Browsing by Author "Tadesse, Nata"

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    Hydrogeology of Jimha Area
    (Addis Ababa University, 1994-06) Tadesse, Nata; Billi, P.(Prof.)
    The studied area is located 335 Km. southwest of Addis Ababa in the southwestern Ethiopian plateau . Over half of the map-area is underlain by thick basalt flows inter layered with tuffs and volcanoclastic sediments . The Jimma Volcanics, which is mainly composed of massive rhyolite in thick flows alternating wjth trachytes, tuffs, ignimbrites and subordinate basalts, underlain small area in the eastern and northeastern part of the basin. Based on the hydrometeorologic data collected for the last 38 years an attempt has been made to calculate the hydrological parameters l i ke precipitation, evapotranspiration, and runoff. The ann ual mean rainfall in the basin is 1476.89 mm . The values of the annual potential evaportanspirtion and actual evapotranspiration are 837.51 mm and 822.29 mm respectively. The only runoff that leaves the studied area comes through Boye river and is 90 .77 million cubic meter of water. The amount of water which is actually available to recharge the groundwater circulation within the hydrogeological basin is 51.14 million cubic meter . The hydrogeological character of the outcropping lithotypes was discussed with particular reference to infiltration and their water bearing capacity. The hydr ogeology of the area varies in different parts of the basin. There is a great potential of groundwater in the valleys which is partly filled with alluvial sediments while aless significant ameunt exist within the velcanic recks ferming the uplifted berders ef the valleys. Determinatien ef the hydraulic parameters like transmissivity and permeability have been carried eut. Due to. the absence ef ebservatien well nearby a discharging wells it was impessible to. calculate the ceeff icient ef sterage, a significant earameter which measures the velume ef water the aquifer releases er take into. sterage. Fer basaltic aquifers a transmissivity value ef 18.39 m2/day and a permeability value ef 3.31 x 10-1 cm/ sec. have been calculated. A transmissivity value ef 1.89 m2 /day . and a permeability value ef 1.62 x 10-5 cm / sec. have been co.mputed fer beth alluvial and rhyelitic aquifers. Because ef the lack ef bere wells in the seuthwest, seuth, and seutheast lewer mest part ef the basin a thereugh study ef the actual variatien ef greundwater level was net pessible. Fer the ether part ef the basin, apprepriate greundwater level centeur lines was censtructed on the basis of the available static level data. Preper chemical analysis ef beth the greundwaters and surfaces water carried eut in the basin shew lew tetal disselved selids. All of the samples have less than 1000 mg / l disselved selids. The analysed samples also. cempared with the acceptable standard for human, agr icul tural, and industr ial use.
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    Hydrogeology of Jimma Area
    (Addis Ababa Universty, 1994-06) Tadesse, Nata; Billi, Paolo
    The studied area is located 335 Km. southwest of Addis Ababa in the southwestern Ethiopian plateau. Over half of the map-area is underlain by thick basalt flows inter layered with tuffs and volcanoclastic sediments. The Jimma Volcanics, which is mainly composed of massive rhyolite in thick flows alternating with trachytes, tuffs, ignimbrites and subordinate • • basalts, underlain small area in the eastern and northeastern part of the basin. Based on the hydrometeorologic data collected for the last 38 years an attempt ha~ been made to calculate the hydrological parameters like precipitatior, evapotranspiration, and runoff. The annual mean rain~All in the basin is 1476.89 mm. The values of the annual ,lotential e,vaportanspirtion and actual e,v apotranspiration are 837.51 mm and 822.29 mm respectively. The only runoff that leaves the studied area comes through Boye river and is 90.77 million cubic meter of water. The amount of water which is actually available to recharge the groundwater circulation within the hydrogeological basin is 51.14 million cubic meter. The hydrogeological character of the outcropping lithotypes was discussed with particular reference to infiltration and their water bearing capacity. The hydrogeology of the area varies in different parts of the basin. There is a great potential of groundwater in the valleys which is partly filled with alluvial sediments while a less significant amount exist within the volcanic rocks forming the uplifted borders of the valleys. Determination of the hydraulic parameters like transmissivity and permeability have been carried out. Due to the absence of observation well nearby a discharging wells it was impossible to calculate the coeff icient of storage, a significant ~arameter which measures the volume of water the aquifer releases~~ take into storage. For basaltic aquifers a transmissivity value of 18.39 m2/day and a permeability value of 3.31 x 10-4 cm/sec. have been calculated. A transmissivity value of 1.89 m2 /day. and a permeability value of 1.62 x 10-5 cm/sec. have been co.mputed for both alluvial and rhyolitic aquifers. Because of the lack of bore w~lls in the southwest, south, and southeast lower most part of the basin a thorough study of the actual variation of groundwater level was not possible. For the other part of the basin, appropriate groundwater level contour lines was constructed on the basis of the available static level data. Proper chemical analysis of both the groundwaters and surfaces water carried out in the basin show low total dissolved solids. All of the samples have less than 1000 mg/l dissolved solids. The analysed samples also compared with the acceptable standard for human, agricultural, and industrial use