Talk:Growing degree-day

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"I'd like to find GDD references for Poinsettias, Roses and Lilies because they are so commonly forced for Holidays" [old request comment removed from text - MPF (talk) 09:52, 20 November 2007 (UTC) ]

Incorrect calculation
There are three methods I've read about to calculate growing degree days. None of them use the method listed here.

I think the calculation should be to find the average of max and min and subtract the base temperature. If that number is negative, then GDD = 0. Otherwise that number is GDD. The method listed here (if Tmin or Tmax is below Tbase, set them to Tbase) would over estimate GDD. —Preceding unsigned comment added by 128.174.75.226 (talk) 23:29, 10 December 2007 (UTC)

I agree that there are other methods, but the method described in the article is quite common. Here is a reasonable explanation from a different source. I have updated the second example to show how the clipping of Tmin would influence the result.Elmore307 (talk) 15:45, 5 April 2017 (UTC)

Scale Should be Specified
The growing degree days listed for corn (maize) appear to be Farenheit degree days. I suspect that the same is true for other plants listed in the chart. This is confusing since the calculation example which preceeds the chart is specifically in degrees centigrade. I did a quick rough calculation and there are about 3800 growing-degree days F during the average year in southern Indiana. This is roughly equal to 2100 growing-degree days C. Since the chart indicates that 2700 GDD are required for corn to mature this must be a GDD F.  — Preceding unsigned comment added by David Paul Ford (talk • contribs) 14:15, 20 August 2012 (UTC)

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GDD - Is the simpler formula correct?
I feel, that the simple formula is not similar to the formula using an integral. The integral takes a time-span into account, whereas the simpler formula is only valid to calculate GDD for a single date, which I would not call GDD but average daily temperature above a base temperature. My suggestion is to change the formula into (sorry the ugly html workaround): GDD = Σundefinedn ((Tmax,d + Tmin,d) / 2 - Tbase)

A similar formula is used in:

Spectral Vegetation Indices to Track Senescence Dynamics in Diverse Wheat Germplasm

J. Anderegg, K. Yu, H. Aasen, A. Walter, F. Liebisch and A. Hund

Frontiers in Plant Science 2020 Vol. 10 Issue 1749

DOI: 10.3389/fpls.2019.01749

https://www.frontiersin.org/article/10.3389/fpls.2019.01749

Do you agree? Moritzca (talk) 16:46, 28 February 2023 (UTC)
 * The day part of GDD is partly the answer there, it's supposed to be for a single day (that is then summed over each day). GDD basically is supposed to be average daily temperature above a base temperature for a given day and if the units are going to change to a different type of GDU like degree-hours, that's a bit more of a special case.
 * Either way, the "simple" version in Growing_degree-day is what is used for GDD at it's simplest for most applications. This is a good note to gather up some sources for the section though and to do some cleanup there. KoA (talk) 19:07, 28 February 2023 (UTC)
 * Yes, I agree. Generally, the article should be called "temperature time" or "heat accumulation" of which growing degree days are a special case of. Still, I think the accumulation is an integral part of GDD and can lead to confusion (as it just did to our students) and the time component lacks completely in the given simple formula. However, it seems difficult to find an exact source for the simple formula, probably because it is common knowledge? The source given for the integral (Prentice et al. 1992) found a similar, practical way to calculate GGD as described in the formula of my first post and commonly used such as in Anderegg et al. 2020. I make suggestion for a change. Moritzca (talk) 20:00, 2 March 2023 (UTC)
 * Remember we are writing for a general encyclopedic audience, not a specialized one. Even my eyes glazed over in that edit to be honest. I did update the text to just simply say the formula is for degree-days for a given day. There are plenty of sources out there that go over how growing degree-days are calculated that don't require sorting through journals. I can try to work through the article a bit someday to clean things up and include more sources later on. KoA (talk) 22:18, 2 March 2023 (UTC)
 * Still, I think it is wrong and leads to a lot of confusion. I think, the way it is described in the intro is correct and splits the calculation of GDD nicely in its two main steps: "GDs are calculated each day as maximum temperature plus the minimum temperature divided by 2, minus the base temperature. GDUs are accumulated by adding each day's GDs contribution as the season progresses.". Therefore, the formula discussed is for GD and not GDD. Of course, GDD for a single day equals GD in the calculation but not in the general concept. Moritzca (talk) 07:42, 3 March 2023 (UTC)