Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Summer 2011
Vapor Pressure Deficit
High temperature was not the only issue in August; the vapor pressure deficit (VPD) reached abnormally high values on one or more dates that month. Vapor pressure deficit is defined as the difference between how much water vapor is in the air, and the amount of water vapor the air can hold at saturation at a given temperature. VPD is a function of relative humidity and temperature and is measured in kilopascals (kPa).
A high VPD occurs when high temperature and low relative humidity occur at the same time. Figures 1 and 2 show hourly temperature and relative humidity data recorded at two WWG stations and the VPD calculated from those data.
Daily maximum VPD values calculated from the most recent 20 years of data recorded by the California Irrigation Management Information System (CIMIS) weather station in Windsor (Sonoma County, CA), indicate a VPD greater than 6 kPa occurred on 16 dates. That calculation assumed the maximum temperature occurred at the same time as the lowest relative humidity on each date. Since that was not likely to occur on all 16 dates, it is safe to say that a VPD over 6 kPa is very rare in Windsor. In the central San Joaquin Valley, daily maximum VPD values can reach 7 kPa, but that is also rare.
Weekly VPD measurements we recorded in 2009 between 1 pm and 3 pm from late July through October in an Alexander Valley vineyard, ranged from 1.94 to 4.36 kPa. We did not record measurements in 2010; however, a new WWG station in Alexander Valley (just a few miles from the 2009 vineyard site) records data every 15 minutes and generates hourly reports. VPD calculated for the same dates and hours in 2010 as we measured the previous year ranged between 0.92 and 6.47 kPa.
On August 24, 2010, the maximum VPD soared to 7.69 kPa (Figure 1). That day in Graton, the maximum VPD was 6.34 kPa (Figure 2). As a point of reference, that day in Parlier (about 20 miles southeast of Fresno)
the maximum VPD was 6.59 kPa (calculated from CIMIS weather station data in the manner previously described).
How is leaf water potential affected by VPD?
At some point during summer, available soil moisture from previous winter rains is significantly depleted and shoot tip growth slows; this is about when many growers start to apply water. The amount of water growers choose to apply from that point to harvest will affect the degree of vine water stress. If a high VPD event occurs, it will usually happen in summer when soil moisture levels have been purposely depleted.
Some growers measure leaf water potential (LWP) weekly during the growing season to monitor water stress in vines and modify an irrigation schedule accordingly. A few growers noted that the LWP during the heat spell was not much different than during the week prior.
Researchers have shown that soil moisture availability of deficitirrigated or dry-farmed vines has a stronger effect on LWP than does VPD. Reduced water in the soil profile causes leaf stomata to become smaller, which reduces conductance and transpiration. In studies, LWP of moderately stressed vines (LWP less than -12 bars) did not change significantly over a wide range of VPD conditions.3
Take home messages
On very hot, dry days in midsummer, vines under moderate (or greater) water stress or unirrigated vines will not transpire at significantly greater rates than they would under milder conditions; therefore, they will not lose significantly more water than during “normal” summer days. On the other hand, clusters can lose a significant amount of water by transpiration in those conditions as compared to “normal” summer days, and depending on severity, sunburn or worse may result.
Things to consider
Cluster exposure, and thus berry temperature, is affected by many factors
including row orientation, trellis design, water management, leaf removal, and sprinkler cooling. If sunburn is a common problem in a block, then the objective should be to reduce direct solar radiation on fruit, and to have adequate soil moisture when needed. That is easier said than done. However, what follows are some things to consider:
Row orientation – This is often determined by the shape and size of the land available for planting, configuration of adjacent blocks, slope, wind direction, land use of adjoining parcels, etc. Practical considerations and logistics usually have the strongest impact on vineyard design; however, the correct row orientation can reduce the risk of sunburn and heat damage, especially in warm, low vigor sites.
Vine rows oriented north-south intercept more sunlight compared to eastwest. In warmer coastal regions, trellis systems designed to confine shoots to a narrow zone, combined with normal production practices of leaf or lateral shoot removal, often result in overexposure of fruit to sunlight. Offsetting vine rows by 25° to 45° from true northsouth to northeast-southwest orientation reduces direct exposure of fruit during the warmest period of the day.1
Trellis design – A vertically shoot-positioned (VSP) trellis is the most common type used in the North Coast and is ideal for low to moderate vigor sites because vine rows can be spaced more closely together.
If row orientation results in excessive cluster exposure, some growers have added cross-arms to widen the canopy, or modified shoot orientation on the afternoon sun side of the vine row to allow foliage to better cover clusters. Such adjustments are limited because of the need for adequate distance between already narrow tractor rows. If (true) north-south rows are the only option, do not use a VSP trellis; cross-arms will be essential to provide fruit shading.
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