Michael Percy writes
Oz wrote:

I would define arid as being where the transpiration rate exceeds the
precipitation by four inches for three months of the year.

That definition can't be useful. It says nothing about three
quarters of the year.

Eh?

Ahh, perhaps not clearly put.
I meant the total precipitation resulted in three month periods with a
constant *soil* deficit exceeding four inches.

An
alternative might be something along the lines of never achieving field
capacity in the top 150mm for two years.

Puts severe bounds on the amount of rain in short term rain events within
those two years. Conversely sevents might trigger a change in status from
arid to semi-arid, even when events are insufficient to allow rainfed
agriculture.

It does rain in arid areas, I know. I stood in the moroccan desert last
year with no visible green (for 100's of km outside oases) being wetted
by modest rainfall.

2" of rain in a desert does not mean you can now call it 'semi-arid'.

By my definition Tinbouctou would be arid. What would it be
according to your definition -- to be precise, in how many years out of 10
would it be arid?

Perhaps you can give the last 10 year's rainfall, although as I
understand it tinbouctou is an oasis.

Remember there are places in the UK with sub 20" average rainfall, and
you would not describe them as remotely arid.

You caught me emptyhanded there. I had left the definition of remotely arid
undecided thinking I'd never need it.

How would you classify 20" rainfall then?

--
Oz
This post is worth absolutely nothing and is probably fallacious.
Note: soon (maybe already) only posts via despammed.com will be accepted.