Hydrogeology of the Alaydegea Plain and its Environs ( Middle Awash Valley, Afar Region )

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Addis Ababa Universty


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



Alaydegea plain