Numerical Groundwater Flow Modeling of Mehoni Sub-Basin in the Raya Valley (Northern Ethiopia)
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Date
2007-02
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Addis Ababa Universty
Abstract
Mehoni sub-basin which is found in the Raya Valley is located in the southern part of Tigray
Regional State of the Northern Ethiopia. The area covers 60% of the Raya valley with area
coverage of 1183km2.The sub-basin water resource is used for irrigation and domestic water
supply and is covered by tertiary volcanic in western and eastern highlands and alluvial deposit
in the valley fill. The western rocks are affected by tectonic movement .The N-S faulting is
responsible for tilting of the rock formations with a general strike NE-SW and dipping to the SE
direction.
As numerical groundwater flow models represent the simplification of complex natural systems,
different parameters were assembled into conceptual model to represent the complex natural
system in a simplified form. The conceptual model was put into the numeric model to examine
system response.
Numerical groundwater flow was simulated in the model by the finite-difference method using
MODFLOW, 1996 (McDonald and Harabaugh, 1988). The finite difference grid consisted of 1
layer, 116 rows and 111 columns. Two dimensional profile model was developed considering the
system to be under steady state condition and assuming flow system view point. Four scenarios
Increase withdrawals by three fold of the average existing withdrawals for whole sub-basin and
local areas, assigned 25 wells with daily discharge 1000m3/d and decrease recharge by 50% were
used. Model calibration was carried out by trial and error calibration method using groundwater
contours constructed from heads collected in 40 observation points. The calibration showed that
about 97.5%of simulated heads were within the calibration target and the overall root mean
square error for simulated hydraulic heads is about 8.52m. The poor fit at some points was due to
numerous limitations associated with the model.
Model sensitivity analysis was conducted by considering the horizontal hydraulic conductivity
and recharge because they are sensitive. A change in hydraulic conductivity by -55%,-45%,-25%
and 55%, 45% 25% resulted in root mean square (RMS) 27.51, 19.293, 12.948, 8.499, 11.86 and 13.356 respectively. A change in recharge by the same amount as the horizontal hydraulic
conductivity RMS head changes by 9.553 ,9.254, 8.854,8.290, 8.385, and 8.571 respectively.
The calibrated model was used to simulate the possible effect of increased groundwater pumping.
Increase of three fold of withdrawals from average existing withdrawals in the whole sub-basin
has resulted in the maximum and minimum decline in head by 2.33m and 0.56m respectively.
Local increase of withdrawal by three fold of an average existing withdrawal indicated that the
two (volcanic and alluvial) aquifer systems are not interconnected.
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Keywords
Mehoni Sub-Basin