Climate Effect on Crop Coefficients

Crop coefficient (Kc) values are commonly used by agronomists and engineers to estimate well-watered crop evapotranspiration (ETc) as: Kc=ETo×Kc, where ETo is a measure of evaporative demand that is estimated from weather data collected over a large grass field using the standardized reference evapotranspiration for short canopies (ETo) equation. The ETo equation has a fixed canopy resistance, and the aerodynamic resistance is an inverse function of wind speed. To resolve a daily Kc factor, weather data is used to calculate ETo, ETc is measured, and the daily Kc=ETc/ETo. Worldwide, the biggest source of Kc information is the UN FAO Irrigation and Drainage Paper 56, and the publication does include a climate correction for Kc values. Theoretically, a crop with the same aerodynamic characteristics as the reference surface, should have Kc = 1.00 regardless of the climate. An analysis of California data, however, showed that the climate correction Kc value for a crop with the same aerodynamic characteristics varied with wind speed. Therefore, a new method to adjust Kc values for climate differences was developed by determining the Kc and the corresponding aerodynamic resistance coefficient in a base climate. It was assumed that the net radiation and canopy resistance does not change, but the differences in aerodynamic resistance coefficient between ETo and the crop allowed for the calculation of ETo, ETc, and Kc values specific to other climates. This analysis showed big differences between the method presented by the UN FAO 56, and the new equation to correct crop coefficients could greatly improve ETc estimation accuracy around the world by making crop coefficients more universally applicable. The results of the study will be presented.