Modeling Grapevine Water Use
The last of a three-year study was completed in 2000. The data collected this past growing season was to establish crop coefficients for grapevines using different trellis systems. The seasonal crop coefficients were derived from the linear relationship between the crop coefficient and percent shaded area beneath a Thompson Seedless grapevine measured at solar noon (between 1230 and 1330 hours, PDT). The slope of this relationship was 0.017. The 0.017 value multiplied by the percent shaded area is the crop coefficient. For example, if the shaded area beneath a vine at solar noon is 20%of the total area allocated to the vine within the vineyard then the crop coefficient is 0.34 (0.017 x 20 = 0.34).
Several different trellis systems were measured at five locations throughout the State of California. These trellises included the lyre, GDC, VSP and vertically split canopies using the Scott Henry and Smart-Dyson trellis systems. Several additional trellis systems were also used such as the ?California Sprawl? and others using various crossarms to support the cordons and foliage catch wires. Shaded area under the vines was determined with the use of a digital camera and software to calculate shade in the image. Results indicated that a trellis that horizontally splits the canopy results in greater shaded area and therefore higher crop coefficients than a trellis that does not. The same is true for canopies that are vertically split compared to a trellis like the VSP. The crop coefficients also take into account row spacing.
At several locations, water meters were installed to measure the amounts of water the grower/cooperators applied and midday leaf water potential was measured. Applied water amounts were compared at the end of the season with estimated vineyard water use (Etc) during specific intervals. Estimated Etc was calculated as the product of Eto times the percent shaded area derived crop coefficients. In most cases when estimated Etc and applied water amounts were similar, midday leaf water potential values were less negative than ?1.0 Mpa (-10 bars). When applied water amounts were less than estimated Etc leaf water potential was more negative than ?1.0 Mpa. These results indicated that the derivation of crop coefficients from percent shaded area were useful in estimating actual vineyard water use at 100%of Etc.
The ability of grape growers to predict potential vineyard water is a necessity in California where water can be in short supply or expensive. One can estimate vineyard water use with the following equation: ETc = ETo x kc. Once this is calculated then deficit irrigation practices can be used such that a fraction of full ETc is applied to the vines either throughout the growing season or during specific phenological events. I have shown over the past several years that one can deficit irrigate grapevines such that water use efficiency is increased, yields can be either maintained or increased with high fruit quality for table, raisin and wine grapes.