Tuesday, 3 September 2013

Mr Goyder and his heritage

I made a smartipants response, invoking Goyder's line, to Denis' comment about the low rainfall in Robertson for the month of August.  For those not familiar with Robbo, aka the home turf of the Big Spud,  it is a bit like Ireland where it is reputed to rain for 15 minutes every quarter of an hour.

That is of course a vile calumny against a very nice town.  Here is what the Bureau of Meteorology has in its records for station 068054 (Calong St, Robertson) since 2005 (there being a large gap in records before that year).
There are few months with low rainfall, but equally the huge amounts are not that common.

But getting back to Goyders Line.  It is a line created by George Goyder, Surveyor General for South Australia, and marks the limit of the area with sufficiently reliable rainfall to permit crops to be grown.  It follows quite closely the long-term 10 inch rainfall isohyet.  Outside that line pastoral activities are the go.

When Goyder defined the line there followed a few good years (for a more recent example, see second map below) and the grain growers expanded beyond the line.  Then reality struck with some dry years.

Following metrication getting anything measured to 10 inches is a bit tough so in this first map from BoM showing a 30 year average rainfall it is approximated by the 300mm isohyet.
The line runs through Mildura and heads more or less North, passing just to the West of Bourke.  Robbo is well to the East of that (about where the second A of 'CANBERRA" is shown).  I presume that the influence of rain coming from the tropics and the coast improves the falls compared to central and Eastern SA.

This second map is for the last three, very moist, years such as might lead the foolhardy to try growning crops without irrigation beyond the line.  In effect the brown area in Central SA is the 10" per year area while the long term 10" per year line is close to the boundary of the yellow and light green areas, which is close to 16" per year.


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