Managing Irrigation to Maximize Grape Production
George Zhuang, UCCE Fresno
Growers that I’ve talked to are concerned about maintaining grape production with less water. The easiest solution to this problem is twofold: Don’t start watering until you need to and, when you do, only apply what you need. Some factors to consider:
When should seasonal irrigations be initiated?
Identifying grapevine symptoms of water stress
Measuring vine water status
Measuring soil water tension or content
How much water to apply?
Identify when deficit irrigation may be used to reduce water use but maximize yield and quality
When to initiate seasonal irrigation
Grapevine symptoms of water stress
Shoot growth is one indicator of a vine’s water status. For mildly stressed vines, shoot growth will slow and internodes will shorten. Under severe water stress, shoot tips and tendrils dry and may die on the primary and lateral shoots.
The angle of the leaf blade to its petiole is another indicator of water stress—the angle decreases as water stress increases.
Becoming competent in the visual detection of vine water stress is a useful qualitative skill that can help you evaluate your irrigation program.
Measuring vine water status
Vine water stress can be quantified by taking measurements with a pressure chamber. These “water potential” measurements are commonly made on leaves at midday (taken ±1 hour of solar noon, so between 12:30 PM to 2:30 PM). Leaf water potential is a direct measurement of vine water status and is an accurate and reliable indicator for vine water stress. Studies indicate that when midday leaf water potentials decline to ≤-10 bars, it is a good time to begin irrigation. Measurements made after irrigations have commenced are also useful for monitoring the effectiveness of the irrigation program.
Measuring water in soil
Soil acts as a reservoir for water, but only a limited amount of water in the soil is available to vines. Therefore, knowing the soil water content or tension is useful in scheduling and monitoring vineyard irrigation events. To determine the soil water content, a grower can simply collect a soil sample using a shovel or auger. Then, the sample is weighed as-is from the field, dried, reweighed, and the moisture content calculated from the difference in weight.
However, tools that give more immediate feedback are available. Water tension can be measured using tensiometers and soil water content can be measured with neutron probes or time domain reflectometry. For soil water tension, place two tensiometers directly beneath the drip line, side by side, (12-18 inches to the side of an emitter) with one monitoring the 1 to 2 foot depth and the other measuring moisture in the lower soil profile, 3 to 4 feet deep (Fig. 1). Generally, a pair of tensiometers for every 20 acres is adequate, but more tensiometers are recommended if you have variable soil. Irrigation should be initiated once pre-determined values of soil tension have been reached at the different soil depths. Generally, irrigation should begin before soil tensions at an approximate 2-foot depth approach -40 to -50 centibars, but the irrigation threshold values will depend on soil type.
Figure 1. The recommended location of tensiometers or neutron probe access tubes to measure soil water content or matric potential (image courtesy of Dr. Larry Williams).
Two access tubes for a neutron probe can be placed side by side directly beneath the drip line to a depth of 6-9 feet (Fig. 1). Placing additional access tubes between vine rows is recommended for more accurate data on water use, since it will indicate the water depletion across the vineyard floor. Soil water content measured by neutron probe reflects the total water content. Irrigation should be initiated when the soil allowable depletion is less than the difference between measurements from the neutron probe and soil water-holding capacity. Soil allowable depletion can be variable depending on soil type (1.4 to 4.4 inches from sandy to clay, respectively).
Overall, soil water content is highly correlated with midday leaf water potential. Growers can use soil and vine measurements to validate irrigation scheduling based on experience and visual observation.
How much water should be applied?
Once irrigation begins, growers should only apply the amount of water that is needed. General vine water use estimates for Thompson Seedless with a large canopy, like a crossarm trellis, can be found in Table 1 (below).
Another way to precisely quantify how much water to apply is to use crop evapotranspiration (ETc) information. ETc can be calculated as ETc=ETo×Kc, in which ETo is the reference evapotranspiration and Kc is the crop coefficient. Currently, UCCE is working with Department of Water Resources to provide weekly ETc values to growers using ETo from CIMIS stations and Kc.
Kc can be estimated by using the following equation: Kc=(SA%×0.017)-0.008, where SA% is the proportion of shaded area on the vineyard floor. Recently, Kc values calculated from aerial and satellite images of the normalized difference vegetation index (NDVI) images have become available for commercial use. An efficiency factor (70%-90%) can also be used to adjust ETc based on the irrigation efficiency of a drip system.
Table 1. Vine water use (drip irrigation schedule) for a large canopy or a crossarm trellis*.
*Raisin vineyard canopy covers 75% or more of the land surface during summer months.
(Raisin Production Manual, UC ANR Publication 3393)
Opportunities to deficit irrigate to maximize yield and quality
Deficit irrigation is a very powerful tool for growers to consider as the drought continues. For raisin and wine grapes, deficit irrigation can be used to save water while maximizing production. Sustained deficit irrigation (SDI) is the practice of purposely deficit irrigating throughout the growing season, where only a fraction of the full ETc will be applied. For Thompson Seedless grapevines, SDI at 60% to 80% of ETc was sufficient to maximize the yield without reducing sugar accumulation by berries or raisin quality. Similarly, 80% of ETc in a Merlot vineyard maintained yield and fruit quality. In terms of wine grape quality, timing deficit irrigation is critical. About 65-75% of final berry size is determined in the time period from fruit set to veraison. Regulated deficit irrigation can be applied during this stage to achieve better fruit quality with reduced berry size. Given that a smaller berry has higher skin/pulp ratio, deficit irrigation can result in more color, tannin, and flavor at harvest. Importantly, deficit irrigation doesn’t decrease bud fruitfulness the following season.
For table grapes, more research is needed to fully understand the potential impact of deficit irrigation on berry size and fruit quality. Generally, deficit irrigation is not recommended until after veraison in table grapes.
Overall, deficit irrigation at approximately 80% ETc appears to maximize yield and quality of wine and raisin grapes while minimizing water use.
I would like to acknowledge the irrigation information provided by Dr. Larry Williams and technical support from Dr. Matthew Fidelibus, Dept. of Viticulture and Enology, UC Davis.