Assessment Of Water Balance And Lake Level Fluctuation Lake Abaya Ethiopia

Lake Abaya is a quasi-endorheic lake which is located 6º 26’ N Latitude and 37º 53’ E Longitude withrnsurface area of 1095 kmrn. It drains with five main rivers (Bilate, Gidabo, Gelana, Hamessa and Harie) andrnmany other intermittent tributaries and covers a total catchment area of 14638.83kmrn2rn2rn. rnThe objective of this study is to assess the water balance and lake level fluctuation of Lake Abaya. Thernlake level is gauged up to the year 2013, but the inflow from the four out of five tributaries is gauged uprnto the year 2005. As a result stream flow is predicted by using HEC-HMS to the year 2013. Inflow fromrnthe un-gauged catchment is estimated using area ratio method. Areal rainfall for the sub-basin and lakernsurface is estimated using Thiessen polygon method. Evaporation from the lake surface is estimated usingrnCropwat 8 software, Penman’s method and Pan Evaporation; pan evaporation is taken for the analysis. rnWater abstraction from tributary rivers and the lake for irrigation is determined by Cropwat 8 software. rnAfter analysis of all components of water balance for the lake, a water balance model has developed usingrncontinuity equation. Summing up of all the components, lake surface rainfall 18.87 %, inflow 32.35 %rn(Gauged 19.18 %, un-gauged 13.16 %), lake surface evaporation 46.08 % and abstraction 2.71 %rncontributes for the lake water balance trend and no observed out flow from the lake. The lake level has arnrelation with climatic and human factors; rainfall and temperature of the area shows a slow increment andrnthat of wind speed, sunshine hours and evaporation shows slow decrement. When comparing the landrncover of 1996 to the land cover of 2006 that the area under intensive cultivation, grassland, urban area andrnriparian has increased by 84.59, 32.64, 1.08 and 0.11 kmrn respectively and that of shrub-land, unidentifiedrnarea, marshland, forest, moderately cultivated and exposed surface has decreased by 69.20, 35.90, 8.00,rn4.92, 0.40, 0.01 kmrn2rn respectively and that of water body and woodland has not changed yet. These all isrnnot in contradict with the increment of the lake level rather it agrees. To irrigate a total land of 9906 harnwith common crops of Banana, Tobacco, Cotton, Vegetables and diversified crops 140.98 MCM of waterrnis abstracted from the lake and all the tributary rivers. Two irrigation scenarios are conducted, scenario 1 rn2rnwith irrigation expansion of 10610 ha on the tributaries needs additional water of 84.10 MCM per yearrnand reduces the lake level by 60 mm, and scenario 2 irrigation expansion of 30,831 ha, it requiresrnadditional water of 528.45 MCM per year and this will reduce the lake level by 420 mm. The simulationrnof lake level has been conducted by excel spread sheet model at monthly time stapes. rnThe causes for the increment of lake level are (1) the nature of the lake (which is quasi-endorheic) (2)rnClimatic factors (the increment of rain fall, decreasing of evaporation, etc.) (3) Anthropogenic factorsrn(deforestation, intensive cultivation, etc.)

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