152 GEOIvOGICAIv SURVEY OF NEW JERSEY. 



into the atmosphere above the tree-tops. Nor, again, can we 

 believe that it would not be found that evaporation from beneath 

 the leaves of a heavy crop of Indian corn or clover would also 

 be less than in the open. The same answer applies to the state- 

 ment that temperature beneath the forest trees being less, on our 

 theory that evaporation is so much affected by the temperature, 

 this would call for a less evaporation from forests. What it does 

 call for is a less evaporation beneath the leaves of the forest, 

 which we have already stated is undoubtedly the fact, but it 

 cannot affect the evaporation measured, as it must be, above 1»he 

 level of the tree-top. It ought not to be necessary to remind 

 scientific writers that it is not proper to measure evaporation 

 from crop areas so as to include water taken up by the plants, 

 and evaporation from forests so as not to include water taken up 

 by the trees, and then attempt to compare these two measure- 

 ments in order to determine relative evaporation from forest and 

 from farm crops. (See Proceedings of the American Forestry 

 Association, Vol. XII, page 157.) 



EFFECT UPON MAXIMUM AND MINIMUM FLOW OF STREAMS. 



While we have pointed out the beneficial effect of forests in 

 equalizing the flow of streams, this effect does not appear in the 

 maximum and minimum flow. The reason is that the severest 

 floods occur under such conditions as that of a warm rain falling 

 suddenly upon a heavy accumulation of snow, or upon ground 

 frozen in such a manner that it cannot be absorbed. Or if it is 

 in summer, the ground may already be so saturated with water 

 as to prevent further absorption. Forests are powerless to 

 prevent floods under such extreme conditions as these, although 

 they do diminish the number of floods. 



In the same manner the extreme low point is reached when 

 the streams have been for a long time drawing upon their 

 springs and have extracted the water from the ground to a 

 considerable depth below the surface. Our investigations show 

 that the lowest discharge is usually reached when the water 

 has been drained from the subsoil to an average depth of over 

 four feet. It should be evident that when the water which 

 feeds the stream is coming from points so far beneath the sur- 

 face, the humus or other surface peculiarities of forests can have 



