Issue link: http://winesandvines.uberflip.com/i/246577
GRAPEGROWING vineyard ET is affected by the evaporative demand of the atmosphere, vine canopy size and availability of water in the vineyard soil. 3) The ratio of the actual vineyard ET to the Reference ET. Assuming the vine canopy area has not changed much since the previous irrigation, a decrease in actual vineyard ET may be due to either a decrease in evaporative demand or a decrease in water availability. The ratio of the actual vineyard ET to the Reference ET accounts for changes in evaporative demand. A decrease in the ratio of the actual vineyard ET to the Reference ET indicates that the vines are responding to lower soil water availability. Grapegrowers may monitor the ratio of actual vineyard ET to the Reference ET to determine a level of vine water deficit and maintain a desired water status level. 4) The run-time for a pump required to irrigate a vineyard at a given ratio of actual vineyard ET to Reference ET. The information facilitates the viticulturist's decision to decrease, maintain or increase the amount of water the vineyard receives relative to what it has been receiving. Growers currently estimate the potential water use of a fully irrigated vineyard by using the Reference ET and crop coef- 145 Jordan Street • ficient (Kc) model. The area of transpiring leaves of a vineyard is typically smaller than the transpirational area of a reference grass crop. The Reference ET is multiplied by the crop coefficient (Kc), which corrects for leaf area differences between the reference crop and the vineyard, to estimate the evapotranspiration of the fully irrigated vineyard. The weekly potential water use of the 25-acre Cabernet Sauvignon vineyard at Wente Vineyards in Livermore, Calif., was 34.3-43.8 gallons per vine per week in July (Table 1). The actual vineyard evapotranspiration was close to the modeled evapotranspiration, demonstrating that the vineyard had sufficient water in the soil to meet the potential demand. MonthWeek EToKcETc ETa (inches) (gal./vine)(gal./vine) July 1 1.770.68 39.5 36.3 July 2 1.810.73 37.5 39.8 July 3 1.660.67 34.3 33.0 July 4 1.780.71 43.8 38.1 Table 1. The Reference ET (ETo), the crop coefficient (Kc ), the modeled vineyard evapotranspiration (ETc ) and the actual vineyard evapotranspiration (ETa ) for the 25 acres of Cabernet Sauvignon at Wente Vineyards. The Reference ET was taken from the local CIMIS station. The crop coefficient was estimated from the shaded area beneath the vines using the method of J.E. Ayers and L.E. Williams.3 The modeled vineyard evapotranspiration (ETa ) was the product of the Reference ET and the crop coefficient (Kc ). The actual vineyard ET (ETa ) was measured with surface renewal. Recent UC Davis breakthrough The turbulent motions of the wind drive the movement of water from a vineyard into the greater atmosphere. As the wind moves across a vineyard, heat and water vapor are transferred from the vineyard into eddies of the wind. When an eddy of wind reaches a surface-renewal station in the vineyard, it reports the heat and water vapor transfer over the area that the eddy has traveled. A UC Davis research group of Drs. Andrew McElrone, Kyaw Tha Paw U, San Rafael, CA 94901 • 415-457-3955 • Fax 457-0304 Rick Snyder and Tom Shapland in the Departments of Viticulture & Enology and Atmospheric Science developed the breakthrough in surface renewal.2 In each field it was deployed, surface renewal previously had required calibration against expensive direct evapotranspiration measurement methods. Once the surface-renewal system was calibrated, it matched the actual evapotranspiration measurements from a • www.boswellcompany.com pr actica l win ery & vin eya r d F EB RUARY 20 14 61