Grapevine water use and the crop coefficient are linear functions of the shaded area measured beneath the canopy

The relationships among water use and the crop coefficient of Vitis vinifera L. cv. Thompson Seedless with several measures of canopy development were determined with the aid of a weighing lysimeter in the San Joaquin Valley of California. At various times during two growing seasons, vine leaf area, calculated leaf area index (LAI) and the amount of shade cast on the ground directly beneath the canopy were determined. Leaf area was estimated by measuring the length of all shoots on the vines within the lysimeter and determining the relationship between length and leaf area per shoot and calculating total vine leaf area or by destructive harvests of vines of similar size surrounding the lysimeter. Shaded area was determined in 1998 using a grid (with 50 cm2 individual sections) on the ground beneath the vine at solar noon and estimating the percent shade within each square. Total shade was calculated as the product of the area of all squares and the percent shade within each square. In 1999 shaded area was determined from an image of the shade beneath the canopy that was downloaded to a computer and the shade digitized with the use of a software program. Daily water use ranged from 4 to 60 L per vine across both years. Leaf area per vine ranged from 2 to 34 m2 per vine during the study. The amount of shade cast on the ground was a linear function of total vine leaf area although there were differences between years. The north and south curtains of the vines' canopies were raised for a 2-week period in 1999 to simulate an overhead trellis system. The percent shaded area increased from 60 to 75% and vine water use increased from approximately 42 L per vine before the curtains were raised to greater than 60 L per vine after being raised. The crop coefficient (K(c)) increased from 0.9 to 1.3. Vine water use and the crop coefficient were linearly related to leaf area per vine, LAI and the amount of shade cast on the ground. However, the greatest R2 value (0.95) of the relationships with the K(c) was that for shaded area compared to a R2 value of 0.87 for leaf area and LAI. The data indicate that due to the structure of a grapevine canopy the interception of light, as measured by the amount of shade cast on the ground, is a more important determinant of vine water use and the Kc than total leaf area or LAI.