Hydrogeology Of Jimma Area

The studied area is located 335 Km. southwest of AddisrnAbaba in the southwestern Ethiopian plateau. Over half of thernmap-area is underlain by thick basalt flows inter layered withrntuffs and volcanoclastic sediments. The Jimma Volcanics, whichrnis mainly composed of massive rhyolite in thick flowsrnalternating with trachytes, tuffs, ignimbrites and subordinate • •rnbasalts, underlain small area in the eastern and northeasternrnpart of the basin.rnBased on the hydrometeorologic data collected for the lastrn38 years an attempt ha~ been made to calculate the hydrologicalrnparameters like precipitatior, evapotranspiration, and runoff.rnThe annual mean rain~All in the basin is 1476.89 mm. Thernvalues of the annual ,lotential e,vaportanspirtion and actualrne,v apotranspiration are 837.51 mm and 822.29 mm respectively.rnThe only runoff that leaves the studied area comes through Boyernriver and is 90.77 million cubic meter of water. The amount ofrnwater which is actually available to recharge the groundwaterrncirculation within the hydrogeological basin is 51.14 millionrncubic meter.rnThe hydrogeological character of the outcroppingrnlithotypes was discussed with particular reference torninfiltration and their water bearing capacity.rnThe hydrogeology of the area varies in different parts ofrnthe basin. There is a great potential of groundwater in thernvalleys which is partly filled with alluvial sediments while a less significant amount exist within the volcanic rocks formingrnthe uplifted borders of the valleys.rnDetermination of the hydraulic parameters likerntransmissivity and permeability have been carried out. Due tornthe absence of observation well nearby a discharging wells itrnwas impossible to calculate the coeff icient of storage, arnsignificant ~arameter which measures the volume of water thernaquifer releases~~ take into storage. For basaltic aquifersrna transmissivity value of 18.39 m2/day and a permeability valuernof 3.31 x 10-4 cm/sec. have been calculated. A transmissivityrnvalue of 1.89 m2 /day. and a permeability value of 1.62 x 10-5rncm/sec. have been co.mputed for both alluvial and rhyoliticrnaquifers.rnBecause of the lack of bore w~lls in the southwest, south,rnand southeast lower most part of the basin a thorough study ofrnthe actual variation of groundwater level was not possible.rnFor the other part of the basin, appropriate groundwater levelrncontour lines was constructed on the basis of the availablernstatic level data.rnProper chemical analysis of both the groundwaters andrnsurfaces water carried out in the basin show low totalrndissolved solids. All of the samples have less than 1000 mg/lrndissolved solids. The analysed samples also compared with thernacceptable standard for human, agricultural, and industrialrnuse

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