Failure of the Yan Yean Reservoir, 127 



whole amount of the watershed as deducible by Dr. Thom- 

 son's method, 



rt. In. 



5,000 gallons per minute -,6 7 



60 hours' floods . - 0 7J 



Two- thirds increase in winter nipnths 1 1 



Total - - 8 3i 



Whole watershed - - 9 9 



Balance unaccounted for 1 5J 



The only other hnportant source of supply is the drainage 

 area of the reservoir itself. Mr. Hodgkinson was kind 

 enough to measure this area for your committee, and his 

 estimate is 3,000 acres, or about twice the surface of the 

 reservoir. 



During the winter months there has generally been more 

 or less water in the reservoir ; but during the summer it has 

 almost always been quite dry. 



In 1851 there was no water in it during the Avhole year; 

 and this fact is not only important as regards the water- 

 shed of the reservoir, but it has a far more extensive and 

 significant importance as regards the watershed of the 

 Plenty. I have estimated the rainfall for the basin of the 

 Plenty at thirty-six inches; but in 1851 there was so little 

 rain in that district, that even during the winter months no 

 water was collected in the reservoir, from a drainage area, 

 including the reservoir, of nearly seven square miles, or 

 one-eighth part of the drainage area of the Plenty. 



It can hardly therefore be alleged that I have under-rated 

 the average watershed of the Plenty. 



After heavy rains and floods, the reservoir has sometimes 

 had as much as two feet of water at the lower end, and then 

 it overflows into a small watercourse or rather swamp, which 

 skirts the ranges for about two miles, and then enters the 

 river by a creek, which has a sectional area of about twelve 

 feet. At the upper end, the reservoir receives a larger 

 creek, which has a length of about three miles, and contains 

 a large quantity of water after heavy rains ; but when there 

 is no rain it is quite dry. 



According to the best information which I have received 

 the reservoir has, on an average, been dry for six months in 

 the year, evaporation, therefore, must be constant during 



