HYDROLOGY OP NEW YORK 



119 



many reports and papers it is assumed that averages of a series 

 of percentages can be safely taken. The following illustration, 

 with five cases drawn from observation, may be taken to show 

 that this is erroneous : 



Runoff, Per Cent of Rainfall 



Case 



Rainfall 



Inches 



1 1 44.3 



2 62.5 



3 16.2 



4 24.3 



5 40.4 



187.7 



Runoff 



Inches 

 20.1 

 35.0 

 1.2 

 2.5 

 13.1 



71.9 



Runoff 



Per cent 



45.37 

 56.00 

 7.41 

 10.29 

 32.42 



5)151.49 



_ 71.9 

 Rainfair~187.7 = 

 Difference= 



30.29 

 -38.31 

 = 8.02 



Runoff coefficient misleading. As a corollary to the preceding 

 proposition, it follows that the ratio between annual rainfall and 

 runoff, known as the " runoff coefficient or factor," is essentially 

 misleading. A realization of this fact has led the writer, in his 

 report to the United States Board of Engineers on Deep Water- 

 ways, to practically expurgate this statement, or anything ap- 

 proximating to it, from his report. The expressions, " average 

 runoff " and " percentage of the rainfall," do not appear. 



Relation between total runoff and runoff of storage period. 

 Attention may also be again directed to the fact that the total 

 runoff of a stream in any given year depends very largely on the 

 runoff of what may be termed the " storage period." Usually 

 about 0.55 to 0.75 of the total rainfall of this period appears as 

 runoff in the stream, while for the summer, or growing period, 

 not more than about 0.1 of the rainfall appears. This great dif- 

 ference is due to greater evaporation, as well as to the absorption 

 of water by plants during this period. The total amount for 

 the year which will appear as runoff in the stream will depend, 



