Comparison of the Daily, Monthly and Annual Time Steps in Hargreaves-Samani and Penman-Monteith-FAO Models to Estimate Reference Evapotranspiration in the ZayandehRud Basin

Reference evapotranspiration (ET0) is the maximum amount of water transpiration for a particular vegetation cover (such as grass, alfalfa, or a standard crop area). FAO 56 report emphasis, is on measuring meteorological parameters at a height of two meters above green grass. Cultivation around the station should be on a wide area and there is no shortage of water. Agricultural meteorological stations covering a certain territory around them record the same meteorological conditions as the surrounding plants. The Penman-Monteith-FAO (PMFAO) equation was proposed to calculate ET0 and the use of alternative method was not recommended. In the present research, the Hargreaves-Samani (HS) method and the PMFAO method were evaluated on daily, Monthly and yearly time steps in the ZayandehRud basin over a period of 25 years from 1995 to 2019. The results showed that in the daily time step, the difference between the two methods is large (Nash coefficient from 0.63 to 0.87). In the monthly time step, the difference is generally negligible (Nash coefficient from 0.8 to 0.96). In annual time step (Nash coefficient above 0.8), the results are very close to each other. But in different stations, the same behavior was not seen, for example, in Kabutrabad station, in contrast to East Isfahan station, almost always, ET0 by HS method is higher than PMFAO. Therefore, in practice, to use ET0 by HS method, it is recommended to verify and evaluate at least with the nearest synoptic station and PMFAO method and then use it.