Weather

Growing Degree Days

In-season grapevine development is strongly influenced by air temperature.  Growing Degree Days (GDD) is a useful unit to measure temperature in the growing sesaon. The summation of daily GDD units can be used for a variety of things: comparing one region to another, comparing one season to another, and predicting important stages in vine development (bloom, veraison, and maturity).

GDD is calculated from a base temperature, can be in °F or °C, and can be summed over different periods of time.  Washington State University calculates all GDD in °F, with a base temperature for grapes at 50 °F. We calculate season total GDD from April 1 through October 31.

Below are charts of accumulated GDD for the Yakima Valley for the current season, and for long-term average GDD across AVAs in Washington. All data is from AgWeatherNet. You can use AgWeatherNet to create your own, station-specific GDD charts.

2024 Cumulative GDD – Yakima Valley AVA

 

 

Long-term Average Cumulative GDD – All AVAs

 

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Precipitation

Precipitation is critical in replenishing winter soil moisture in irrigated eastern Washington. See the annual precipitation record for the Yakima Valley AVA below, and how it compares to the long-term average.  For precipitation values for other locations in the state please visit AgWeatherNet.

Current Year Cumulative Precipitation – Yakima Valley AVA

 

Long-term Average Cumulative Precipitation – All AVAs

 

 

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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.

There are several ways to calculate and present ET. It is important to understand their distinction if you are using ET to determine irrigation needs. See our webpage on Irrigation for more information on irrigating wine grapes in Washington.

Evapotranspiration Values:

• Epan: A measured value of how much water was evaporated from an open pan of water on a given day
• ETo: A calcuated value using the Penman-Monteith equation and approximates an ‘open’ pan  or grass.
• ETr: Also a calculated value that references alfalfa water use. ETr is typically 20% greater than ETo as alfalfa uses more water than grass.
• ETc: A calculated, crop-specific value (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; but water needs change throughout the season.

Current Year Cumulative ETo – Yakima Valley AVA