The Determination of Crop Coefficients for Central Coast Vineyards

This project has further developed and applied a novel method for determining the site-specific irrigation crop coefficient, based on measurements of the midday canopy shaded areas using large solar panels. The canopy shaded areas were measured at eight vineyards throughout the growing season in San Luis Obispo County, and converted to crop coefficients. Together with local reference evapotranspiration and irrigation application data, the irrigation applications were expressed as a percentage of the full potential vine water use.

This technique makes it truly practical to fully utilize the equation developed by Williams and Ayers, which correlates the midday canopy shaded area to the irrigation crop coefficient. Prior to the development of this solar-panel method, the use of this equation was largely limited to research purposes, as the effort required for measuring the canopy shaded area using the existing manual or photographic techniques was impractical for commercial operations. The site-specific crop coefficients that a grower can collect with this method will allow one to apply climate-based irrigation scheduling with more confidence, less risk, and more consistency season after season. The large differences between the crop coefficients measured at sites that had similar trellis systems indicate that there is a value in measuring these coefficients locally.

With accurate crop coefficients it becomes possible to perform detailed evaluations of irrigation management at a given site, quantifying the season?s irrigation applications as percentages of the full potential vine water use. These results show, likely for the first time, just what degree of irrigation deficit the test plot vines were subjected to during the growing season.