Determination of the Vineyard Irrigation Crop Coefficients within the Paso Robles Groundwater Basin ‘Area of Concern’

This project evaluated the irrigation crop coefficient at 84 vineyards east of Paso Robles, which lie within a region previously designated as a groundwater ?Area of Concern? due to declining groundwater levels. The mid-summer measurements, taken during the peak irrigation water use period, indicated an average crop coefficient of 0.53, with a range from 0.27 to 1.03. The significant degree of variability indicates the wide disparity in theoretical irrigation water requirements for vineyards in the area, and supports the practical observations that ?not one size fits all? with respect to vineyard water requirements. The survey results, while of value in and of themselves, will be used primarily in conjunction with the results on an ongoing UCCE vineyard water use survey, to add value to those measurements. Comparing the volumes of water used between vineyards is not a fair assessment of how efficiently water is managed, because it does not take into account the different water requirements for different vineyards. By having site-specific crop coefficient values in addition to the measured irrigation use data, it is then possible to compare water use as the percentage of ETc applied, which is a more robust comparison between different locations.