The Impact of Existing and Proposed Irrigation scheme on Hydrology of lake Ziway

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Addis Ababa University


Lake Ziway is located in Oromia Regional State near the Town of Ziway some 150 km south of Addis Ababa at the northern end of the southern Rift Valley. The lake covers an area of some 450 km2 at its average surface level of 1,636.12 m and has a maximum depth of 8 m. The two major rivers flowing to the lake are Meki and Katar Rivers and there is Bulbula river as an outflow from the lake. The Lake is important water resource in the study area beside its importance; the level of the lake has changed dramatically over the past decades due to the rapid change of inputs and output components of the lake water balance. To alleviate the existing development constraints, the national government of Ethiopia has designed region based irrigation development polices and strategies, from this development the one which needs more attention is the Ziway-meki pressurized irrigation development project which planned to irrigate a gross area of 15500 hectare (net 14657.2 ha) land. This study is conducted to determine water balance of Ziway lake based on the available water balance components and by considering existing and proposed irrigation scheme of the Ziway-Meki irrigation project by creating different scenario to reflect the different stage of irrigation development and variation in the river flow patterns that flow to and out of the Ziway Lake and the rainfall pattern within the Ziway Lake basin. The water balance was formed on the basis of excel spreadsheet which handles all the inflow (from rivers and runoff alongside the lake shore) and outflow patterns from the lake (irrigation and water supply need and Bulbula river). The water balance was basically seen via the lake water level by applying existing abstraction and proposed irrigation scheme of 14657.2 ha land. From the result of water balance model the lake drops its level from the full level (1638m) by 0.568m from September to august of which 0.33m is by the existing irrigation scheme and 0.238 m drop is due to inflow rivers variation. From model result the existing abstraction and the total irrigation area of 14657.2 ha decreases Bulbula outflow by 20 m3/s of which 17.51 m3/s due to the proposed scheme. Such yearly variation would result in a drastic decrease in Lake water level and Bulbula river flow in upcoming years.



Oromia Regional