Dry Region.
This is bounded somewhat arbitrarily by the 10-in. annual isohyet. Two areas within the tract as plotted have more than 10 inches—in the north-east, the highlands of the MacDonnell Ranges, and in the north-west the Peak Hill Gold-field.
These two relatively wet areas may be related with the two favorite paths of the tropical "lows"; which often curve southward and south-eastward over Pilbara and Western Queensland respectively.
Owing to lack of information the northern boundary of the dry region cannot be accurately plotted, so that the line given—from Winning Pool to Barrow Creek—will, no doubt, need considerable revision later. Although the scattered rains of the north fall chiefly in January, and of the south in May and June, this variation does not determine the type of vegetation which is almost wholly controlled by absence of rain (xerophilous), and not by the season when it falls. The dry region, which consists mainly of an undulating tableland 1,000-3,000 feet high, forms therefore a united whole.
If the isohyets for 0.5 inches are plotted in each month (not shown in Figs. 39 to 50), the MacDonnell Ranges are seen to form an "oasis" in the arid region. In May, June, October, and November, when the districts not only south but east (Sturt's Desert, &c.) are receiving less than half-an-inch this region of uplands (3,000 feet) is favoured by more rainfall. Possibly the Musgrave Ranges—which are said to reach 5,000 feet (Mt. Woodroffe, 5,230 feet), also benefit in a similar degree, but no data are available.
The driest region so far furnished with rain-gauges lies east and north-east from Lake Eyre, where less than 5 inches is the average annual rainfall. This minimum rainfall is coincident with the lowest elevation, Lake Eyre being actually below sea-level, 39 feet.
