Hydrology, Hydrogeology, and Hydrochemistry of the Lakes System Haiq-Ardibo, Northern Ethiopia
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Date
2000-06
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Addis Ababa Universty
Abstract
The geology, hydrology, and hydrogeology of Haiq-Ardibo catclilllent was studied. The main
objectives is to understand and map the different lithologic units and characterize them in to
hydrostratigraphic units, to analyze the water balance of the catchment including lakes Haiq
and Ardibo. Hydrochemical and bathymetric studies were also conducted.
The geology of the area was mapped at the scale of 1: 50, 000 and this was used as a basis to
establish the hydrogeological map by taking into account the permeability, geomorphic
setting and recharge conditions.
Two major hydrostratigraphic units are mapped. These are the Quaternary unconsolidated
sediments and the fractured volcanic rocks, both having a good permeabitlity. Due to
morphologic setting, the fractured volcanics are refen'ed to as poor aquifers, but good
recharge zones, recharging the QuaternalY unconsolidated sediments that occupy low lying
flat areas in the studied catchment.
Land use/land cover, soil, and slope maps of the area were prepared so that the rate of
evapotranspiration for a vegetation-soil complex is computed, and the runoff into lake Haiq
and Ardibo estimated.
Areal mean arumal rainfall was analyzed using arithmetic mean of the long telID mean alillual
rainfall of four stations, Thiessen weighted averages of these stations and Isohyetalmethods;
a value of 1158.5 mm depth was obtained by taking the average of the first two methods.
Open water evaporation on the lakes surface was estimated using Pan evaporation data,
radiation, Penman , and mass transfer approaches. The first three methods gave reasonable
values and the average was taken and is 146.5 mm depth over the surface of the lakes.
Potential evapotranspiration (PET) computed using modified Perunan gave 1200 nun depth
over the area of the studied catchment.. Using the computed PET and effective rainfall,
actual evapotranspiration and groundwater recharge over the catclunent using Thornthwaite
and Mather (1957) soil-water balance method obtained is 805 and 43.2 mill respectively Runoff into the lakes was also calculated using a weighted average mnoff coefficient of the
Haiq and Ardibo subcatchments taking in account soil, slope, and vegetation cover. An
annual nmoff into lake Haiq and Ardibo is 18.82 and 12.68 million cubic meter (mcm)
respectively.
Taking into consideration all the inflows and outflows into the catchment and lakes, the
water balance of the studied area was analyzed. The annual inflow into lake Ardibo is
estimated to be 30.98 mcm and outflow is 26.3 mcm. The residual found is 4.68 mcm.
Representing the groundwater net flux. Using a Darcy, the groundwater outflow from lake
Ardibo is estimated to be 3.2 mcm. The groundwater inflow into this lake therefore is 1.48
mcm. In the same way the atmual input into lake Haiq is 46.83 mcm. and the outflow is 33.4
mcm, the residual remaining in the balance is 13.43 representing the net groundwater flux.
The water balance computed for the entire study area is estimated to be 162.3 mcm. inflow
and 138.434 mcm out flow. The residual found is therefore 23.87 mcm, assuming that the
groundwater divide coincides with the surface water divide, the residual tenn is the annual
groundwater outflow from the studied catchment.
The current bathymetric survey indicates that both lakes have experienced significant lake
level reduction. This lake level change is attributed to land use changes, neotectonics and
general decrease in the rainfall of the area.
Hydrochemical study of the surface and groundwater indicated two major water types. The
surface water being magnesium bicarbonate type, while the groundwater is calcium
bicarbonate type . Further observation of the cheInistry of the ground and surface water
bodies indicated that the waters are suitable for irrigation. Except for the trace non metals,
the chemical analysis of the inorganic constituents show that the water is potable provided,
bacteriological analysis should be done
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Keywords
Haiq-Ardibo