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NEW YORK STATE MUSEUM 



We have seen in the preceding that it is somewhat uncertain 

 whether difference in soil due to difference in character of rocks 

 has much influence on the runoff, although casually it appears that 

 sandy soils, from their porousness, do considerably affect the 

 result. Recent European studies of this subject have shown 

 (1) that in many river basins the annual runoff stands in a nearly 

 constant relation to the rainfall, and (2) that this constancy is 

 more marked when the excess rainfall above a certain minimum 

 annual depth is considered. This latter statement is equivalent to 

 saying that if the yearly rainfall is less than such minimum depth 

 little or no runoff will take place. 1 



The general truth of this proposition is shown by many western 

 streams where the runoff is little or nothing. In New Jersey 12 

 inches of rain during the summer season produce a runoff of 1.5 

 inches, though others have stated a somewhat different relation. 

 In the State of New York from 1.7 to 2 inches may be considered 

 the general range. As to the amount of rain required to produce 

 any runoff at all, from 12.5 to 16.5 inches have been given. For 

 this minimum many western streams do not run more than 0.25 

 to 0.5 inch, and some even are perfectly dry. These statements 

 indicate that the character of the soil, nature of vegetation, the 

 elevation, etc. are of comparatively small importance as regards 

 relation between the yearly volumes of rainfall and runoff. If, 

 however, we consider the rainfall and runoff of the several periods, 

 as shown by the accompanying tables, it is not entirely certain 

 that these propositions are other than approximately true. The 

 weight of evidence indeed is, on the whole, negative. Mr Ver- 

 meule is disposed to attribute nearly all of the differences between 

 streams to difference in geology, and accordingly gives a geologic 

 classification for the New Jersey streams. Mr Vermeule says : 



As a rule, the watersheds which lie upon the same geological 

 formation will be found to have a strong resemblance, both in the 

 character of flow and in the chemical composition of the waters. 



Yet, as will be shown later, the Genesee and Oswego rivers, two 

 streams with approximately the same runoff, lie mostly in differ- 

 lBarge Canal Report, p. 708. 



