Water Balance and Effect of Irrigated Agriculture on Groundwater Quality in the Wonji Area / Ethiopian Rift Valley
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Date
1993-06
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Addis Ababa Universty
Abstract
Wonji basin covers a total area of 357 Km2 and has a population of 290,703 out of
which 50,000 are sugar estate farm workers. The main source of domestic water in the
area is groundwater. Awash river passes in the area dividing in to two equal parts
whose water is mainly used for cattle drinking and industrial purposes. Meteorological
and hydrological data were used to establish and calculate water balance in the Wonji
basin. Groundwater recharge was estimated to be 557.75 x 106m3 per year. Different
methods some with many parameters and others with few were checked side by side to
obtain more realistic actual evapotranspiration. Their average gave 702 mm mean
annual actual evapotranspiratin. The mean annual rainfall is 785 mm in the area.
Based on joint data along with mathematical treatment and pumping test results,
lithological units have been characterised into different permeability groups. From the
results of water chemistlY and compositional diagrams, 5 groundwater systems with
some interconnections were identified. Direction of regional groundwater flow of the
basin is from west to east. Two horizons of cold water of deep aquifers occur at an
average depth of 120 and 170 in the region. Using temperature of thermal springs,
depth of geothermal water circulation has been calculated to be 242 m below the
surface. This has been also confirmed by the drilling data of borehole 26. However this
depth could vary up to 400-500 III under Gadamsa caldera rim, Boku ridge and Koka
escarpment high lands. The estimated groundwater demand in the basin ranges up to
4.7xI06m3
, which is Illuch less than yearly recharge.
Fluoride concentration showed zonal variability from low lying parts of the basin to the
high lands of the study area corresponding to compositional variation of the rocks and
thermal activity. Concentration of total dissolved solids in the groundwater, exhibited
positive relation with irrigated sugar plantation. Pollution from inorganic fertiliser
(NO,) is not however, observed under sugar cane farm. This is suggested to be due to
high groundwater inflow dilution from Koka reservoir. Subsurface water in the region
is generally of good quality for drinking purposes. But some dug wells and hot springs
are of non promising quality for irrigation.
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Keywords
Wonji