Bathymeteric Survey And Estimation Of Water Balance Of Lake Zeway, Central Rift Valley Of Ethiopia

Abstract

Lake Zeway is locally called “Hara Dambal”, is naturally existing exoreic reservoir located in the central Rift valley of Ethiopia with a total drainage area of 7444km2 and at elevation of 1636 m.a.s.l. It is drained with two main rivers flowing in to the lake which are Meki and Katar and at the same time Bulbula River spills form the lake. The Lake is currently used for irrigation, water supply, fishing, transportation and recreation. Global change of climate, newly development activities around the lake, uncontrolled abstraction of water and human interference of the catchment inevitably changed the hydrological and ecological balance of the lake. The objective of the study was focused on bathymetric survey and assessing the water balance components of the Lake Zeway. The bathymetric map of the lake was established at a contour interval of 0.5 meter based on depth measurement of 652 geographic co-ordinate points using Arc GIS software while water balance components of the lake was analyzed using spread sheet . Average rainfall for the areal rain fall was quantified using stations arithmetic average method. Evaporation has been estimated using Penman Montheith method. The irrigation water abstraction was estimated using Cropwat. Bathymetric survey results showed that maximum depth of the lake, average depth, maximum length, maximum width of water lake perimeter, area and volume were 0.0062 km, 0.0026 km, 31.74 km, 20.05 km, 95.91 km, 469 km2and 1174.23 MCM respectively. The water balance analysis of the lake revealed that, the inflow components such as gauged runoff (Katar and Meki), un-gauged runoff and precipitation on the surface of the Lake were 699.08 MCM, 121.33 MCM, and 356.51MCM per year respectively. While, the out flow components such as gauged runoff (Bulbula river), evaporation from the lake surface, and abstraction (irrigation and livestock consumption) were 173.45 MCM, 933.23 MCM, and 82.19 MCM respectively. The net groundwater flux estimated as a residual of the other water balance components was +11.95 MCM, indicating that the ground water inflow was substantial to contribute the flow to the lake Zeway having a positive impact to the storage of the reservoir system. The change in storage (∆S) was estimated to be -11.93 MCM out of the total annual inflow percentage. The evaporation, Bulbula river outflow and abstraction (irrigation and livestock) consumption were 78.49%, 78.49% and 6.91% respectively out of the total out flow. Therefore; evaporation was highly responsible for the depletion of Lake Zeway annual water budget. In general, the mean annual inflow to the lake and mean annual out flow from the lake was estimated as 1176.93 MCM and 1188.86 MCM respectively. Annual inflow component of the lake was showed decreasing trend, while outflow components (evaporation and abstraction) showed increasing trend. This leads to the declining effect of Lake Zeway water level and flow of Bulbula River which supplies Abijata Lake. The rising trend of annual evaporation rate and siltation rate is responsible for this effect and will change this lake to Endoerhic Lake in near future and will attain the fate of Lake Haromaya and Abijata in the long run. Therefore, it is recommended to look a solution for minimization of the extent of out flow components (rate of evaporation and over utilizations of water from this lake) as well as applying important catchment development policy to minimize siltation for future sustainability