Hydrogeology Of The Alaydegea Plain And Its Environs ( Middle Awash Valley Afar Region )

The Alaydegea plain is an extensive flat south north running plain in the middlernAwash valley with an average width of 26-30Kms and length of more thanrn70kms, situated between 9 0- 9 0 52’ north and 40o – 40o 30’ east. the averagernmonthly temperature and the mean annual precipitation in the area is aroundrn25.7oc and 597.5mm respectively. Potential evapotrspiration estimation for thernarea from empirical methods of Penman and Thorenthwaite gives annual PETrnvalues of around 1427mm and 1000mm respectively. The arid climatic nature ofrnthe area results in the occurrence of high potential evapotranspiration over thernprecipitation amount. Actual potential evapotraspiration for the area has beenrnestimated from soil water balance and Turc method. Annual AET values (595mmrnfrom Turc and 615mm from soil water balance are almost close or nearly equal tornthe annual precipitation amount resulting in no direct recharge or surplus in thernarea where almost all the rainfall is lost by evaporation. Deep and shallow groundrnwater systems exist in the area where their flow , occurrence and distribution inrngeneral is controlled by the topographic and geologic situation of the area. ThernAlaydegea plain and related pediments are characterized by a deep groundrnwater system encountered at a depth of 60 to 75 meter. Fractured basalts andrnignimbrite interbaded with old outwash plain gravels and volcanic sand is thernmajor aquifer unit in the area. Groundwater recharge to these aquifers is mostrnpossibly from precipitation on the eastern mountain range. Areas on the westernrnmargin of the plain close to the wonji fault lines are ground water potential sitesrnwith aquifer of better tranmissivity (800 m 2 /d and above) .The observed regionalrntrend for the groundwater flow in the area is north west wards from the easternrnhighland beneath the Alaydegea plain. The shallow aquifer zones in the riverrnvalley system are fed indirectly by percolation or transmission loss from thernAwash river where the Awash river loses on annual base a total volume of waterrnnearly 382 Mm3 of water between Melka worer and Awash Sebat. Water type onrnthe Alaydegea plain is Na – Ca- HCO3 type evolving towards more Na –HCO 3rntype in the Awash river valley, controlled by Cation exchanges, rock waterrninteraction and evaporation factors along the flow path

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