AEID POKTIONS OP SOUTH AUSTRALIA. 49 



the percentage of what may be called the non-effective rainfall in- 

 creases with the decrease in the total average rainfall. At Quom, for 

 example, it is 18 per cent, at Copley it is 19 per cent, and at Oodnadatta 

 it is 30 per cent for the years under consideration. Assuming that 

 these figures represent the average percentage of the non-effective rain- 

 fall, which may or may not be the case, and knowing the actual average 

 rainfall for the three stations, the expected efficient rainfall can be 

 easily calculated. For example, the average rainfall at Oodnadatta is 

 4.85 inches. This being the case, a correction for the percentage that 

 does no good to the native vegetation would give an efficient average 

 rainfall of 3.42 laches. The average rainfall for Copley is 8.70 inches, 

 which would amount to 7.13 inches effective rainfall, and that at Quom 

 is 13.72 inches. At the latter station the effective amount would be 

 11.11 inches. The relation of the total, the effective and the non- 

 effective, rainfall at Oodnadatta, Copley, and at Quorn is shown 

 graphically in figure 11. In 1906 the non-effective rainfall at Oodna- 

 datta was 43 per cept of the total recorded for the year. Thus the 

 effective amount of precipitation for the year at Oodnadatta was 

 1.7 -laches. This exceeds the percentage of non-effective rainfall of 

 each of the other stations whose records were studied and indicates 

 (together with the average percentage of non-effective rainfall for the 

 stations given above) that the curve of aridity falls more rapidly, 

 as one goes from a less to a more arid station, than the curve of the 

 rainfall itself. 



