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NEW YORK STATE MUSEUM 



The constant 0.844 is equal to the number of feet in a square 

 mile divided by the seconds in a year. 



In these formulas the sliding coefficient is also recognized. The 

 results, however, are based on averages, although it seems clear 

 enough that, in either power or water supply works, what is 

 wanted is the minimum runoff for a year or a series of years. 

 For instance, the minimum rainfall at Lake Minnetonka in 1889 

 was only 18.36 inches, while the maximum in 1892 was 37.90 

 inches, or a little more than double the minimum. It is evident 

 enough to any person who has gaged streams extensively that the 

 runoff in 1889 must have been very much less than in 1892. In 

 the absence of statements as to the amount of runoff in 1889, the 

 writer can only estimate it, but he doubts if it were over 10 per 

 cent to 12 per cent of the rainfall. Probably about 2 inches is 

 not far from the mark. What is wanted, therefore, is a concise 

 statement, not only in this case but in every other, of the runoff 

 of the year or series of years of minimum rainfall. 



Danger of using averages. The writer has dwelt upon the fore- 

 going point somewhat because only a few of the more advanced 

 students of hydrology have thus far fully appreciated its import- 

 ance. A very large proportion of all the papers and reports pre- 

 pared in the last ten years have proceeded on the supposition that 

 safe deductions could be made from an average runoff. It is 

 needless to say that all such are, without exception, erroneous. 

 What is wanted is a clear statement of the minimum, together 

 with the longest period which such minimum may be expected 

 to occupy. A study of the meteorological records of the State of 

 New York shows that the minimum period may be expected fre- 

 quently to extend over three years. In the writer's report to the 

 United States Board of Engineers on Deep Waterways, in the 

 chapter on the Meteorology of New York and the relation of pre- 

 cipitation to runoff, a large number of specific cases are cited, 

 but space will not be taken here to discuss them. This proposi- 

 tion is true for other regions than the State of New York. 



Danger of using percentages. A much greater danger arises 

 from the use of percentage of rainfall appearing in runoff. In 



