Estimation of reference crop evapotranspiration by Hargreaves Samani and modified Hargreaves Samani methods and evaluating them using the water requirement system of plants in Kermanshah province

Accurate estimation of reference evapotranspiration is essential in many studies such as water hydrological balance, design and management of irrigation systems, simulation of plant growth stages and yield, and planning and management of water resources. Accurate estimation of this parameter can lead to proper management and planning of water resources. The aim of this study was to estimate the values of reference evapotranspiration in synoptic stations of Kermanshah province during the statistical period of 2010-2020, evaluation of Hargreaves Samani and modified Hargreaves Samani methods and zoning of reference evapotranspiration in the province. The standard Penman Monteith-FAO method resulting from the water requirement system of plants was used to determine the reference evapotranspiration. The results showed that the highest and lowest annual reference evapotranspiration among the meteorological stations in the amount of 1770 and 1341 mm per year belonged to Qasr-e-Shirin and Sararood stations, respectively. The most important effective parameter in the higher reference evapotranspiration in Qasr-e-Shirin station was high temperature and wind speed compared to other stations. The results of zoning of reference evapotranspiration showed that the western regions of the province have a higher intensity than the eastern regions. Evaluation of Hargreaves Samani and modified Hargreaves Samani methods showed that the mean of R2, RMSE and NRMSE statistics in Hargreaves Samani method were 0.9, 0.78 mm day-1 and 0.19, respectively. The mean values of these statistics in the modified Hargreaves Samani method were 0.71, 2.59 mm day-1 and 0.65, respectively. Therefore, the application of Hargreaves Samani method to calculate the reference evapotranspiration in this province is recommended.