Hydrogeology of Jimma Area
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Date
1994-06
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Addis Ababa Universty
Abstract
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
Description
Keywords
Jimma Volcanics