Comparative Evaluation of Shallow Groundwater Dynamics of Becho and Koka Plains, Upper Awash Basin, Central Ethiopia
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Date
2018-06-22
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Addis Ababa University
Abstract
The target area (Koka and Becho plains) are situated in the upper reach of the Awash
River basin, geographically, Koka plain located between 38.750 – 39.120E and 8.380 –
8.760N and Becho plain is located between 38.190 – 38.610E and 8.540 – 9000’N. The
main objective of this work is evaluating shallow ground water dynamics of Becho and
Koka plains comparatively from water level fluctuation of monitoring wells and recharge
estimation of the two plains by Water balance method. The shallow groundwater flow
direction is identified from water table contour map and cross sectional map of well
lithology in the target area. The water table contour map indicates that the groundwater
flow direction of Becho plain is toward the Awash River from the recharge area.
However, in Koka plain the groundwater flows toward Koka reservoir by the effect of
topography and geological structures. The water balance method give recharge of the
Becho plain is 235.029mm/yr., this means 23.38% of annual precipitation was recharged
to groundwater and the recharge of Koka area is 22.2627mm/yrs., and i.e. only 2.65% of
the annual precipitation is recharged to groundwater. The selected River gaging stations
for baseflow separation are; Awash at Melka Kunture, Awash at Bello, Mojo at Mojo
village and Mojo at Koka dam. Volume of baseflow, total volume and baseflow index
were calculated from baseflow separation of the selected stations. The baseflow index of
Awash at Melka Kunture, Awash at Bello, Mojo at Mojo village and Mojo at Koka dam
are 0.65, 0.787, 0.18 and 0.343 respectively.
The index of base flow Calculated at Awash Bello and Awash at Melka Kunture was
much greater than the estimated BFI at Mojo village and Koka Dam; this implies that the
Awash river catchment has more stable flow regime and able to sustain river flow during
extensive dry period. Piezometers were used to monitor groundwater from Grofutures
project. Based on their distance from the surface water and effect of irrigation, monitoring
wells of the study area are grouped into four; (i) piezometers located far from a surface
watercourse and outside any irrigated area, (ii) piezometers close to a surface
watercourse but outside any irrigated area, (iii) piezometers located far from any
surface watercourse but located in irrigated areas and finally, (iv) piezometers close to
a surface watercourse and located in irrigated areas. The piezometer data indicates
that the temperature of the shallow groundwater is more or less the same with that of
surface temperature and has an inverse relationship with water level.
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Keywords
Becho, Koka, Upper Awash, Groundwater Dynamics, Monitoring Well