Hydrogeology Of The Borkena River Basin Hollo Ethiopia

The Borkena Hiver Basin is located in Wollo governorate general.rnIt is part of a major graben related to the Ethiopian Rift System,rnwhich extends north to Korbeta graben around Maichew and southrnupto Debre Berhan, and approximately stretches in a north-southrndirection. There are significant contrasts of topography in thernbasin where the structural control is very strong.rnThe hydrology of the area varies in different parts of the basin.rnConsequently,the hydrogeology of the basin alse differs, such thatrnthere is a great potential of groundwater in the flat low lyingrnalluvial sediments while a less signi fican t amount exists wi thinrnthe volcanic rocks forming the uplifted borders of the graben.rnSeveral springs with discharges of less than 5 lilt/sec. occur in thernarea issued from the volcanic rocks. There are eleven waterrnsupplying drilled wells in the area, seven located in the alluviumrnaquifers and the remaining four on basaltic aquifers; Pump testsrnhave been carried out in some of the wells which gave transmissibilirnty coe fficient of 91+ to 28.1 m3 per day per meter. However,rnit was impossible to calculate the coefficient of storage, arnsignificant parameter which measures the volume of water, thernaquifer releases or takes into storage.rnProper chemical analysis of both the ground and surface watersrncarried out in the area show low total dissolved solids. All of the samples in tho area have loss thun 1000 mg/l (1 mg/l = 1 ppm)rndissolved solids, except for the hot spring waters. Few of thernbacteriological tests of the waters in the area show low pathogenicrnorganism though most of the water samples, (particularlyrnthe ground waters ) contain high ni tra te con ten t.rnThe water balance for the area has been determined. The totalrnannual rainfall calculated on the basis of the mean annual precipitationrnfor the basin is 1846.41 x 106 m3 • The groundwaterrnrecharge for the area has been calculated based on thernevapotranspiration data at Cheffa, where the water loss isrnmaximum to approximate the minimum infiltration, and amounts torn185.11 x 10 6 m3 . This is 10.02% of the precipitation. The actualrnevapotranspiration in the area amounts to 872, 834.8 and 836.1 mm.rnfor Cheffa, Dese and Kombolcha respectively. 218.5'5 x 106 m3 ofrnwater leaves the basin at the BOrkena River outlet as runoff andrnis equivalent to 11.83% of the precipitation •

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