Hydrogeology of the Alaydegea Plain and its Environs ( Middle Awash Valley, Afar Region )
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Date
2006-06
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Addis Ababa Universty
Abstract
The Alaydegea plain is an extensive flat south north running plain in the middle
Awash valley with an average width of 26-30Kms and length of more than
70kms, situated between 9 0- 9 0 52’ north and 40o – 40o 30’ east. the average
monthly temperature and the mean annual precipitation in the area is around
25.7oc and 597.5mm respectively. Potential evapotrspiration estimation for the
area from empirical methods of Penman and Thorenthwaite gives annual PET
values of around 1427mm and 1000mm respectively. The arid climatic nature of
the area results in the occurrence of high potential evapotranspiration over the
precipitation amount. Actual potential evapotraspiration for the area has been
estimated from soil water balance and Turc method. Annual AET values (595mm
from Turc and 615mm from soil water balance are almost close or nearly equal to
the annual precipitation amount resulting in no direct recharge or surplus in the
area where almost all the rainfall is lost by evaporation. Deep and shallow ground
water systems exist in the area where their flow , occurrence and distribution in
general is controlled by the topographic and geologic situation of the area. The
Alaydegea plain and related pediments are characterized by a deep ground
water system encountered at a depth of 60 to 75 meter. Fractured basalts and
ignimbrite interbaded with old outwash plain gravels and volcanic sand is the
major aquifer unit in the area. Groundwater recharge to these aquifers is most
possibly from precipitation on the eastern mountain range. Areas on the western
margin of the plain close to the wonji fault lines are ground water potential sites
with aquifer of better tranmissivity (800 m 2 /d and above) .The observed regional
trend for the groundwater flow in the area is north west wards from the eastern
highland beneath the Alaydegea plain. The shallow aquifer zones in the river
valley system are fed indirectly by percolation or transmission loss from the
Awash river where the Awash river loses on annual base a total volume of water
nearly 382 Mm3 of water between Melka worer and Awash Sebat. Water type on
the Alaydegea plain is Na – Ca- HCO3 type evolving towards more Na –HCO 3
type in the Awash river valley, controlled by Cation exchanges, rock water
interaction and evaporation factors along the flow path
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Alaydegea plain