WATER REQUIREMENTS DURING AND AFTER VINEYARD ESTABLISHMENT FOR CHARDONNAY GRAPEVINES GROWN IN THE CARNEROS DISTRICT
A study was conducted to determine the water use or vineyard evapotranspiration (ET) of Chardonnay grapevines during vineyard establishment. ET is the combined loss of water by evaporation from the soil and transpiration by the vine. The experimental vineyard was located in the Carneros District of Napa Valley. In addition, the vines were grafted onto two different rootstocks (110R and 5C) to determine if there were differences in water use between them. Vineyard water use was determined by measuring soil water depletion and the addition of water during an irrigation. Soil water content was measured with a neutron probe (also called a hydroprobe or soil moisture gauge). Access tubes, needed to measure soil water content, were placed at eight sites throughout the vineyard (four sites per rootstock). At each site six access tubes were placed such that the soil water content in one quarter of the vine’s root volume could be quantified down to a depth of 3 m (10 ft) . The tubes were placed equidistant from one another: two tubes in the vine row, two tubes midway between rows and two tubes midway between the four tubes mentioned previously. The tubes also extended midway between vines within a row. The vineyard was planted with the rootstocks in June of 1990. In the Fall, the rootstocks were chip budded with Chardonnay scion buds. Starting the first week in July, soil water content (SWC) was measured at all eight sites (four sites per rootstock) weekly. Soil water content the first year declined slightly from the first measurement date throughout the remainder of the season. This indicated that the vineyard was irrigated at slightly less than vineyard ET during that time. Approximately 3 0%of the water used by the vines was supplied by the soil, with the rest being provided by the irrigation water. The total amount of water used by the vines (averaged across both rootstocks) from July until the end of October was approximately 117 mm (4.6 inches) of water. This was equivalent to 380 liters (100 gallons) of water per vine during that time. Potential evapotranspiration (ETJ , determined at the CIMIS (California Irrigation Management Information System) weather station at Oakville was 575 mm (22.6 inches) of water from July to the end of October. Potential ET is the calculated water use of a short, green, well irrigated reference crop (usually a grass) . The use of water by the grass is used for comparison with the use of water by other crops. ET? is needed to develop crop coefficients. In 1991, SWC increased during the first six weeks of the season indicating that the vines were being over-irrigated. After that, SWC decreased for the next six weeks and then remained constant from then until the end of October. These results indicated that the vineyard was irrigated at close to vineyard ET during the major portion of the growing season. Again, approximately 3 0%of the water used by the vines in this vineyard was supplied by the soil. The total amount of water used by the vines (average of the two rootstocks) from April until the end of October was 175 mm (6.9 inches) of water. This was equivalent to 563 liters (150 gallons) of water per vine. ET0 measured at the Oakville CIMIS weather station was 810 mm (31.9 inches). Leaf water potential was measured several times during the 1991 growing on the Chardonnay scions within the vineyard. Leaf water potential is a measure of the water status of a plant. It previously has been demonstrated that leaf water potentials less negative than -1.0 MPa indicate that the vine is not under water stress. In this study, leaf water potential values for vines on both rootstocks were less negative than -0.9 MPa, indicating that during the growing season the vines were probably not under stress. It also indicates that the vines had been irrigated at amounts close to what the vines actually used. The water used by the vines in this vineyard was less than half that used by grapevines growing in the San Joaquin Valley during similar periods of vineyard establishment. For example, vineyard ET was 300 mm (11.8 inches) and 400 mm (15.7 inches) of water the first and second years, respectively, for a Thompson Seedless vineyard compared to 117 (4.6 inches) and 175 (6.9 inches) for this vineyard during the same years of vineyard establishment. This would be expected as evaporative demand is much greater in the San Joaquin Valley compared to the Carneros district. In addition, this Chardonnay vineyard was not planted until the last week in June compared to an April 10 planting date for the Thompson vineyard (therefore, a shorter time period for taking measurements in the first year of vineyard establishment at the Carneros site). Another reason for the lowered water use reported here was that vine growth was much less for the Chardonnay vines compared to the Thompson Seedless vines grown near Fresno. The data contained in this report is the first the authors are aware of in which vine water use was quantified for vines grown in a cool climate during vineyard establishment. This study will continue for another four years, including years in which vines will be in full production. Lastly, crop coefficients will be calculated for each year of the study and then be used for future irrigation management practices.