Vineyard Irrigation Scheduling: ET-Based Methods and Stem Water Potential

The Goal of Vineyard Irrigation Scheduling

Wine grape vines are deficit-irrigated in almost all commercial production regions. Unlike most other crops, you do not aim to keep soil moisture at field capacity throughout the season. Moderate water stress at specific growth stages improves berry quality, controls excess vigor, and produces more concentrated flavor compounds. The art of irrigation scheduling in the vineyard is knowing how much stress to impose, when, and for how long.

The typical approach for most wine grape production is to irrigate at or near field capacity from bud break through fruit set to support canopy establishment and berry set, then impose progressive deficit irrigation from post fruit set through veraison to control berry size and accumulate flavor compounds, then monitor carefully from veraison through harvest to maintain stress at an appropriate level without triggering premature senescence or berry shriveling.

Evapotranspiration-Based Scheduling

Evapotranspiration (ET) scheduling uses reference ET data from weather stations combined with crop coefficients for grapevines to estimate how much water your vineyard is losing each day. California growers can access CIMIS (California Irrigation Management Information System) stations throughout the state. Washington growers use AgWeatherNet. Oregon's AgriMet network covers much of the state.

The basic calculation is: Vineyard ET = Reference ET x Crop Coefficient (Kc). Crop coefficients for grapevines vary from roughly 0.15 to 0.30 during the early season to 0.45 to 0.70 during peak canopy development. Published Kc tables from UC Cooperative Extension provide regional guidance, but your actual coefficient will vary based on your trellis system, canopy size, row orientation, and the cover crop situation in your midrows.

ET-based scheduling works well as a baseline tool but requires calibration to your specific site. If you're consistently seeing stress symptoms you didn't intend or observing that stem water potential readings are more negative than you're targeting, your Kc may need adjustment upward. If vines are maintaining more water than you want, adjust downward.

Stem Water Potential

Stem water potential (SWP) measured with a pressure chamber is the most direct measure of vine water status available to growers without a laboratory. The measurement is taken on a shaded, bagged leaf in midday on a clear day, and the result is expressed in bars or megapascals (MPa). Well-watered vines typically read -3 to -5 bars at midday. Moderate deficit is -6 to -10 bars. Severe stress is below -12 bars.

Target SWP ranges vary by region, variety, and growth stage. UC Cooperative Extension guidelines for California coastal wine grapes suggest maintaining -5 to -8 bars from fruit set to veraison for quality red varieties, with slightly less stress for white varieties and sparkling base wine production where retaining acidity matters. Oregon research from OSU suggests Pinot Noir in the Willamette Valley performs well with -6 to -9 bars from fruit set through harvest in most years.

Record your SWP measurements by block with the date, time, vine used, and result. Tracking these readings over the season and across years gives you a calibration between SWP and irrigation response that becomes specific to your site and varieties.

Drip Irrigation Systems

Drip irrigation is the standard for most new vineyard plantings because of its water use efficiency and ability to deliver water precisely. Subsurface or surface drip lines deliver water directly to the root zone, minimizing evaporation from soil surface and eliminating wet foliage that can increase disease pressure.

For scheduling purposes, drip systems need to be calibrated for emitter flow rate and distribution uniformity across the system. An emitter rated at 1 gallon per hour that is actually delivering 0.7 gph due to pressure variation means your irrigation calculations will be off. Run system uniformity checks annually and keep records of your system capacity, operating pressure, and any maintenance done.

Overhead Systems and Frost Protection

Overhead sprinklers serve a different purpose in vineyards than drip. In frost-prone regions, overhead risers are used for frost protection by coating emerging buds with a layer of ice that maintains temperature at 32°F even as air temperature drops below freezing. For this application, timing and rate are critical: the system must be running before temperature reaches the critical freeze point and must continue until air temperature rises above freezing.

Some older vineyards in California and the Pacific Northwest use overhead sprinklers for summer irrigation as well, though this is increasingly uncommon in wine grape production because of the disease pressure created by wetting foliage during the growing season.

Record Keeping for Irrigation

Irrigation records should include: date, block or zone irrigated, run time, estimated volume applied (calculated from flow rate and run time), and any notes about system issues. This data feeds into water use calculations for permits, sustainable certification programs, and your own efficiency analysis. Many California growers operating under water district restrictions need irrigation records to demonstrate compliance with their allocation. VitisScribe tracks irrigation events alongside other block-level records so water use is visible in context with pest and disease management decisions.