Deficit irrigation of wine grape vineyards Larry E. Williams Department of Viticulture and Enology UC-Davis and Kearney Agricultural Research and Extension Center 9240 S. Riverbend Ave., Parlier, CA 93648 [email protected] The majority of grapevines (Vitis vinifera L.) grown world-wide are cultivated in Mediterranean type climates having warm to hot temperatures and little rainfall during the summer. The cultivation of grapevines in arid and semi-arid regions of high evaporative demand would indicate that water stored in the soil profile would more than likely be insufficient to meet a vineyard’s consumptive water use. For example, the soil water content (SWC) of a Hanford fine sandy loam soil in the San Joaquin Valley is approximately 22% by volume at field capacity while SWC at a soil moisture tension of -1.5 MPa (-15 bars) is approximately 8.0% by volume. Total available water to a depth of 2.9 m (9.5 ft.) for this soil at field capacity is approximately 400 mm (15.7 in,). Seasonal vine water use of the Thompson Seedless grapevines average 844 mm (33.2 in.) per growing season. Therefore, supplemental irrigation is necessary if one is to produce a harvestable crop of high quality even during years with average or above average rainfall. In general, vines receiving no supplemental water or that are deficit irrigated will have less vegetative growth, smaller berries and lower yields than vines that are irrigated or irrigated with greater amounts of water (Williams 2010, 2012, 2014b; Williams et al., 1994; Williams et al., 2010a, 2010b; Williams and Matthews, 1990). Deficit irrigation and/or moderate vine water stress has been associated with increased fruit quality, especially for red, wine grape cultivars (Williams and Heymann, 2017; Williams and Matthews, 1990; Williams et al., 1994). Several methods can be used to determine when to start irrigating (Williams, 2017). One can measure the depletion of water in the soil profile using various methods. Once a critical soil moisture value has been reached, seasonal irrigation would commence. One can use plant-based methods to determine when to start irrigating. This would include the measurement of vine water status (such as midday leaf water potential), leaf stomatal conductance, canopy temperature or thermal imaging and remote sensing. Again, when a pre-determined critical value had been reached, irrigation would commence. I generally start irrigating vineyards in the San Joaquin Valley when midday leaf water potential drops to -1.0 MPa (-10 bars) and in coastal vineyards when it drops to -1.2 MPa (-12 bars) or slightly lower (more negative). The initiation of irrigation each year is dependent upon the amount of water in the soil profile; a function of rainfall amount, soil type and rooting depth. Once the decision has been made to irrigate there are several methods with which to deficit irrigate vineyards. Sustained deficit irrigation (SDI) is the practice of purposely deficit irrigating beginning with the first irrigation of the season and irrigating such throughout the remainder of the growing season. Regulated deficit irrigation (RDI) is the practice of purposely 1 creating water deficits during specific times of the season to conserve water while minimizing or eliminating negative impacts on yield or crop revenue. The timing of RDI in vineyards is usually associated with phenological events of the vine such as between berry set and veraison (berry softening/color change) or veraison and harvest. Both SDI and RDI are based upon knowing what full evapotranspiration (ETc) for the vineyard is and then irrigating at some fraction of that amount once irrigation commences. One last technique used to deficit irrigate vineyards is called Partial Rootzone Drying (PRD). It is an irrigation regime whereby vines are watered on one side of the vine’s trunk (receiving 50% the amount of water of the control treatment (generally full ETc)) during a two- week period and then irrigating the next two weeks on the other side of the vine. During the two-week period roots on the other side of the vine would experience water deficits. Normally two drip lines are placed down a row with one emitter on one side of the vine’s trunk and a second emitter (in the other drip line) on the other side of the vine’s trunk. However, based upon my experience (see Table 1) and that of Gu et al. (2004) the effect of PRD on vine growth and productivity was no different than irrigating at 50% of full ETc (using SDI). In addition, Sadras (2009) concluded that the economic justification for the use of PRD was only suitable in a few rare conditions. One means to estimate ETc for use in a deficit irrigation management program would be to use the following equation: ETc = ETo * Kc (1) where ETo is reference ET and Kc is the crop coefficient (Allen et al., 1998). Reference ET is a measure of evaporative demand and can be obtained from the California Irrigation Management Information System (CIMIS) or other entities. The Kc is the fraction of water used by a specific crop compared to that of ETo at a given location. The above equation predicts ETc under standard conditions where no limitations are placed on crop growth or ET due to water shortage, crop density or disease, weed, insect or salinity pressures (Doorenboos and Pruitt, 1977). Crop coefficients are derived from measurements of ETc which can be determined from energy balance and microclimatological methods, soil water balance technique and lysimeters (Allen et al. 1998). Crop coefficients have been developed on Thompson Seedless grapevines grown in a weighing lysimeter at the Kearney Agricultural Research and Extension (KARE) Center and farmed as raisin grapes (Williams et al., 2003a; 2003b) or as table grapes (Williams and Ayars, 2005a). Row spacing for the vineyard was ~ 3.5 m (11.5 ft.), a 0.6 m (24 in.) crossarm was used and canopy type a sprawl. The maximum Kc values obtained (0.95 to 1.15) across years for these vines were greater than those previously published for raisin or table grapes. The seasonal Kc values were expressed as a function of calendar day and degree-days (base of 10○C) from budbreak. By using degree-days, it was anticipated that the crop coefficients developed near Fresno could be adapted to other locations where date of budbreak may differ. The author has also developed seasonal Kc values for Chardonnay vines grown in the Carneros district of Napa Valley and trained to a VSP trellis on 2.13 m (7 ft.) rows using the soil water balance method (Williams, 2014a). The seasonal progression and maximum Kc values 2 (0.74) developed for that vineyard differed from those for Thompson Seedless grapevines due to differences in trellis type and row spacing. Williams and Ayars (2005b) found that the Kc was a linear function of the amount of shade (also referred to as fraction of ground cover by others) measured beneath the lysimeter vines at solar noon. The relationship was: Kc = -0.007 + 0.017x (2) where x is the percent of the ground shaded beneath the canopy (a whole number) per area allocated to an individual vine within the vineyard at solar noon. Others have also found that the Kc was a linear function of the fraction of ground cover with a slope similar to that given in Equation 2. The estimated slope from Ayars et al. (2003) on peach trees grown in a weighing lysimeter was 0.016. A slope of 0.018 was calculated for Colombard grapevines with data from Stevens and Harvey (1996). Picón-Toro et al. (2012) and López-Urrea et al. (2012) found the slope for that relationship to be 0.02 and 0.017, respectively, for Tempranillo grapevines grown in weighing lysimeters. Lastly, Ferreira et al. (2012) calculated a slope of 0.019 for their relationship between the Kc and percent ground cover. López-Urrea et al. (2012) concluded that measuring canopy cover is a reliable approach to estimate Kc values in grapevines and that the use of growing degree-days should improve the precision of the estimate by removing year to year variation in crop development. The above had been advocated in earlier papers by Williams et al. (2003b) and Williams and Ayars (2005b). The usefulness of this technique to derive Kc values for other crops is expanding (Allen and Pereira, 2009). Williams and Ayars (2005b) concluded that it was the orientation of the canopy in space and not the actual leaf area per vine or leaf area index (LAI) that determined vine water use. Doorenbos and Pruitt (1977) concluded that the Kc values for vineyards would vary considerably due to cultural practices (i.e. differences due to row spacing, trellis type, pruning pattern and differences in growth among cultivars and perhaps scions grafted onto different rootstocks). Based upon the above it would appear that the seasonal Kc should differ due to trellis used and row spacing, factors not considered for Kc values used in the past. Across numerous growing seasons in vineyards around California, shaded areas under different trellis systems were measured. The wine grape trellises included the lyre, GDC (Geneva Double Curtain), VSP (Vertical shoot positioning trellis) and vertically split canopies (Scott Henry and Smart-Dyson trellis systems). Several additional trellis systems were also measured such as the ‘California Sprawl’, overhead trellises and others using crossarms of various lengths. Shaded areas under the vines were measured with the use of a digital camera at solar noon, software was used to calculate shade in the image (Williams and Ayars, 2005b) and the percentage of shade per area allotted per vine in the vineyard converted to a crop coefficient using Equation 2.