Evaluation of irrigation management based on local crop coefficient measurements

This project continues earlier work in the use of solar panels as shade-measuring devices, for estimating the percentage of the vineyard floor shaded by the vine leaf canopy, which is the same as the amount of direct sunlight intercepted by the leaf canopy. The primary use of the shade area value is to calculate the irrigation crop coefficient, based on a relationship developed by Dr. Larry Williams at the Kearney Ag Center.

In the 2007 work, the method was used to track the evolution of the midday canopy shaded area in an irrigation trial near Paso Robles. Companion readings of other water-related measurements, including leaf water potential, soil moisture, and leaf temperature, were also made throughout the season.

The shaded area values were used to calculate the site-specific crop coefficient throughout the season, and then to express the actual applied irrigation amounts for the different treatments as percentages of the full vine water requirement, e.g the %of Full ETc.

Comparisons of the %of Full ETc values to leaf water potentials showed some correlation, but not a very strong one. Comparisons with the leaf temperature readings made throughout the season will be done in early 2008.

Further evaluation and testing of the shade-measuring device increased our understanding of the possible degree and types of error associated with using the method. Tests of the effects of the height of the shade source above the solar panel indicated that a relatively small error occurs as the shade height increases, due to reflected light from surrounding objects. Leaf tissue was found to have similar shade-producing effects as opaque cardstock, as far as the response of the solar panel was concerned. Lastly, alterations of the electronic circuit in the device to improve reliability also led to fundamental a change in the device response, meaning that a device-specific calibration will be required.