ESTIMATION AND REGIONALIZATION OF EVAPOTRANSPIRATION IN ABAYA CHAMO BASIN

Abstract

The study area, the Abaya chamo Basin is a sub basin of the Ethiopian Rift Valley lakes Basin .It comprises two lakes Abaya and Chamo, perennial rivers, and other ephemeral rivers are entering at Chamo and an over flow from chamo joins Sermale river to join Segen river, which in turn ends up in Lake Chew-bahir at border of Ethiopia with Kenya.The study thus investigates Evapotranspiration (ET) of the basin which is a major component of the hydrologic cycle. As one of universally accepted and well understood evaporation estimation method, it has been assumed that FAC Penman-Monteith method perform better than other available method. Thus in this study. In order to synchronize estimates from various evaporation methods the concept of Penman equivalent estimate (PEE) is introduced. Naturally, due to^the difference in the conceptualization and its intended original use, different evaporation methods give different estimates. The concept of PEE is coined to reduce part of error introduced as a result of using different evaporation methods in estimation of a single time series of evaporation. In this regard, estimates from various methods were standardized to penman method by introducing regression coefficient to each method. Therefore, in this study a standardized value of the PEE is used in the estimation of the Evapotranspiration of the basin. The mean Daily estimates of Evapotranspiration by FAC Penman -Monteith was obtained as 3.7mm/day at Hageremariam to 4.5mm/ day at Mirab Abaya. After methods (Hargreaves and Blanney-Criddle) have been regressed with penman estimate improvement estimate are done for stations. For Mirab Abaya, Dila, Arbaminch and Konso the improved estimate for Hargreaves produces R^ value of 0.98,0.96,0.98 and 0.93 respectively. Calibration with the FAC Penman- Monteith was done by improving the Khs coefficient of the Hargreaves equation. Similarly calibration with the FAC penman -Monteith was done by improving the P value of the Blaney and Criddle equation.