Evapotranspiration and Grapevine Water Use

What is Evapotranspiration?

Evapotranspiration (ET) is a measurement of the amount of soil water lost to the atmosphere via evaporation from the ground surface and from the plant leaves. The latter process is also called transpiration. Historically, ET was measured by how much water was evaporated from an open pan of water on a given day (Epan); now, it is calculated via the Penman-Monteith ETo equation (‘open’ pan / grass). ETr (‘reference’) is a different reference value based on alfalfa water use. ETr is typically 20% greater than ETo as alfalfa uses more water than grass. Although alfalfa reference (ETr) is commonly used in the Pacific Northwest for other crops, grass reference (ETo) is often used by wine grape growers because of its historical use in California. Note of caution: ETr and ETo are not interchangeable.

How does Reference Evapotranspiration Relate to Grapevine Water Use?

Since ETo represents water loss over a period of time, it can be used to estimate grapevine water use, which in turn, can be used to make irrigation scheduling decisions. However, different plants have different rates of water loss, as seen with differences between ETo (grass) and ETr (alfalfa). When talking about crop-specific water needs, a new ET term is introduced, ETc, or crop evapotranspiration.

ETc is calculated by multiplying ETo by a crop-specific coefficient (Kc). In cases where a crop demands a lot of water relative to grass, Kc will be greater than 1. In cases where the crop is more water efficient than grass, Kc will be less than 1. Wine grapes are a fairly efficient plant in terms of water use, and their Kc is typically less than 0.8.

Crop Water Use Calculation: ETc = Kc x ETo 

In most systems, a single Kc is used throughout the growing season. However, the amount of water a grapevine needs changes as the canopy and fruit develop.  As such, many systems use an adjustable Kc as the season progresses, and that adjustable Kc is based on the size of the canopy.  For example, at the beginning of the growing season when the canopy is small, a Kc might be 0.3, whereas in mid-season when the canopy is at maximum size, the Kc might be 0.8.

Current-Season and Long-Term ETc for wine grapes in eastern Washington.

Both the current-season and long-term ETc for wine grapes in eastern Washington are presented below (Figures 3 and 4), using growing degree day information and ETo from the “WSU-Headquarters” AgWeatherNet station near Prosser, WA.  Conditions in your vineyard may vary depending on the vineyard’s mesoclimate.

Currently, this specific calculation of ETc for grapevines is not directly available on AgWeatherNet stations, but ETo is available.

 

 

 

Regulated Deficit Irrigation

Regulated deficit irrigation (RDI) is a form of water stress applied after fruit set to aid in controlling grapevine canopy size and berry size. It works by only partially replacing ETc for grapevines during watering regimes. In many cases, 70-90% ETc during fruit set and veraison is used, but regimes below 50% ETc may be applied b for premium wine grape production. RDI is NOT recommended prior to fruit set, or after veraison. Water stress pre-fruit set can result in cluster abortion, and in irrigated regions, if soil moisture is not sufficiently replaced prior to irrigation shut-off, then insufficient soil moisture over the winter season can increase the risk of root cold damage and poor development the following spring. Typically, due to the timing of irrigation source shut off, this replenishment must begin prior to harvest.

To calculate the amount of water that should be replaced when using RDI, simply take ETc (usually calculated as the accumulation of water loss between irrigation sets), and multiply it by the desired RDI level for that growth state (i.e., 70% = 0.7, 80% = 0.8).

Water Replacement using RDI (crop-specific) = ETc x RDI (as a decimal)

Note – There is often a lot of confusion as to how to calculate the amount of water needed when applying RDI. There are many recommendations in handbooks, websites, and conversations that use ETc, ETo or ETr as the base ET calculation for water replacement. The specific ET used can have major consequences on the final result.

For example, what happens to the total amount of water replaced under 70% RDI, when different ET values are used? The final amount of water is listed as % ETo to help with comparisons:

Example 1: RDI is applied to ETc. Grapevine Kc = 0.8; (full-canopy, mid-season).

Water replacement = Kc x ETo x RDI = 0.8 x ETo x 0.7 = 0.56 ETo

or 56% ETo

Example 2: RDI is applied to ETo.

Water replacement = ETo x RDI = ETo x 0.7 = 0.7 ETo

or 70% ETo

Example 2: RDI is applied to ETr.  ETr = Kc (alfalfa) x ETo. Kc (alfalfa) = approx. 1.15

Water replacement = Kc x ETo x RDI = 1.15 x ETo x 0.7 = 0.8 ETo

or 80% ETo

When following RDI recommendations, understand how those recommendations were developed, and how the recommender would like it applied; misunderstandings can result in either under-watering or over-watering a crop.