Refining crop coefficients using surface renewal, eddy covariance and energy balance

The standardized reference evapotranspiration equation was recently presented by the American Society of Civil Engineers – Environmental Water Resources Institute to provide evapotranspiration researchers with a standardized method to estimate reference evapotranspiration. The standardized equation was developed mainly to provide a universal method for researchers to develop generalized crop coefficient (Kc) values that are more universally applicable. Crop coefficients are determined by measuring the evapotranspiration of a well-watered crop (ETc), estimating reference evapotranspiration (ETo) from weather data measured over a well-watered grass surface, and computing the ratio ETc/ETo. The assumption is that one can estimate ETc in the future by using calculated ETo and a Kc that was developed over a crop with the same surface and canopy characteristics. Although ETo is an estimate of the evapotranspiration from a virtual crop surface having a known canopy and aerodynamic resistance, it is approximately equal to the ETc of a 0.12 m tall, cool season grass. The aerodynamic resistance is relatively high for ETo, whereas taller, rougher crops tend to have lower aerodynamic resistance, and the aerodynamic resistance will vary more with wind speed for the taller, rougher canopy. Therefore, one would expect the Kc = ETc/ETo to vary depending on the wind climatology of a region. Thus, Kc values should be corrected in general for wind speed climatology and more specifically for the combination of temperature and wind speed. For example, a high wind speed in the afternoon, when temperatures are hot, is likely to increase ETc more than a high wind speed during a cool morning. In California, a joint project between the University of California and the California Department of Water Resources is underway to refine Kc values for a wide range of crops relative to the new ETo equation. In this paper, some results from the California project will be presented to discuss the accuracy of the ETo equation, some of the developed Kc values, things we learned about using the surface renewal and energy balance approach to assess ETc, and some results indicating the importance of wind speed on Kc values.