EELATION OF FORESTS TO STREAM FLOW. 283 



days without even moistening the soil. In so far as forests check the 

 winter winds and provide shade, they lessen winter evaporation. 

 This lessening" of the evaporation from snow, surfaces, through the 

 action of forests, is seen in the fact that snows linger much later in 

 spring in well-wooded regions than in open areas. 



It appears, then, that forests materially retard evaporation, both of 

 soil moisture and of snowfall. 



DO FORESTS INFLUENCE TRANSPIRATION? 



When land is covered with vegetation a certain amount of the rainfall 

 is taken up by the growing plants. A small part, through chemical 

 change, becomes incorporated into the plant, but the larger part is 

 returned to the atmosphere through transpiration. Although those 

 who have investigated this subject are by no means in accord, there is 

 reason to believe that considerable difference exists in the amount of 

 water taken up by the different types of vegetation in the process of 

 growth. On the whole, the forest probably takes up less water from 

 the soil than the average agricultural crop. Risler, from a lengthy 

 series of investigations, reached the conclusion that forests actually 

 take up less than one-half as much water from the soil as the average 

 agricultural crop. 



The above would lead one to infer that where the soil, if not covered 

 with forest growth, is clothed with grass or some other low form of 

 vegetation, the return of moisture to the atmosphere, through evapora- 

 tion and transpiration, or, in other words, the "flj^-off," is less from 

 the forest than from the open. But in regions having a short wet 

 season followed by a long dry one the return of moisture to the atmos- 

 phere is probably greater from a forested area, because in the open 

 for a large part of the year there is very little to evaporate, and the 

 scanty growth of grass and other low forms of vegetation gives little 

 opportunity for loss through transpiration. 



THE INFLUENCE OF FORESTS IN REGULATING THE RUN-OFF. 



Stream flow consists of both surface run-off and seepage run-off. 

 Although these two can not be separate^ determined, total run-off 

 admits of accurate measurement. Surface run-off may be considered 

 as flood water, while seepage run-off is that portion of the drainage 

 which gives the streams a sustained flow. It is evident that any factor 

 which decreases the surface or superficial run-off and increases the 

 seepage run-off' is of the utmost importance in regulating the flow of 

 streams. 



The proportion of flood water to seepage is influenced by the rapidity 

 of the rainfall. It is well known from direct observation that a slowly 

 falling, prolonged rain, even on the naked soil of steep slopes, is all 

 taken up by the soil. On the other hand, a heavj^ shower of short 



