Deficit of Larry E. Williams Department of 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 profile would more than likely be insufficient to meet a ’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 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 -based methods to determine when to start irrigating. This would include the measurement of vine water status (such as midday water potential), leaf stomatal conductance, 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

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creating water deficits during specific times of the season to conserve water while minimizing or eliminating negative impacts on or crop revenue. The timing of RDI in vineyards is usually associated with phenological events of the vine such as between set and (berry softening/color change) or veraison and . Both SDI and RDI are based upon knowing what full (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 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, 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 (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 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

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(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 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 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, pattern and differences in growth among cultivars and perhaps scions grafted onto different ). 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. The Kc values for different trellises, estimated from shaded area measurements in various vineyards across years did differ from one another. For example, the shaded area at full canopy (and calculated maximum Kc) for a Lyre trellis was greater than that of a VSP trellis at the same row spacing. This would be expected as there was more canopy per unit land area for the Lyre system. The Kc values increased as the season progressed for all trellises. Maximum canopy

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size was obtained between 750 and 1000 degree-days (base of 10°C) after a starting point, except for VSP trellises which took longer to achieve maximum canopy size. Lastly, estimated Kc values derived from shaded area measurements at Carneros (VSP trellis) were similar to those developed earlier via the soil water budget method (Williams, 2014a). The results from the data collected in 2000 and in subsequent years/studies (Williams, 2010, 2012, 2014a; Williams and Fidelibus, 2016; Williams and Heymann, 2017) indicate that the derivation of crop coefficients from percent shaded area is a reliable means to estimate vineyard ETc. Best estimates of the seasonal crop coefficients for various wine grape trellises and row spacings are summarized in Table 2. The equations in Table 2 derive the Kc using degree-days (>10°C) from a starting point (budbreak). Note the degree-days used in the equations are from the Celsius scale with a base of 10°C. The use of Fahrenheit temperature (base of 50°F) will not work with these equations (the UC-IPM website will calculate degree- days using the Celsius scale). Since budbreak generally occurs around the middle of March in the San Joaquin Valley, March 15th is used as the starting point. Budbreak in the coastal valleys of California generally occurs around April 1st. In the Coachella Valley, January 15th is used as the starting point since that is when table grapes initiate growth there. With the exception of the VSP trellis, the shaded areas measured beneath the canopy of east/west and north/south rows at solar noon are similar to one another. The seasonal crop coefficients I’ve developed for a VSP trellis given in Table 2 are valid for most row directions. Using a 3D model that calculates light interception by the canopy (Iandolino et al. 2013) it was demonstrated that a VSP trellised vineyard on true north/south rows will intercept greater light on a daily basis than a similar vineyard with true east/west rows. Lastly, the values presented in this table do not account for cover crop water use in the vineyard. A study at the KARE Center demonstrated that water use of vines within rows planted with cover crops was 40% greater than vines in rows without a cover crop (LE Williams, unpublished data). Crop ET can be derived from the ‘dual crop coefficient’ method (Allen et al., 1998): ETc = (Kcb + Ke) * ETo (3) where Kcb is the basal crop coefficient (the portion of ETc due to plant transpiration) and Ke is the coefficient used for soil evaporation. The use of this method is more complicated than that of Equation 1. One must calculate daily Ke values for soil evaporation. A study was conducted by Williams and Fidelibus (2016) where the soil surface of the weighing lysimeter was covered with plastic intermittently to minimize soil evaporation and the Kcb determined. The trellis used was an overhead arbor, vines were farmed as table grapes and irrigated whenever they used 2 mm (8 L [2.11 gallons] per vine) of water (therefore they were not stressed for water). Calculated soil evaporation across a 100-day period at full canopy was 13% that of ETc. The seasonal Kcb was 87% that of the Kc. Despite the high frequency of drip irrigation and the continuously wetted soil surface, soil evaporation only ranged from 0.76 to 0.84 mm/day (0.03 to 0.033 inches/day). Soil evaporation in vineyards with longer intervals between irrigation events may be less than that reported for the high frequency irrigated, large vines in the lysimeter.

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Do grape growers need to apply water at 100% of estimated ET? It would depend upon production goals. Once full ETc is calculated via Equation 1 or 3 then deficit irrigation practices can be used such that a fraction of full ETc is applied to the vines either throughout the growing season (SDI) or during specific phenological stages (RDI). The following equation could also be used to calculate a Kc for use in a deficit irrigation strategy using the dual Kc method: Kc = KsKcb + Ke (4) where Ks is a dimensionless ‘stress’ coefficient whose value can be set by the grower. The seasonal crop coefficients for wine grapes in Allen and Pereira (2009) include an implicit Ks factor of 0.7. If one has access to appropriate Kc values, based upon degree-days and considering trellis and row spacing, then applied water amounts at various fractions of ETc should result in similar vine water status readings from one year to the next. Such was the case in a study conducted on Chardonnay grapevines where estimated ETc varied from a high of 500 mm (~ 20 in.) one year to 350 mm (13.5 in.) the next (Williams, 2014a). Values of leaf water potential at various fractions of applied water amounts at estimated ETc were very similar at harvest from one year to the next across the 8-year study (Figure 1). This indicates that if your goal is to grow vines with a uniform degree and pattern of water stress every season, then scheduling vineyard irrigation using reliable crop coefficients is beneficial. I have shown across many years and locations that one can deficit irrigate grapevines such that water use efficiency is increased and yields can be maintained for wine, raisin and table grapes (Williams, 2010; 2012; 2014b; Williams and Heymann, 2016; Williams et al., 2010b). For example, Williams et al. (2010b) demonstrated that yield of Thompson Seedless was maximized at applied water amounts between 60 and 80% of full ETc, with the actual amount dependent upon year. There were no significant differences in yield of Chardonnay grown in Carneros, a cool grape growing region, at applied water amounts from 25 to 125% of estimated ETc (Williams 2014b). Lastly, berry size and yields of Perlette and Flame Seedless grapevines grown in the Coachella Valley were maximized when deficit irrigation at 50% of estimated ETc (RDI technique) was not initiated until after berry set had occurred (unpublished data). In general, applied water amounts at 50% or less of ETc throughout the growing season (SDI strategy) will reduce yields of grapevines (Table 1). It is thought that berry size may be an important quality factor in red wine grapes. Smaller berry size may be preferred since the skin contains most of the color and flavor producing compounds. Deficit irrigated vines will have smaller berries than more well-watered vines (Table 1). Vines experiencing water deficits during the period between berry set and veraison will have smaller berries at harvest than vines experiencing the same degree of stress but only between veraison and harvest. On average, 65 to 75% of the final berry size is determined between set and veraison and that growth during this period is due to both cell division and cell elongation. Water deficits will reduce cell division. Berry growth after veraison is due only to cell elongation.

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Sugar concentration in grape berries will determine the alcohol content in the wine. In general, sugar accumulation is less affected by water deficits than berry growth (Table 1). Excessive stress may delay the accumulation of sugars. A study was conducted on 17 red, wine grape cultivars at the KARE Center for four years. Irrigation treatments included: A.) applied water at 100% of estimated ETc between berry set and veraison and then no applied water to harvest, B.) no applied water between berry set and veraison and then applied water at 50% of estimated ETc through harvest and the last treatment was C.) applied water at 50% of estimated ETc season long. Across all years of the study and cultivars, sugar accumulation occurred more rapidly for treatment A (late deficit) than for treatment B (early deficit). In fact, the early deficit treatment delayed the start of veraison for several of the cultivars. Final sugar concentration in the berries of treatment A was somewhat greater than that for vines irrigated at 50% of ETc season long. A moderate decrease in organic acids of the fruit is generally found where soil or vine water status indicates water stress. The effect of water deficits on the decrease of acids in grape berries is due more to a reduction in malic acid, than for tartaric acid. In the study on 17, red wine grape cultivars at the KARE Center, vines which received the early deficit treatment (B – no applied water between set and veraison, 50% ETc thereafter) or the 50% ETc season long treatment had much less titratable acidity when measured at harvest than vines in the late deficit treatment (A – full ETc between set and veraison, no water after that). Matthews and Anderson (1988) found that malic acid in the fruit of their early deficit irrigation treatment was significantly less than that in the fruit of their late deficit irrigation treatment. Deficit irrigated vines usually have greater total phenols and than those receiving more applied water (Williams and Heymann, 2017; Williams and Matthews, 1990). Matthews and Anderson (1988) reported that both early and late season deficits were equally effective in increasing total phenolic content compared to vines receiving more applied water. The changes in the composition of the fruit due to water deficits are reflected in the wine (Matthews et al., 1990; Williams and Heymann, 2017). The effects of early and late water deficits on total anthocyanins in the wine of vines grown at the KARE Center are less conclusive. Total anthocyanins in made from fruit in 2013 for the early deficit irrigation treatment was 14% greater than those from the late deficit treatment while the reverse was true in 2014 (the late deficit was 18% greater than that of the early deficit treatment). Wine total anthocyanins of the 50% season long treatment were intermediate between the other two treatments both years. The degree of vine stress one imposes and its timing can be dependent upon location. Soil water deficits have less of an effect on productivity in a cooler growing region (Williams 2014b) than a hot region (Williams, 2010; Williams et al., 2010b). Based upon the red, wine grape cultivar study conducted at the KARE Center, the lack of irrigation between set and veraison delayed/decreased sugar accumulation and acid content in the fruit and return fruitfulness (cluster number the following growing season) and final yield to a greater extent than the lack of applied water from veraison to harvest. In addition, the frequency of irrigation events

6 could possibly mitigate the effects of deficit irrigation on productivity under certain conditions (L.E. Williams, unpublished data). Therefore, such information should be taken into account when devising a deficit irrigation strategy for vineyards.

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Table 1. The effects of applied water amounts at 56 and 112% of estimated ETc on berry weight, soluble solids and yield measured on . The grapevines were grown at J. Lohr vineyards near Paso Robles. Both sustained deficit irrigation (SDI) and partial rootzone drying (PRD) were employed as deficit irrigation strategies. Values are the means across the four-year study with those values within a column followed by a different letter significantly different from one another.

Irrigation Treatment Berry weight Soluble Solids Yield (% ETc) (g/berry) (°) (kg/4 vines)

112% 1.17 a 22.9 42.8 a 56% SDI 0.97 b 23.6 31.7 b 56% PRD 0.94 b 23.5 33.1 b

Table 2. The effect of row spacing on estimated seasonal Kc values for a VSP trellis system, a California Sprawl type canopy, quadrilateral cordon trained vines and Lyre (or ‘V’) type canopies. The x value in the equation is degree-days (base of 10°C) from a starting point. The ‘e’ value in the equation is 2.71828. Note that row spacing only changes the numerator in the equation, the maximum Kc value. Trellis/ Row Spacing Canopy type (feet) Crop coefficient equation

(-(x – 525)/301) VSP 6 ft. Kc = 0.87/(1+ e ) (-(x – 525)/301) 9 ft. Kc = 0.58/(1+ e )

(-(x – 325)/105) CA Sprawl 10 ft. Kc = 0.84/(1+ e ) (-(x – 325)/105) 12 ft. Kc = 0.70/(1+ e )

(-(x – 300)/175) Quad-cordons 11 ft. Kc = 0.93/(1+ e ) (-(x – 300)/175) (or GDC/Wye) 12 ft. Kc = 0.85/(1+ e )

(-(x – 300)/150) Lyre Types 10 ft. Kc = 0.84/(1 + e ) (-(x – 300)/150) or ‘V’ 12 ft. Kc = 0.70/(1 + e )

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Figure 1. Leaf water potential measured across 8 growing seasons (1994 – 2001) taken close to harvest each year (dates varied from 28 Aug. to 21 Sept). Irrigation commenced when midday Ψl was ~ -1.0 MPa (-10 bars) each year. Values are the means across years (vines grafted onto two rootstocks, 5C and 110R) + standard error (n = 4 for the 0, 0.25, 0.75 and 1.25 treatments and 8 for the 0.5 and 1.0 treatments).

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References:

Allen RA and Pereira LS. 2009. Estimating crop coefficients from fraction of ground cover and height. Irrig Sci 28:17-34. Allen RA, Pereira LS, Raes D and Smith M. 1998. Crop evapotranspiration: guidelines for computing crop water requirements. FAO irrigation and drainage paper 56, FAO, Rome. Ayars JE, Johnson RS, Phene CJ, Trout TJ, Clark DA and Mead RM. 2003. Water use by drip- irrigated late-season peaches. Irrig Sci 22:187-194. Doorenbos J and Pruitt WO. 1977. Guidelines for predicting crop water requirements. FAO Irrigation and drainage paper 24, FAO, Rome. Ferreira MI, Silvestre J, Conceição N and Malheiro A. 2012. Crop and stress coefficients in rainfed and deficit irrigation vineyards using sap flow techniques. Irrig Sci 30:433-447. Gu SL, Du GQ, Zoldoske D, Hakim A, Cochran R, Fugelsang K and Jorgensen G (2004) Effect of irrigation amount on water relations, vegetative growth, yield and fruit composition of grapevines under partial rootzone drying and conventional irrigation in the San Joaquin Valley of California, USA. J. Hortic. Sci. Biotechnol. 79:26-33. Iandolino AB, Pearcy RW and Williams LE. 2013. Simulating three-dimensional grapevine canopies and modeling their light interception characteristics. Austral J Grape Wine Res 19:388-400. López-Urrea R, Montoro A, Mañas F, López-Fuster P and Ferreres E. 2012. Evapotranspiration and crop coefficients from lysimeter measurements of mature ‘Tempranillo’ wine grapes. Agric Water Manag 112:13-20. Matthews MA and Anderson MM. 1988. Fruit ripening in vinifera L.: responses to seasonal water deficits. Amer J Enol Vitic 39:313-320. Matthews MA, Ishii R, Anderson MM and O’Mahony MO. 1990. Dependence of wine sensory attributes on vine water status. J Sci Food Agric 51:321-335. Picón-Toro J, González-Dugo V, Uriarte D, Mancha LA and Testi L 2012. Effects of canopy size and water stress over the crop coefficient of a “Tempranillo” vineyard in south-western Spain. Irrig Sci 30:419-432. Sadras VO (2009) Does partial root-zone drying improve irrigation water productivity in the field? A meta-analysis. Irrg. Sci. 27:183-190. Stevens RM and Harvey G. 1996. Soil water depletion rates under large grapevines. Austral J Wine Grape Res. 2:155-162. Synder RL, Lanini BJ, Shaw DA and Pruitt WO. 1987. Using reference evapotranspiration (ETo) and crop coefficients to estimate crop evapotranspiration (ETc) for trees and vines. Cooperative Extension University of California, DANR Leaflet 21428, Berkeley. Williams LE. 2010. Interaction of and applied water amounts at various fractions of estimated evapotranspiration (ETc) on productivity of Cabernet Sauvignon. Austral J Grape Wine Res 16: 434-444. Williams LE. 2012. Interaction of applied water amounts and leaf removal in the fruiting zone on grapevine water relations and productivity of Merlot. Irrig Sci 30:363-375. Williams LE. 2014a. Determination of Evapotranspiration and Crop Coefficients for a Chardonnay Vineyard Located in a Cool Climate. Am J Enol Vitic 65:159-169. Williams LE. 2014b. Effect of applied water amounts at various fractions of evapotranspiration on productivity and of Chardonnay grapevines. Am J Enol Vitic 65:215 – 221.

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Williams LE. 2017. Physiological tools to assess vine water status for use in vineyard irrigation management: Review and update. Acta Hortic 1157:151-166. Williams LE and Ayars JE. 2005a. Water use of Thompson Seedless grapevines as affected by the application of gibberellic acid (GA3) and trunk girdling – practices to increase berry size. Agric For Meteor 129: 85-94. Williams LE and Ayars JE. 2005b. Grapevine water use and the crop coefficient are linear functions of the shaded area measured beneath the canopy. Agric For Meteor 132: 201-211. Williams LE, Dokoozlian NK and Wample RL. 1994. Grape. p 83-133. In: B. Shaffer and P.C. Anderson (eds.), Handbook of Environmental Physiology of Fruit Crops. Vol. 1. Temperate crops. CRC Press, Orlando, Florida. Williams LE and Fidelibus MW. 2016. Measured and estimated water use and crop coefficient of grapevines trained to overhead trellis systems in California’s San Joaquin Valley. Irrig Sci 34:431-441. Williams LE and Heymann H. 2017 Effects of applied water amounts and trellis/training system on grapevine water relations, berry characteristics, productivity and wine composition of Cabernet Sauvignon. Acta Hortic 1150:413-426. Williams LE, Grimes DW and Phene CJ. 2010a. The effects of applied water at various fractions of measured evapotranspiration on water relations and vegetative growth of Thompson Seedless. Irrig Sci 28: 221-232. Williams LE, Grimes DW and Phene CJ. 2010b. Effects of applied water at various fractions of measured evapotranspiration on reproductive growth and water productivity of Thompson Seedless grapevines. Irrig Sci 28:233–243. Williams LE and Matthews MA. 1990. Grapevines. p. 1019-1055. In: B.A. Stewart and D.R. Nielsen (eds.) Agronomy Monograph #30 Irrigation of Agricultural Crops. ASA-CSSA- SSSA Publishers, Madison, Wisconsin. Williams LE, Phene CJ, Grimes DW and Trout TJ. 2003a. Water use of young Thompson Seedless grapevines in California. Irrig Sci 22: 1-9. Williams LE, Phene CJ, Grimes DW and Trout TJ. 2003b. Water use of mature Thompson Seedless grapevines in California. Irrig Sci 22:11-18.

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