COMPARISON OF EVAPORATION BY PENMAN METHOD AND MEASURED PAN METHOD: CASE STUDY AT LAKE HAWASSA

Abstract

Evaporation from lakes and reservoirs is an important component of the water cycle. Size and level of Lake Hawassa is changing due to the influence of many natural and anthropogenic factors. Climatic changes such as decreasing trend of precipitation and increasing of temperature that consequently increase the rate of evaporation are the most important factors that cause a decline in the size and level of Lake Hawassa. This study was conducted to compare evaporation estimated by Penman, Hargreaves and measured pan in order to determine the overall water loss from the lake. The Penman method, Hargreaves and pan evaporation method were used to carry out the water loss. Evaporation measured at three different sites using nonstandard pan shows similarity for all sites most of the days but did not agree with results obtained using the standard pan. The percentage error between standard and nonstandard pan and Penman method shows maximum of 53% and 54%, respectively. Comparisons of standard pan evaporation with calculated Penman and Hargreaves evaporations show variations. Monthly-averaged rainfall and pan evaporation values were high at May and September for rainfall and January, November and December for evaporation. Linear correlation between pan evaporation and Penman evaporation as well as pan evaporation and Hargreaves evaporation shows R2=0.9 and R2=0.61 respectively. Net annual water loss due to evaporation from Lake Hawassa is 1582 Mm3 per year.