Phred writes

Each to his own of course, but I know of plenty of places around here
where farmers getting 80 to 100 inches of rain per year still find it
useful to irrigate at times to ensure optimum production of pawpaws,
bananas, and sugar cane.

I'll believe that, in fact I'd expect it. The total rainfall is a bit of
a red herring since it cannot distinguish between jim, in a constant
rain 24/7/365 and with low transpiration rates, and a seasonal monsoon
with a 10 month dry period and huge transpiration despite both getting a
60" rainfall. One would love to see the sun and occasional days over
20C, the other needs irrigation!

Mind you, I must admit they probably don't decide when or how much by
using a water budget -- more likely kick the ground, look at the sky,
and say "Hmmm... better put on a couple of inches tomorrow." Much as
Gordon says above.

I don't know. When I grew spuds (not my farm) I had a shallow, wide
cylindrical glass container that I filled up and left outside sometime
in early may when the field drains had stopped running for a week or so.
When about 1.5" had evaporated I started considering irrigation
(depending on likely weather to some extent) and when I had applied an
inch, I topped it up with an inch. This gave me a crude estimate of
moisture deficit. Of course it quite often overflowed if a wet spell
came. The most interesting thing was that locally it was known that one
side of the farm had a 36"+ rainfall, and the other a 25". I therefore
had one in each field (probably 1/2 mile apart one year). One side (in
quite a dry year, admittedly) was irrigated pretty well on about a
1"/month basis for six weeks or so but the other side never even reached
the trigger point.

To be honest though, after only two years I hardly needed it, being able
to judge about as well just by digging an 18" hole in the field and
seeing how dry it was at depth. Probably better, now I think about it as
this made allowance for soil holding capacity.

--
Oz
This post is worth absolutely nothing and is probably fallacious.
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