Numerical Groundwater Flow Modeling Of the Gerado River Catchment
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Date
2018-12
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Addis Ababa University
Abstract
In this study, a one dimensional groundwater flow model under steady state condition was
constructed as a tool to understand the aquifer system of Gerado River catchment using
numerical groundwater flow modeling. The study was ambitious in the sense that the modeling
was conducted with limited data, particularly for the highland tertiary volcanic part of the basin
due to absence of wells in the highland. The groundwater storage is in the alluvium and in the
fractured volcanics. The aquifers in the rocks and alluvial sediments are assumed to be
hydraulically linked or interconnected.
The groundwater flow system in the two aquifers of the basin was modeled using MODFLOW
(McDonald and Harbaugh, 1988). The model was run for steady-state conditions in for
unconfined aquifer. The grid cell size of the model was taken 250 x 250m. Model area and the
layer top elevation were delineated by the DEM processing and use of topographic maps. The
hydraulic conductivity values were found from pumping test data analysis and literature review
for the alluvial sediment aquifer and the volcanic aquifer respectively.
Recharge for the area was estimated using the water balance approach and gave 148.95mm/year
or 13% of the rainfall. The model Calibration was using trial and error approach results were
evaluated using statistical lumped sum description of the average differences of residuals between
simulated and measured heads (mean error, mean absolute error, root mean square error & R2) were
calculated for the residuals and found to be 1.5m,5.29m,6m and 0.88m respectively.
The average recharge obtained by the model is about 151mm/year which closer to the manually
calculated value of 148.95mm/year. The total groundwater in Gerado catchment is 39.3
MM
3
3
/year. This volume of water compared with the total abstraction in the basin (about 2.6 MM
/year) is good storage and can sustain well abstraction at the existing rate or more than the
existing in the future.
Simulated heads were sensitive to recharge and hydraulic conductivity in similar fashion. As
observed from the values, the model was most sensitive to both decrease in the recharge and
hydraulic conductivity values, mainly beyond 50 percent and less sensitive to both recharge and
hydraulic conductivity increasing of the value.
The responses of the aquifer to different scenarios were investigated with increased withdrawals and
decreased recharges. The effects of increased in groundwater withdrawal rate by 50% cause decline
in groundwater level with a maximum of the water level by 1.08 m and a minimum of
0.19m,decreas groundwater discharge through general head boundary and Base flow to river. The
second scenario is effect of decrease in groundwater recharge by 25 % due to less than normal
precipitation cause maximum decline of the water level by 2 of the water level by 23.09m and a
minimum of 2.98m , decrease of groundwater discharge through the general head boundary & Base
flow.
Description
Keywords
Groundwater Flow, Gerado River Catchment