Water Requirements During and After Vineyard Establishment for Chardonnay Grapevines Grown in The Carneros District
A study is being 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 is 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 the two. The rootstocks were planted in June of 1990 and Fall chip budded with the Chardonnay scion. Vineyard water use was determined by measuring soil water depletion and monitoring the addition of water via irrigation or measured rainfall. 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 places such that the soil water content in one quarter of an individual vine’s root volume could be quantified down to a depth of 3 m (approximately 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 v\Lnes within a row. The year 1992 represented the third growing season for this vineyard. Soil water content decreased rapidly early on in the growing season until the end of May. After this time soil water content remained constant until the end of October. The depth of water extraction from the soil extended down 1.3 m (4.25 ft). The constancy of the soil’s water content from June until October indicated that the water applied via irrigation equaled the amount of water used by the vineyard. The average seasonal water use of both rootstocks was 236 mm (9.3 inches) of water. This is equivalent to approximately 750 1 (200 gallons) of water per vine. The amount of water used by the vines supplied from the soil resevoir was 42%of the total amount used. Midday leaf water potentials were measured every two weeks during the season. Leaf water potential is a measure of a vine’s water status and would give an indication as to whether the vine experienced stress at any time. Of the eleven times measurements were taken, leaf water potential readings on nine dates indicated that the vines were not under stress (values were less negative than – 0.8 MPa or -8 bars) . However, on two of the dates leaf water potentials became more negative than -1.0 MPa. Values more negative than -1.0 MPa are associated with vine water stress in mature vines. It should be pointed out that on both of those dates ambient temperatures were greater than 32°C (90°F) and relative humidities were quite low. This information would indicate that irrigation amounts should have been increased during those periods when evaporative demand was greater than normal. One of the primary objectives of this study is to determine crop coefficients for vines grown in a cool climate during vineyard establishment. The crop coefficient for this vineyard started out at 0.5 and decreased thereafter until day of year 220 (approximately the first week in August) when the crop coefficient reached a minimum of 0.2. Subsequent to this date the crop coefficient increased and reached a maximum of 0.4 at the end of the growing season. The relatively high crop coefficient early in the season could have been due to the percolation of water below the depth of the access tubes (which would have been measured as vineyard water use) and the fact that a natural cover crop had been established during the winter months. The cover crop would have used a considerable amount of water until the vine rows started drying out. The increase in the crop coefficient from August until the end of the growing season was due to the continued growth of the vines up to November. An increase in the crop coefficient up to the end of the season has previously been reported for vines grown in the San Joaquin Valley during the second year of vineyard establishment.