Groundwater Recharge Estimation and Aquifer Characterization. a Case Study of Walga Catchment Upper Gibe Basin, Central Ethiopia
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Date
2020-11-24
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Addis Ababa University
Abstract
Groundwater is mainly used for irrigation and water supply in Walga catchment as other parts of
the country. The main objective of this study is to quantify the amount of groundwater recharge
using WetSpass model and characterization of aquifers using Aquifer test in the upper Omo-Ghibe
basin of Walga catchment. Meteorological data like rainfall, wind speed, temperature, relative
humidity, sunshine hours were collected from meteorological stations located within the catchment
and nearby area. Potential evapotranspiration was calculated using Penman-Montieth based FAO
CROPWAT software. Slope and topography map were generated from Shuttle Radar Topographic
Mission elevation data while Land use land cover were prepared from Landsat OLI8 satellite
images downloaded from United States Geological Earth Explorer. Parameter table (dbf data) and
grid maps were prepared for WetSpass model input with the help of ArcMap 10.4. Using the model
the mean annual recharge of the catchment was found to be 7.5% of the precipitation whereas
surface runoff was 38.25% of the annual precipitation. The aquifer hydraulic properties were
estimated from the pumping test by fitting mathematical models (type curves) to response data
(water level changes) through computer software known as AQUIFER TEST. Yield or discharge
of bore holes values range from 0.15 l/sec. to 61 l/sec, whereas Transmissivity ranges from 3.5
x10-4 m2/day to 290 m2/day with an average of 50.94 m2/day. Hydraulic conductivity values ranges
from 0 to 16.04 m/se with mean values of 1.2 m/s. The sum of the yield from 72 boreholes is 430
l/s (119.4 m3/hr), this gives a total of 37152000 liters per day. 65.8% of Walga catchment coverage
is under Satisfactory ground water potential while 0.72% is classified under very good
groundwater potential. The mean descriptive statics value of groundwater quality of Walga
catchment is below WHO limit and also good for irrigation water quality. The study area is
characterized by lowest groundwater recharge relative to surface runoff and actual
evapotranspiration due to effect of impermeable soils, morphology of the land and land use land
cover of the area. The lowest transmissivity value is indicative of the poor permeability in the
pyroclastic lithologic formations and low discharge to wells. It also implies that it will take a
considerable time for the aquifers to replace water into wells removed during pumping.
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Keywords
Walga Catchment, Wetspass, Groundwater Recharge, Aquifer Characterization