Assessment of Water Balance and Lake Level Fluctuation, Lake Abaya, Ethiopia
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Date
2019-10
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Addis Ababa University
Abstract
Lake Abaya is a quasi-endorheic lake which is located 6º 26’ N Latitude and 37º 53’ E Longitude with
surface area of 1095 km
. It drains with five main rivers (Bilate, Gidabo, Gelana, Hamessa and Harie) and
many other intermittent tributaries and covers a total catchment area of 14638.83km
2
2
.
The objective of this study is to assess the water balance and lake level fluctuation of Lake Abaya. The
lake level is gauged up to the year 2013, but the inflow from the four out of five tributaries is gauged up
to the year 2005. As a result stream flow is predicted by using HEC-HMS to the year 2013. Inflow from
the un-gauged catchment is estimated using area ratio method. Areal rainfall for the sub-basin and lake
surface is estimated using Thiessen polygon method. Evaporation from the lake surface is estimated using
Cropwat 8 software, Penman’s method and Pan Evaporation; pan evaporation is taken for the analysis.
Water abstraction from tributary rivers and the lake for irrigation is determined by Cropwat 8 software.
After analysis of all components of water balance for the lake, a water balance model has developed using
continuity equation. Summing up of all the components, lake surface rainfall 18.87 %, inflow 32.35 %
(Gauged 19.18 %, un-gauged 13.16 %), lake surface evaporation 46.08 % and abstraction 2.71 %
contributes for the lake water balance trend and no observed out flow from the lake. The lake level has a
relation with climatic and human factors; rainfall and temperature of the area shows a slow increment and
that of wind speed, sunshine hours and evaporation shows slow decrement. When comparing the land
cover of 1996 to the land cover of 2006 that the area under intensive cultivation, grassland, urban area and
riparian has increased by 84.59, 32.64, 1.08 and 0.11 km
respectively and that of shrub-land, unidentified
area, marshland, forest, moderately cultivated and exposed surface has decreased by 69.20, 35.90, 8.00,
4.92, 0.40, 0.01 km
2
respectively and that of water body and woodland has not changed yet. These all is
not in contradict with the increment of the lake level rather it agrees. To irrigate a total land of 9906 ha
with common crops of Banana, Tobacco, Cotton, Vegetables and diversified crops 140.98 MCM of water
is abstracted from the lake and all the tributary rivers. Two irrigation scenarios are conducted, scenario 1
2
with irrigation expansion of 10610 ha on the tributaries needs additional water of 84.10 MCM per year
and reduces the lake level by 60 mm, and scenario 2 irrigation expansion of 30,831 ha, it requires
additional water of 528.45 MCM per year and this will reduce the lake level by 420 mm. The simulation
of lake level has been conducted by excel spread sheet model at monthly time stapes.
The causes for the increment of lake level are (1) the nature of the lake (which is quasi-endorheic) (2)
Climatic factors (the increment of rain fall, decreasing of evaporation, etc.) (3) Anthropogenic factors
(deforestation, intensive cultivation, etc.)
Description
Keywords
Water Balance, Lake Level, Lake Abaya, Fluctuation