128 



Failure of the Van Yean Reservoir. 



the six winter montlis ; now from other data we can estimate 

 the amomit 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 

 apphcation 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 3^ inches, will 

 represent the absolute quantity of the watershed for the six 

 summer months. Now, as there is one-third less ram, 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 ram, and, 



