By Erica Lundquist, Ph.D.
LCWC Viticulturist

Of all vineyard cultural practices, irrigation management has one of the greatest influences on winegrape quality. Limited water applications often improve winegrape quality. The concept of “regulated deficit irrigation” to maximize winegrape quality is discussed below. Using water as efficiently as possible is also an important part of sustainable winegrowing. Water is a limited resource both locally and statewide. If those aren’t enough reasons to ‘fine-tune’ your irrigation program, you may also find that it will help save on water pumping costs.

The irrigation system Before a grower can effectively schedule irrigation and develop an irrigation program to maximize winegrape quality, it is essential to ensure that water is being delivered uniformly throughout a vineyard block and that this water delivery capability does not deteriorate with time. Uniform water application determines irrigation efficiency, or the proportion of total applied water that is used by the crop, and therefore directly affects the amount of water needed for irrigation.

Soils can vary substantially in their water-holding capacity, and frequently sections of a vineyard block are more drought- prone or wetter than other sections. It is important, however, not to introduce more differences in vine water status due to non-uniformity of water emission. (Solutions to the problem of soil variability may be to add emitters, to use different rootstocks, to alter canopy management and crop load, or ideally, to modify the irrigation system to irrigate the section as a separate block.)

Contributing factors to high distribution uniformity include; (1) an appropriately designed irrigation system that minimizes pressure differences throughout an irrigation block and provides effective filtration, (2) routine monitoring of emission uniformity, (3) routine flushing of the system and treatment to prevent clogging.

Evapotranspiration, ET, is the amount of water that is lost from soil and other surfaces (evaporation ) and from the stomata or pores of leaves during photosynthesis (transpiration). ET changes over the growing season due to changes in canopy size, day length and weather conditions. By looking up potential ET measured at a weather station and converting it to grapevine ET by multiplying it by a crop coefficient, a good estimate of vine water use during a given period of time can be determined. This estimate can then be used to guide irrigation rates. Lake County is fortunate to have a network of weather stations sponsored by University of California Cooperative Extension in Lake County, the Lake County Air Quality Management District, the Fruit Frost District and the Lake County Winegrape Commission. Weather data from the stations can be accessed from the LCWC web-page. The University of California also has the California Irrigation Management Information System (CIMIS), a statewide network of weather stations. The nearest one to Lake County that measures potential ET, is located in Hopland.

Many studies have shown that grapevines produce the highest quality fruit when they receive less than the full amount of water the vines can use. Some of the benefits to fruit quality include enhanced color pigments (anthocyanins) in red grapes, a higher ratio of tartaric to malic acid, and enhanced flavor compounds. Berry size is frequently reduced by deficit irrigation, which can contribute to wine color through a larger skin to berry volume ratio.

Water deficits can be too severe. Moderate water deficits tend to increase the rate of sugar accumulation, but severe deficits delay sugar accumulation. Under severe deficits berries may shrivel and produce undesirable flavors. When water deficits lead to excessive fruit exposure, prolonged heat spells can raise berry temperature, causing poor red berry color and reduced fruit quality.

The term regulated deficit irrigation, RDI, is used to describe the practice of regulating the application of irrigation at some level below full vine water use. It implies monitoring soil and plant water status, and scheduling irrigation based on weather conditions of the current season. This approach will allow you to achieve consistent, regulated vine water deficits. It is essential to keep records of plant and soil water measurements, the irrigation schedule, observations on canopy growth and crop load, and fruit and wine quality. These records will enable you to develop an RDI program tailored to your site. The many factors that influence an RDI program for a given site include grapevine variety and rootstock, soil depth and water holding capacity, and the local climate.

In general RDI strategies strive for water deficits that occur before veraison in order to restrict canopy growth and improve light penetration in the fruit zone. After veraison, water applications remain at a deficit level to prevent re-initiation of canopy growth that can lead to vegetative flavors in the fruit and wine. White winegrapes benefit from moderate water deficits, and red winegrapes, depending on the variety, from more severe deficits.

There are many ways to monitor plant and soil water status, and learning to schedule irrigation based on ET data can seem complicated. The LCWC is organizing irrigation workgroups where we will discuss these concepts. The workgroups will meet on a monthly basis at locations throughout the county. To start off the local meetings, we will hold pressure bomb demonstrations throughout the county. Pressure bombs, or pressure chambers, are a widely used tool to monitor vine water status. (See the Calendar of Events on this website for Pressure Bomb meeting dates.) If you want to join a workgroup, please give me your contact information; name, address, and e-mail (e-mail is preferable, but phone number will work), and I will keep you up to date on when workgroups will meet. Contact me at erical@lakecountywinegrape.org, or (707) 245-8384.

Just a few of the many sources of information on irrigation are listed below:

Cal Poly Irrigation Training and Research Center, www.itrc.org. In their Publications For Sale Section find titles such as Drip and Micro irrigation for Trees, Vines and Row Crops (English and Spanish editions), and Fertigation.

University of California Agriculture and Natural Resources Division, www.ucanr.org. In their Publications Section find titles such as Micro-irrigation of Trees and Vines, Scheduling Irrigations: When and How Much, and many more. For a comprehensive look at winegrape irrigation available as a free download, look for the draft version of Irrigation of Quality Winegrapes Using Micro-Irrigation Techniques at the following site: here.