AKID POKTIONS OF SOUTH AUSTRALIA. 



9 



which has the least reliable rainfall, according to the same author, is in 

 Western Australia. At Roeburn, for example, the following extremes 

 in precipitation have been recorded : In the year 1891 the rainfall was 

 0.13 inch only; in the year 1900, at the opposite extreme, the ramfall 

 amounted to 42 inches. The region of greatest rain variability is 

 roughly equal in extent to that which lies within the 10-inch isohyet, 

 but it is situated somewhat farther north and hence is largely in the 

 region of summer rains. 



Fio. 2. — Mean annual rainfall map of Australia, adapted from Hunt. 



It can be seen that with the possible exceptions of the dry interior 

 on the one hand and the humid regions on the other, the precipitation 

 of the continent can be characterized as periodic. There are thus 

 many days in the year when no rain falls and, as just suggested, these 

 may occur in large degree consecutively. The actual number of rainless 

 days may be surprisingly large, as the following will indicate; the 

 figures are for the year 1912 only: In the humid regions the average 

 number of days without rain for 26 stations is 306.2. In the semi-arid 

 regions the average of 24 stations is 332 rainless days, the average for 

 16 stations in the arid regions is 328.4, and finally, the average number 

 of rainless days in the desert, 6 stations, is 346.4. It is possible that the 

 regular recurrence of rainless periods over most of Australia is a very 

 important factor, althpugh a very complex one, in giving the vegetation 

 as a whole the xerophytic stamp it bears. 



