Washington AVA Growing Degree Days
The progression of grape phenological stages (sap flow, bud break, veraison, etc.) is strongly influenced by ambient temperature and so are vegetative growth and fruit ripening. The calculation of daily Growing Degree Days (GDD) is a tool that can be used to predict important stages such as bloom, veraison and crop maturity.
GDD are calculated by subtracting 50 from the average daily temperature (°F). If the resulting value is less than 0, then it is set to 0. Thus GDD are always 0 or greater.
We calculate all GDD in °F. Values in °C can be obtained as follows: GDD[°C]=5/9 x GDD[°F].
Cumulative GDD is a running total of GDD from April 1 through October 31.
AgWeatherNet is the Washington Agricultural Weather Network and can be used to monitor temperatures, precipitation, pan evaporation, and other parameters at weather stations at over 130 locations in Washington. Conditions in your vineyard may vary depending on the vineyard's mesoclimate. If you have a weather station in or near your vineyard, you can calculate your own GDD.
Evapotranspiration (ETA) is a measurement of the amount of water lost to the environment via evaporation from the ground surface and from the plant leaves. It can be used to estimate grapevine water use when making irrigation decisions.
ETA is calculated by multiplying the Penman ETo grass (pan evaporation) value for your site by our grapevine crop coefficient (KC) which was derived from fully irrigated Cabernet Sauvignon so it will give an estimate of grape ETA under full irrigation. If you are using deficit irrigation to manage your canopy, you will want to use less than full irrigation replacement. Your final irrigation decision should be based on several factors: grapevine ETA, specific knowledge of your mesoclimate, and your individual goal for your vineyard.
The following table shows a comparison of seasonal cumulative GDD and precipitation, and weekly cumulative evapotranspiration taken from AgWeatherNet stations in ten of the eleven American Viticultural Areas (AVA)s in Washington. Since the Columbia Valley AVA covers such a large area it is impossible to pick one representative weather station.
2009 Growing Season
Long Term Average Yearly Cumulative GDD
2008 Cumulative GDD
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