Failure of the Yan Yean Reservoir. 127 
whole amount of the watershed as deducible by Dr. Thom- 
son’s method. 
Ft. In. 
5,000 gallons per minute - 6 7 
60 hours’ floods - = 0 ve 
Two-thirds increase in winter months 1 1 
Total - - 8 3h 
Whole watershed : - 9 9 
Balance unaccounted for il 54 
The only other important 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 whole 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 im 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 dramage 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 
ereek, 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 
