Water Use and Effects of Deficit Irrigation on Chardonnay

A study is being conducted to determine the water use or vineyard evapotranspiration (ET) of Chardonnay grapevines subsequent to vineyard establishment. ET is the combined loss of water by evaporation from the soil and transpiration by the vine. The experimental vineyard is located in the Carneros District of Napa Valley. In addition, the vines are grafted onto two different rootstocks (110R and 5C) to determine if there are differences in water use between them. Lastly, five irrigation treatments were imposed to determine the effects of both under- and over-irrigation on vine physiology and growth and wine quality. Vine water use is determined by measuring soil water depletion and the addition of water during an irrigation. Soil water content is being measured with a neutron probe at eight sites throughout the vineyard, four sites per rootstock. Vineyard water use in 1994 and 1995 at full ET (i.e. water applications equal to vine water use) were approximately 470 mm (18.5 inches). Deficit irrigation of vines resulted in the depletion of soil water, the amount dependent upon water application amounts. To date there has been no difference in water use between the two rootstocks. Water applications less than full ET result in more negative leaf water potentials and lower stomatal conductance and photosynthesis. The percent reduction is a function of the amount of water applied. There are highly significant correlations between the grapevine’s water status and soil water content when measured on the same day. There were significant reductions in berry size for vines that were deficit irrigated in 1994 but not in 1995. The difference between the years was due to an irrigation event in 1995 at berry set that resulted in no stress among treatments for a period of approximately three weeks. There has been no differences among irrigation treatments with regard to juice pH. Titratable acidity increased with increasing water applications both years. There were minor differences in berry composition among the rootstocks in 1995, however, there were no significant interactions among rootstocks and irrigation treatments. Experimental wines made in 1994 and 1995 are currently being analyzed. Preliminary results indicate a preference for wines made from the 75%and full ET irrigation treatments.