Vine Lines: Vineyard Water Management
Recently the Department of Viticulture and Enology hosted a timely seminar on vineyard water management. For the benefit of those unable to attend, I’d like to devote this month’s column to reviewing some of the program’s highlights from speakers including Larry Williams, Department of Viticulture and Enology at UC-Davis; Andrew McElrone, USDA-ARS; Lars Pierce, CSU Monterey Bay; Mark Greenspan, Advanced Viticulture; and Mark Battany, Viticulture Farm Advisor, UC Cooperative Extension, San Luis Obispo County.
Larry Williams focused on two key questions; 1) when should irrigation be initiated, and 2) how much water should be applied? Williams noted that most California grape growing areas have relatively dry summers and wet winters. Usually winter precipitation satisfies the low water needs of dormant vines but, during a drought, soil may become too dry in winter, negatively affecting budbreak, shoot growth, and cropping in the following season. Thus, fall and winter irrigations are beneficial when precipitation is lacking.
When the season starts with adequate soil moisture, either from precipitation or irrigation, in-season irrigation should be delayed until needed to save water and help manage vegetative growth and fruit yield and quality. Williams noted that vines only use about 10% of their annual water budget between budbreak and bloom, so many San Joaquin Valley soils have enough stored water to supply the vines through bloom. A pressure-chamber (which measures water potential, an indicator of the degree to which vines are water stressed) can help guide irrigation decisions.
The water potential threshold various growers will use to initiate irrigation will vary according to many factors, but most people agree that vines with water potentials of -1.0 MPa or higher (water potential data are negative, so “higher” means less negative, or closer to 0) are not particularly stressed. Williams and several other speakers also recommended measuring soil water content, often using 30% and 50% of the available water in the root zone as an initiation target. Visual observations of shoot growth can also be helpful in deciding when to start irrigating. As the vines become more stressed, their growth rate will slow, and the shoot tips and new tendrils will appear smaller.
Determine The Right Irrigation Amount
Once irrigation is initiated, how much water should be applied? Williams noted that row spacing, trellis type, and canopy size are the main factors affecting vineyard water use. In fact, evapotranspiration (ETc), the water lost from a vineyard due to evaporation from the soil surface (typically about 11% or less of ETc according to Williams) and transpiration from the vine’s leaves, is a mathematical function of the percent shaded area of the vineyard at noon (see How To Calculate Evotranspiration).
Williams’ equation to estimate ETc has been widely adopted by growers and also serves as a standard by which new measures of vineyard water use may be compared. Actual vineyard water use could be measured, rather than estimated, but such measurements involve the deployment of scientific equipment that is generally expensive and difficult to use.
For example, the surface renewal method can determine transpiration from the energy and water vapor in parcels of air that move across a vineyard. Andrew McElrone discussed how he and his co-workers have simplified surface renewal data measurements and interpretation methods to the point where this technology could be widely employed in the future. Lars Pierce also presented information on new technologies for measuring vine water stress, using satellite imagery and special software to provide vineyard water stress information to growers. This technology is also in the testing phase.
Regardless of the irrigation scheduling method used, growers should monitor vine and soil water status for quality control. Mark Greenspan discussed various soil moisture sensors, stressing the importance of repeatability. He also noted that placement in the soil (including distance from an emitter and depth in the ground) profoundly affects measurements, so whoever is monitoring the data needs to understand that to interpret the data properly. Greenspan stressed that soil based measurements should be combined with plant-based measurements, including visual observations, as discussed earlier, and analytical measurements.
The analytical measurements he uses include vine water potential, and stomatal conductance. As the vines become more water stressed, their water potential measurements will become more negative, and their stomatal conductance will also decrease, since stomata, the pores on leaves that vines use to regulate transpiration, constrict in response to water stress.
Frost And Salinity Protection
While most of the speakers focused on irrigation, Mark Battany reviewed frost and salinity protection measures that make minimal use of water. His suggestions included site selection and preparation to promote cold air drainage, consideration that increasing trellis height may put buds in a zone of higher ambient temperatures, delayed pruning, selecting varieties that commence growth later in the spring, and employing proper vineyard floor management (bare, dark-colored, moist soil surface is ideal).
Wind machines are active frost control methods that are only effective when a temperature inversion is present. Battany shared a design for a small weather station growers could use to determine temperature inversion strength, and noted he is working on a statewide online temperature inversion database for growers. To minimize sprinkler water use for frost protection, Battany suggested using wet-bulb temperatures to guide start and stop times, and he recommended sprinklers with a fast spin rate, as they can provide more protection than sprinklers with a slow spin rate.
Managing salinity with limited water is challenging, but Battany noted that certain scion and rootstock varieties are more resistant to salt than others, and this should be considered before planting. He noted new management ideas are needed, and mentioned Australian research with novel soil covers that concentrate rainfall in uncovered portions of the vineyard to help flush salts. Soil amendments such as gypsum can also be useful to displace sodium, he noted.
How To Calculate Evapotranspiration
Specifically, evapotranspiration is equal to ETo x Kc, where ETo is reference evapotranspiration, which can be downloaded from an online weather database such as this one, for example: http://ucanr.edu/u.cfm?id=92, and Kc is a crop coefficient that corrects the weather station ETo, which is based on the water use of a grass-covered soil, for use in vineyards. Kc is equal to the percent shaded area of the vineyard (expressed as a decimal; 40% = 0.40, for example) multiplied by 1.7.
ETo units are typically acre inches, in which case ETc will also be in acre inches; one acre inch equals 27,154 gallons. Williams’ equation estimates 100% of full potential vineyard water use, but some growers may want to apply a fraction of ETc to help control vine growth and improve fruit quality. This can be done by multiplying ETc by the decimal fraction desired. Additional information and sample calculations may be reviewed online: http://ucanr.edu/u.cfm?id=91.