Correcting Midseason Crop Coefficients for Climate

It is well known that crop coefficients are not necessarily transferrable from one location to another, for a variety of reasons. For example, the United Nations Food and Agricultural Organization (UN FAO) 24 publication on evapotranspiration listed ranges of midseason crop coefficients for particular crops, depending on wind speed and humidity. In the more recent UN FAO 56 publication, an equation was presented that adjusted crop coefficients for wind speed, humidity, and crop height. Climate correction is important for sharing and adjusting crop coefficient data. However, it was found that the FAO 56 equation gave inaccurate corrections for a crop having the same characteristics as the reference crop surface, and it is likely that it also will be inaccurate for other crop surfaces. Consequently, the aim of this research was to develop and test a new method to correct midseason crop coefficients for climate differences. Climate data were used from the California Irrigation Management Information System and a similar approach was used to develop standardized reference evapotranspiration equations for short and tall canopies, to develop and test the new method. The method estimates the aerodynamic resistance of crops in a base climate as an inverse function of wind speed. This function of wind speed was used in a modified Penman-Monteith equation to account for climate effects on midseason crop coefficients in nonbase climates. The methodology to account for climate data and estimate midseason crop coefficient values in a nonbase climate from base climate crop coefficients is presented in this paper.