ARID PORTIONS OF SOUTH AUSTRALIA. 47 



Effective Rainfall. 



It does not require long experience with the chmate of South Austra- 

 Ua, particularly with the drier portions of the state, to recognize the 

 probability that not all of the rainfall reported in the official summaries 

 is of direct benefit to the native flora. Also, what may be said to be an 

 effective rainfall, from this point of view, at one period of the year 

 may not be so at another period. And, further, the time relations of 

 successive storms must also be considered. 



In order to determine what might be considered a minimum rainfall 

 for the native plants, the actual penetration following rains of known 

 but small amounts was observed and measured on a few occasions. 

 Thus, at Copley, on a morning following a slow rain amounting to 

 0.21 inch, it was found that the ground was freshly moistened to a 

 depth of 4 to 9 cm. At another time, when a rainfall of 0.27 inch was 

 reported, the soil was found wetted to a depth of 5 to 8.5 cm. The 

 last observations were made 4 hours after the rain had ceased falling. 



On the plain near Copley there are numerous diminutive drainage 

 channels, 15 cm. deep, or less. These may form separate systems of 

 considerable extent, or they may be tributary to larger drainage sys- 

 tems. Whichever it might be, it was noticed that in none of the small 

 channels were there pools following a rain of 0.27 inch. The rain 

 penetrated where it fell. At Ooldea, also, after a rainfall amounting 

 to 0.21 inch, the soil was found to have been moistened to a depth not 

 exceeding 3.5 cm. In both situations there was a large percentage 

 of sand in the soil. 



As will be shown in another place, the depths given would include 

 the horizon within which the roots of many winter annuals are to be 

 found and also the most superficially placed roots of certain of the 

 perennials. Deeply rooted plants of all sorts would not benefit 

 directly by falls of rain of these amounts; and it may be questioned 

 whether in summer such small rains would benefit the annuals or the 

 superficially rooted perennials, owing to the rapid evaporation from 

 the surface of the soil. The foregoing remarks have been made with 

 respect to individual storms. 



If separate storms follow one another with little time intervening, 

 it is clear that there would be a progressive moistening of the soil, with 

 the effect that the total depth moistened might be considerable. It is 

 necessary, therefore, to take into account the time which elapses be- 

 tween successive stormy periods. However discouraging for exact 

 statement such considerations, and others that will readily occur, 

 may be, it seems worth while to establish tentatively what may be con- 

 sidered the minimum effective rainfall. And before making the defini- 

 tion it should be pointed out that it concerns the native vegetation 

 and not that of agricultural value, inasmuch as the amount in the latter 

 case would necessarily be greater. Musson (1904:3) states that only 



