"Oz" wrote in message
...
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.

I just got through having coffee wiht the guy that invented the neutron soil
moisture probe. He is an retired agronomy professor here and fellow amateur
radio operator. I spent the better part of 6 months working on a capacitive
probe to measure soil moisture for Ag Engineering. The current most popular
method is time domain refectomotmy. My wife's farmer uses a steel probe
forced into the ground and a lot of experiance. Cotton also is self
indicating. It normally shuts down in the heat of the day and wilts even
with plenty of water so you can tell by the time of day and the temperature
pretty well when it needs watering.

There is a very large market for a good soil moisture probe and it is a very
difficult problem because it is impossible to keep the soil in contact with
the probe and all the electrical method the inverse square law applies so
any small change close to the probe is greatly magnified. The Neutron probe
is the gold standard but getting the permits for it is a real bitch and
there is no easy way to test for radiation leaks.

I highly reliable soil moisture probe that cost $50 would increase the
bottom line of most irrigated crops 5 to 15%.