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|Title: ||NUMERICAL GROUNDWATER FLOW MODELING OF THE NORTHERN RIVER CATCHMENT OF THE LAKE TANA|
|Authors: ||Nigussie, Ayehu|
|Advisors: ||Tenalem Ayenew (Prof.)|
|Keywords: ||NUMERICAL GROUNDWATER|
|Copyright: ||Jun-2010 |
|Date Added: ||16-May-2012 |
|Publisher: ||ADDIS ABABA UNIVERSITY|
|Abstract: ||The study area is found North Western plateau in the North Gondar zone, Amhara regional state. Its total
surface coverage is 1887km2.The study area boundary was delineated from 90m Shutter Radar Terrain
Mapping (SRTM) digital elevation model (DEM) using Global Mapper 8 software. This boundary was served
as the divide line of groundwater flow while stream networks were used as internal drainage lines. Input
parameters such as hydraulic conductivity and recharge were obtained from past studies and modelers
knowledge. Based on geologic information of the study area, unconfined subsurface flow condition was
considered and simulated using MODFLOW 2000.
The model calibration accounts the matching of the 58 observation point with simulated head with a
permissible residual head of ±10m. 75% of the difference the observed and measured water level head in the
study area is 5m. . The model was calibrated with mean error 0.506, absolute mean error 4.431m and
standard deviation 6.083m.
The sensitivity of the major parameters of the model was identified during calibration process. Based on the
calibration process, the model is very sensitive in decreasing order change in recharge, hydraulic
conductivity, and stream bed conductance respectively.
The simulated water budget has been computed for the study area. The simulated out flow of the model is
205733827.88m3/year which is nearly equal to simulated inflow with difference 2887.5 m3/year. The base
flow simulated discharge holds 35.75% of the out flow. It also contributed as recharge in to the aquifer that
accounts to 15.30% of the inflow. This share of base flow implies the discharge of the groundwater to the
dominantly gaining streams and high interaction of surface and aquifer systems.
Two scenarios of increased groundwater withdrawals have been conducted. In the first scenario, fife
increased withdrawals amounts were distributed among exciting wells in proportion to the current
contribution of each source to the daily withdrawal rate. Steady state withdrawal rates were increased by
15%, 35%, 55%, 75% and 100% to study the response of the system in this scenario. From the above fife
simulation results, one can observe that the development of a new groundwater sources would not pose
appreciable impact in case of 15% and 35% withdrawal the head declines in this case is insignificant relative
to the steady state withdrawal rate and the natural discharges were not altered highly. In the second
scenario, increased groundwater withdrawal in Gondar-Azezo town and its periphery well fields were
simulated. The simulation result indicated that the stream leakage decreased by 7.9% relative to the whole
steady state value, but showed 14.9% decrease for Angereb, Keha, and Shinta river segments near the well
field area. The water tables decline by 3.57m to18.81m in head observation in the well field area. The lower
Angereb well field head decline is significant when compare with other near well fields.
This scenario simulates also decreasing recharge to aquifers that result from environmental changes,
expansion of agriculture, deforestation and town expansion. The steady state simulated recharge was
decreased by 32% and the simulation results showed on average head decrease of 8.06m over the whole
area; with the highest fall 32m in wells to north and a minimum of about 1m in wells to the south. In addition,
the stream leakage, compared to the simulated steady state value and it was decreased by 75.36%.
The effect return flow of irrigated water and development of Megech reservoir were simulated simultaneously
to see the effects on groundwater level changes and stream leakage. In the simulation the response of the
system was compared with the steady state simulated water levels and stream leakages. The differences
showed the effect of development of Megech reservoir and irrigation on the groundwater. The simulated
value showed an average 2.74m increased head over the whole area. High difference values were observed
at Tseda (7.83m) and Koladiba (7.3m). The minimum difference 1.08m was recorded at Angereb well field
(observation 94). In addition, the stream leakage increased by 87.43%.|
|Appears in:||Thesis - Earth Sciences|
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