46 PLANT HABITS AND HABITATS IN THE 



the south. The storms of winter are cyclonic in character. Areas of 

 low pressure (cyclonic) alternate with areas of high pressure (anti- 

 cyclonic.) The former are ascending, expanding currents and therefore 

 deposit their moisture; the latter are descending, condensing currents, 

 cold and therefore dry. 



From this it will be seen that the rains in the southern portion of 

 South Australia and in the highlands extending north into the center of 

 the state, which project the southern climatic conditions to the north, 

 are periodic and also dependable. But the rains of the northern poi tion 

 of the state are not regularly controlled by the winter climatic con- 

 ditions, or those of summer, and hence are not dependable and are 

 scarcely periodic. Further, as indicated in an earlier paragraph, the 

 midstate region has a rainfall of an intermediate character, both as 

 regards the amount and as regards the periodicity. 



The lowest rainfall reported in South Australia is at Kanowana, 

 Lake Eyre Basin, where it is 4.33 inches, the average of observations 

 covering a period of 18 years. The rainfall in all of the basins is low, 

 and that of the Murray, although of larger amount than the others, 

 is nevertheless relatively low. This may be because of the proximity 

 of highlands to the west. Lake Ejrre is in the midst of the 5-inch 

 isohyet. The basins of Lake Torrens, Lake Gairdner, and Lake 

 Fromme have about 2 or 3 inches more rainfall than that of Lake Eyre. 



Over a very considerable area in the drier portions of the state the 

 rainfall is to a very considerable degree uncertain as to amount. 

 This is true both as regards that for the entire year and for separate 

 storms. Thus, it will be shown below that a relatively large percentage 

 of the precipitation in the northern parts of South Australia occurs in 

 amounts too small to directly benefit plants. But, on the other hand, 

 a very large portion of the yearly rainfall sometimes occurs in a single 

 storm. In the latter instance much of the precipitation does not pen- 

 etrate the ground, but runs off and does not benefit plants directly. 

 In the latter rain-type it is diflicult to estimate the proportion which 

 can be said to be effective, since it depends on the nature of the soil, 

 the plant cover, and (not to mention other features) the slant of the 

 surface, as well as the character of the storm itself. For these reasons 

 only the smaller precipitation amounts, as presented in the follow- 

 ing section, are deducted from the entire rainfall in order to arrive at 

 the amount of rainfall which can be said to be "effective." 



It will be seen in table 10 that about 25 per cent of the yearly rainfall 

 may occur in a single storm, for example that of May 8, 1905, at 

 Quom. And at Oodnadatta, in 1901, the relative amount was even 

 greater, although the actual precipitation in the single large storm of 

 the year was less. The greatest single rain recorded at the three sta- 

 tions given in the table and for the years referred to was on December 

 28, 1903, at Copley, when 3.20 inches were reported. 



