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NEW YORK STATE MUSEUM 



into the stream, forming surface or flood flows; finally, another 

 part sinks into the ground, to replenish the great reservoir from 

 which plants are fed and stream flows maintained during the 

 periods of slight rainfall, for the rainfall is frequently, for months 

 together, much less thaD the combined demands of evaporation, 

 plant growth, and stream flow. These demands are inexorable, 

 and it is the ground storage which is called upon to supply them 

 when rain fails to do so. 



All of these ways of disposing of the rain which falls upon the 

 earth may be classed as either evaporation or stream flow. 

 Evaporation we take to include direct evaporation from the sur- 

 face of the earth, or from water surfaces, and also the water 

 taken up by vegetation, most of which is transpired as vapor, bui 

 a portion of which is taken permanently into the organisms of 

 the plants, Stream flow includes the water which passes directly 

 over the surface to the stream, and also that which is temporarily 

 absorbed by the earth to be slowly discharged into the streams. 

 A portion, usually extremely small, passes downward into the 

 earth and appears neither as evaporation nor as stream flow. It 

 is usually too small to be considered, and we may for our purposes 

 assume that all of the rain which falls upon a given watershed 

 and does not go off as stream flow is evaporated, using the latter 

 word in the broadened sense which we have above described. 



Probably one very important effect of forests is that upon the 

 ground-water flow of streams. The stream with a catchment area 

 wholly or largely in forests will show, without exception, a much 

 better ground flow than one with the area denuded of forests. 

 Neshaminy and Tohickon creeks may be cited as streams with 

 the smallest amount of forest and the lowest curve of ground- 

 water flow. Possibly this is not entirely due to forests, but it 

 may be assumed that they bear some relation to the result. 1 



Units of measurement. Clemens Herschel, member American 

 Society Civil Engineers, in his paper on Measuring Water 2 lias 

 defined the essential elements of this question in the following 

 terms : 



For most purposes the unit of volume, when using English 

 measures, has been agreed upon in favor of the cubic foot, and 



l Examples of ground-water curves for the chief streams therein con- 

 sidered may be found in Mr. Vermeule's lteport on the Flow of Streams, 

 etc., in Final Repft State Geologist of New Jersey, Vol. III. Trenton, 1894. 



2 Measuring Water, i>y Clemens Hersohel ; An address to the students of 

 Rensselaer Polytechnic Institute, Troy, N. Y. 



