Evaluation of Hargreaves Equation for ET₀ Calculations at Selected Synoptic Stations in Iran

Evapotranspiration (ET) is a major component of water budget in most regions of the world especially in semi arid areas where irrigated agriculture is a major land use. Accurate determination of ET is very important with respect to crop water demands and decision-making in the management of water resources for different regions of a country like Iran. In this study thirty nine years of data (1967–2005) from 35 synoptic stations in different parts of Iran were used to evaluate reference ET. One of the most common ways for calculating reference Evapotranspiration (ET₀) is the Penman-Monteith method that includes many parameters. However, there is a constraint in using Penman-Monteith method in many regions due to lack of requisite data such as long term sunshine hours (several years). This constraint forced us to investigate the applicability of Hargreaves Equation for calculating the ET₀ values. To this end, the parameter α coefficient was derived for defining the relationship between the two methods. The results of the analyses showed that Hargreaves' Equation can be used instead of Penman-Monteith method where certain parameters are not available. Also, there is an advantage in Hargreaves Equation because it just needs monthly maximum and minimum air temperatures. After derivation of α coefficient for any station, one can calculate ET₀ even in the absence of fundamental parameters needed for the Penman-Monteith method.