128 - Failure of the Yan Yean Reservoir. 
the six winter months; now from other data we can estimate 
the amount of water thus evaporated, which is equal to two 
feet five inches, and the rainfall, for the same period, is 
twenty-two inches, leaving a balance of seven inches to repre- 
sent the surface drainage that is evaporated. If the watershed, 
therefore, exceeds seven inches it must escape by the small 
creek at the lower end, and may be approximately ascertained. 
This watercourse has been generally observed to run after 
heavy rain, but not otherwise. If, therefore, we assume 
that there are, during the winter months forty days of heavy. 
rain, and that the creek is, on an average, half full, and that 
its velocity is about half a mile per hour, the contents would 
amount to 1,900,800 cubic yards, which would give seven 
inches in the reservoir. By this estimate the water shed 
cannot exceed fourteen inches. 
It is important, also, in this enquiry, to estimate the water= 
shed according to Dr. Thomson’s method. The ratio of the 
drainage area to the reservoir being as two to one, the whole 
rainfall would give a depth of six feet, and one-ninth would 
give a watershed of eight inches; but as there is no water- 
shed in the six summer months, and as the rainfall of these 
months is less than that of the winter months, in the propor- 
tion of two to three, the rainfall of the drainage area must 
be taken at three feet seven inches for the winter months; 
and as the area is chiefly composed of ranges of clay slate, 
which are much less favourable for absorption than the level 
country, three-ninths or one-third instead of one-ninth of the 
rainfall may be estimated as the watershed, and this will 
give fourteen inches, which exactly corresponds with the 
former estimate. 
I shall here notice some considerations which would seem 
to prove the correctness of Dr. Thomson’s estimate, in its 
application to the basin of the Plenty. 
Mr. Blackburn’s highest estimate of all the tributaries is 
5,000 gallons per minute, or 6 feet 7 inches in the reservoir. 
One half of this amount, therefore, or 3 feet 34 inches, will 
represent the absolute quantity of the watershed for the six 
summer months. Now, as there is one-third less rain, two- 
fifths of 88 feet, or 33 feet in the reservoir, will represent the 
whole of the rainfall for this period. Therefore, the water- 
shed is equal to one-tenth of the rainfall, and the remaining 
nine-tenths are evaporated. 
In the six winter months there is one-third more rain, and, 
