WATER BALANCE ANALYSIS OF RIFT VALLEY LAKES BASIN (ABAYA & CHAMO LAKES WATER BALANCE)

Abstract

Rift valley Lakes Basin (RVLB) is one of the river basins in the country. It is an area of significant ecological and environmental interest through the lakes system and the wild life parks and reserves. The basin covers a total area of approximately 52,000 km2 and comprises three sub basins; lakes Ziway-Abiyata- Langano; Awassa and Abaya - Chama- Chew Bahir.The Abya Chama Drainage Basin is a sub-basin of the rift valley that crosses through Ethiopia midway in the north south direction. The basin comprises mainly the two lower lying lakes, Lake Abya and Lake Chama. In this study analysis of the water balance of Abaya and Chama Lakes has been done from the year of 1990-2004 (for 15 years) after determining the water balance components, runoff using Rainfall-Evaporation- Soil moisture model, aerial rainfall using Thiessen polygon method and evaporation using Penman combination method, Among many natural conditions that can have impact on the water resources of any region natural climatic factors would play great role. In Abaya Chama Lakes' basin high evaporation and low rainfall characteristics have considerable influences on the lakes water volume and over all water balance of the region. In the simulation result of water balance of the lakes, the mean total annual input to Abaya Lake is 2626.2 MCM, out of which the composite runoff from partially gauged, input is from runoff where as the remaining 32% is from direct rainfall on the lake and the lake is in problem losing 101.8% of the total input of water annually. Both Lakes are experiencing receding in level though the case of Abaya Lake is as worse as Chama Lake. This is due to the rainfall decreasing trend of the area besides high evaporation though its variation is not as such sounding.