|Ayars, James - Jim|
Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/19/2005
Publication Date: 11/12/2005
Citation: Williams, L.S., Ayars, J.E. 2005. Grapevine water use and crop coefficient are linear functions of the shaded area measured beneath the canopy. Agricultural and Forest Meteorology. Vol. (132) pgs. 201-211. Interpretive Summary: Scientific water management is critical for extending water supplies in irrigated agriculture and irrigation scheduling is an important part of scientific water management. An irrigation schedule determines both the time and depth of irrigation application based on a water balance calculation. The critical element in the water balance calculation is the determination of the crop water use. This is routinely done by estimating the reference evapotranspiration and multiplying it by a crop coefficient that characterizes plant development at the time of computation. This paper describes the development of crop coefficient for grapevines that is based on the estimate of shaded area under the canopy. It was determined that there was a linear relationship between the crop coefficient and the shaded area. This provides a very simple technique for water managers to determine the appropriate crop coefficient their vineyard by simply putting a grid under the canopy and counting the shaded squares. The shaded area data along with the relationship developed in this paper will provide the crop coefficient. The proposed relationship provides a crop coefficient for crop canopies on simple to complex trellis systems and with varying row spacing. Given that grapevine is the number on horticultural crop grown in the world, there is a significant opportunity for improvement in water management throughout the world.
Technical Abstract: 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 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. Daily water use ranged from 4 to 60 L per vine across both years. Leaf area per vine ranged from 2 to 34 square meters 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 ~ 42 L per vine before the curtains were raised to greater than 60 L per vine after being raised. The crop coefficient (Kc) increased from 0.9 to 1.3. Vine water use and the crop coefficient were linearly related to the 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 Kc was that for the 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 the total leaf area.