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How to calculate growing degree days (or thermal time) - A Chalk Talk by METER - YouTube
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Hello my name is Dr. Colin Campbell. I am a senior聽
research scientist here at METER group and today聽聽
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we're just going to give a brief primer on thermal聽
time. When I talked to some of my colleagues about聽聽
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that, they mentioned that this is really just a聽
way to match a plant clock or a plant's clock with聽聽
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our clock. So we understand what's happening with聽
the plant and can predict things like emergence,聽聽
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like maturity, etc. And the way we do聽
this is through this equation here that's聽聽
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actually, when we look at it, pretty聽
simple. It just says that we can sum聽聽
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thermal time by taking this summation of聽
day one to day n of the average temperature,聽聽
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meaning t max plus t min divided by two, minus a聽
base temperature and then multiply by a time step,聽聽
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and in this case our time step is just one聽
day. So basically the whole analysis is simply聽聽
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average temperature minus the base temperature. We聽
get that value each day and we keep summing until聽聽
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it reaches a value that tells us that we've聽
progressed from one stage to another stage.聽聽
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A good example of this is just wheat. And when聽
I was young I actually did an experiment on this聽聽
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in biology and I was super amazed to see that it聽
actually worked. This idea is that for emergence聽聽
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the wheat plant needs 78 day degrees to actually聽
step through from planting to emergence. And so I聽聽
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just used this equation here and when I had summed聽
enough of these day degrees I knew it was stepping聽聽
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from that planting stage to a post-emergent聽
stage and I went and measured it and it actually聽聽
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matched up really well. Not every wheat plant聽
emerged at that point but the average actually聽聽
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was quite close. So what does that mean in terms聽
of graphical data? So I just wanted to show you聽聽
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what this equation actually did and then plant a聽
little seed in your mind for our next discussion,聽聽
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which is how good is this analysis? Because if聽
you think about it, with modern technology like聽聽
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METER group's ATMOS 41, we can get temperature聽
measurements that are every five minutes or every聽聽
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one minute so wouldn't it possibly be better聽
if we collected our thermal time information by聽聽
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taking the sum over each day like we did in the聽
other equation, but instead taking the integral聽聽
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of temperature at any at a small time step,聽
like five minutes minus the base temperature聽聽
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and then just integrating this across the day. So聽
we're going to learn about that in another talk,聽聽
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but here let's go to this graph. We have聽
temperature on the y-axis and time on the x-axis.聽聽
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The total time here of course is just聽
24 hours so this is our daily step聽聽
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where we're collecting this information about聽
thermal time. And here are all these parameters聽聽
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from the equation. So here's the maximum聽
temperature, here's the minimum temperature,聽聽
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this one here is the average temperature, and then聽
this is a base temperature. And I just want to say聽聽
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just to familiarize you with what we're talking聽
about, this is a temperature below which that聽聽
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progress is not made in the development of this聽
plant. It's not like we reverse the progress,聽聽
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so if it's below the base temperature it's not聽
backing up on its development, but it just doesn't聽聽
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progress. Now this black line here is just what聽
I've drawn as a typical diurnal temperature swing,聽聽
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so it's going from a minimum of course in the聽
early morning up to a maximum sometime in the聽聽
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afternoon. And what I've done is try to compare聽
these two approaches. So on this side we have the聽聽
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average temperature, we have the base temperature,聽
and so all of this in this rectangle here,聽聽
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that is our thermal time for that day.聽
But the question is if with all our data,聽聽
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like from the ATMOS 41, where we have pretty聽
small time increments, could we instead just聽聽
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integrate over the day and then collect all the聽
information on that thermal time here that's below聽聽
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this black line, the actual temperature, and of聽
course we subtract out the base temperature so聽聽
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is the in this integral here, how much difference聽
does that make compared to this here? And what is聽聽
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the implications of maybe not being able to聽
measure our temperature terribly accurately?聽聽
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So we're going to talk about that in our next聽
discussion and I look forward to seeing you then.
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