystate
simulation, the root mean square error for simulated hydraulic heads for all wells
was 9.038m. Simulated hydraulic heads were within ±10m for observed values of 80%
of the wells. For the transient simulation, the difference between the simulated and
observed for the four wells was within 10m. The potentiometeric surface calculated by
steady-state simulation established initial conditions for the transient simulation.
Water management alternatives were evaluated by simulating scenarios of
groundwater pumping for the next five years under two pumping conditions. Result of
predictive simulation indicate that in five years, if the pumping condition of Dec, 2005
continues, groundwater decline by around 6m for Haromaya sub-basin which may
affect the groundwater flow system of the area.
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Keywords
Adelle – Haromaya Dry Lakes Catchment