UKTC Archive

Re: Irrigation Needs

Subject: Re: Irrigation Needs
From: Adam Hollis
Date: May 05 2004 10:41:21

On Tuesday, May 4, 2004, at 10:28 PM, wrote:

So, to Adam's question, if all else is equal water use is proportional to the square of the crown spread. So the difference in water use between a small young tree with a 1m spread and a mature oak with a 20m spread is a factor of
400. Increase the spread to 30m (big, but not unheard of), use the
0.82kg.m-².day-¹ figure, you get 2318.5¹, which is about 510 gallons.

Chris Hastie
Strategy Officer (Arboriculture)

Dear Bill, Chris & Rupert

Thanks for your replies

I  rather imagined it would involve the squaring of the circle.

A couple of thoughts though:

My trees have 2-3m spreads (not 1m), if we're talking dm (which we must be at 30m) and I shouldn't be at all surprised, if the researchers (with the 500 gallon demand) used "mature" trees in the Mitchell sense of 50% of final size - say 50cm dbh x 15m spread, for the sake of convenience (if they actually measured a tree empirically, rather than extrapolating mathematically). So my trees might be twice the size and the researchers' tree, half that of Chris's estimate, and of course, there are one or two assumptions / adjustment factors in the above formula.

Even without making the leap from semi-matures to matures, the ISA UK newsletter published an article last year referring to far greater irrigation needs for recently planted street trees and most of the manuals for irrigation systems refer to similar copious amounts.

I mostly plant containerized stock on free-draining green sand in rural sites. I wouldn't say the plants were in shock, nor that they derived much water from the surrounds in the first season, because those that I don't get around to planting are left in their containers and don't need a vast amount more water. Instead, they need to be watered more often - 2 lots of 5 gallons per week. If I tried to apply 10 gallons, most of it would seep through the container.

I generally arrive at my irrigation requirements by hand, using a tensiometer, and creating as little run-off as possible. I have even faffed around with 2 gallon cans to be quite sure of how much I'm applying. Evidently, I've calculated correctly, because all trees planted / looked after have thrived and the last few summers have certainly had their dry spells.

So I can't help suspecting that the assumptions / adjustment factors need tweeking....either that or I keep dozing off at the end of the hose.



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