Evapotranspiration is one of the basic parameters for agricultural water planning. There are various formulas to estimate this parameter such as FAO Penman-Montith method and Evaporation pan method. In the pan method, the amount of reference evapotranspiration (ETo) estimated by multiplying pan coefficient to amount of water vapor from evaporation pan. One of the limitations of pan method is the need to weather parameters (e. g., relative humidity and wind speed) to estimate Kp. In case we have access to these weather parameters other methods of evapotranspiration estimation are preferred. The aim of this study was to determine monthly and seasonal values of Kp in the Kermanshah province. Kp was estimated by dividing calculated ETo (It is calculated based on daily weather data) over pan evaporation. Spatial and temporal variations of Kp have been investigated based on daily weather data (2000 – 2009) in four synoptic weather stations in Kermanshah province. Results showed that Kp in summer is lower than in autumn and spring. Moreover, results indicated that Kp has inverse relation with altitude. In addition daily and monthly pan coefficients have been calculated for Kermanshah airport station based on the FAO method and compared with the estimated pan coefficients in the current study. However, there was low correlation between daily results, but correlation coefficient between monthly results was high (R2=0.89).
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