Abstract

Reference crop potential evapotranspiration (ET_o) is needed for calculating crop water requirement, however its direct measurement by using lysimeter is difficult and time consuming. Although, many equations have been derived for estimating ET_o, however the Penman-Monteith equation is the most common method that has been used for different climates and locations of the world. However, the Penman-Monteith equation needs full weather data such as solar radiation or sunshine hours, therefore the use of this equation has been limited. In this study, in Fars province in south of Iran, monthly sunshine hours were estimated based on minimum, maximum and mean monthly air temperature, and minimum, maximum and mean monthly air humidity, and the obtained equations were used for estimating monthly ET_o based on the Penman-Monteith equation and the results were compared with the Penman-Monteith equation by using actual sunshine hours data. The results indicated that the derived equations for estimating sunshine hours had high accuracy for estimating monthly ET_o with the Penman-Monteith equation. Therefore, it is possible to estiamte monthly ET_o with Penman-Monteith equation in the study area without using the sunshine hours data.

Key words: Air humidity, air temperature, evapotranspiration, sunshine hours.