Run-off from the Avoca River Basin. 



149 



great demands upon the soil moisture, as well as obstructing the flow 

 of water down the slopes of the drainage area. 



Flood Prediction. 



As the months of June, July, August and September are obviously 

 the flood months, it may be worth while giving in tabular form some 

 average results based upon the whole 32 years' record. All Junes and 

 Julys preceded by months with less than 2 inches of rain are excluded. 

 The flood rains are grouped as follow: — under 1 inch, from 1 inch to 

 149 points, from 150 points upwards. While the variation is great for 

 all months, that for June is particularly so. 





Rainfalls under 100 

 points. 



Rainfalls of 100 to 149 

 points. 



Rainfalls of 150 points 

 and over. 



Month. 



>"= '"■°- >;Hi 



^= ''°- .;H^ 



ni cusecs. 



June - - 

 July - - 

 August - 

 Sept. - 



76 pts. 6 600 

 80 pts. 12 1400 

 83 pts. 8 1740 

 87 pts. 7 1460 



126 pts. 6 2190 

 129 pts. 5 1870 

 122 pts. 6 2340 

 125 pts. 7 3160 



170 pts. 6 2500 

 235 pts. 2 3600 

 230 pts. 3 3970 

 196 pts. 9 3590 



Annual Run-off Increasing. 



That the volume as well as constancy of the flow of the Avoca 

 River over the Coonooer Weir is increasing greatly is shown deeisively 

 by the official gaugings. As previously remarked, so many factors 

 powerfully affect the run-off that unless the chief of these are known 

 it is hopeless to attempt to estimate the flood height from any par- 

 ticular rainfall. But by taking a suflSciently long period it may be 

 assumed that average results will provide data for reasonably re- 

 liable deductions. The official guagings cover 32 years. These pro- 

 vide three decades, exclusive of the years 1920 and 1921, The first 

 of these, 1890-1899, was the wettest, giving an average of 20*1 

 inches annual rainfall, and an average annual run-off of 59,278 

 acre feet. The next decade had an average rainfall of 19 • 4 inches, and 

 a run-off of 44,230 acre feet. The last decade 1910-1919, had a rainfall 

 of 19:5 inches, and a run- off of 74,439 acre feet. Taking a mean of 

 the first two, we get an average run-off of 51,700 acre feet for a rain- 

 fall of 19:7 inches. As the average rainfall for the last decade was 

 0:2 inches less, and the average run-off 22,700 acre feet more, there 

 seems no reason to doubt that the run-off is increasing. This is 

 made even more emphatic by taking in the years 1920 and 1921, with 

 rainfalls of 19 and 22 inches respectively, and run-off totals of 94,909 

 and 93,155 acre feet. These make for the last 12 years the average 

 run-off 77,700 acre feet, for an average rainfall of 19:7 inches. The 

 rainfall therefore averages out the same as for the previous 20 years, 

 but the run-off has increased by 26,000 acre feet, or by over 50 per 

 cent. (The rainfall data here used are those published by the Water 

 Commission.) 



