Estimation and introduction of the best coefficients of monthly evaporation pan for calculations of water requirement (Case study Kermanshah province)

Evapotranspiration calculations by the FAO Penman-Monteith method require many parameters, the lack of some of which makes the calculations difficult. On the other hand, it is necessary to use methods that are easier to calculate and are easily understood by farmers. One of the indirect methods of estimating reference evapotranspiration is the pan evaporation method, which can be considered as a suitable index to estimate the evapotranspiration of the reference plant and finally the main plant. In this method, in order to calculate the potential evapotranspiration, the results of pan evaporation must be multiplied by the pan coefficient. Accurate calculations related to the estimation of the pan coefficient are very important because it can be used for irrigation planning when there is no lysimeter equipment. In this study, by using of daily, hourly meteorological data, and data from class pan A evaporation from meteorological stations of Kermanshah province in the long term, potential evapotranspiration values of the FAO Penman-Monteith and then pan coefficients were calculated. The index coefficients as a measure of estimation methods for Cuenca, Allan, Snyder, Modified Snyder, and Orang were used. The results showed that the, Orang and modified Snyder method had a higher level of accuracy compared to the other methods. The R2 with Orang method for different cities of Islamabad Gharb, Qasr-e Shirin, Kermanshah, Sararood, Sonqor, Gilan-e Gharb, Sarpol-e zahab, Kangavar were determined as: 0.84, 0.77, 0.72, 0.76, 0.85, 0.79, 0.77, and 0.73, respectively. Finally, the values of pan coefficients calculated by all methods in study stations in different months were calculated and presented. It is hoped that the results of this study will be used by managers and farmers in the region.