REVIEWS 031 



third to one-half as much as that in the stream in the lightly forested 

 watershed for the same intensity and amount of rainfall. The total 

 flow from the forested watershed in heavy rainfall was usually only 

 half as much as that from the lightly forested. If both watersheds had 

 exactly the same configuration and if forest and shrub cover were 

 entirely lacking in the lightly forested watershed, the difterence of 

 course would be still greater. The height of the water stages and the 

 total run-off during the heavy rainfall of short duration, and this is 

 true of all other rains, depended largely upon previous weather condi- 

 tion which held equally good for both watersheds. 



In general heavy rains the retaining power of the forest and soil 

 varied greatly according to the previous weather or according to the 

 water content of the soil and the duration of the rain. When the 

 moisture content of the soil prior to rain was great, the forest cover 

 proved ineffective and the run-off' was the same as from the lightly 

 forested watershed. If, however, the soil was comparatively dry, the 

 forest cover had a decided eff'ect in reducing the amount of run-off". 



In general rains of variable intensity the forest cover showed a ten- 

 dency to retard the amount of run-oft*. 



In the spring months when the snow melted and in the fall when 

 evaporation in the open was comparatively small, the run-oft' from the 

 lightly forested Rappengraben was on an average greater than from 

 the Sperbelgraben. In summer and winter, on the other hand, more 

 water ran oft' from the forested Sperbelgraben than from the lightly 

 forested Rappengraben. In the summer months, which in that region 

 is the season of dry and variable weather, the stream in the forested 

 watershed was fed more abundantly, more continuously, and more 

 uniformly, than the stream from the open watershed. 



The amount of run-oft' per second and per day fluctuated during 

 each season much more in the stream from the lightly forested water- 

 shed than in the stream from the completely forested watershed. As a 

 general rule, therefore, the forest cover had a controlling influence in 

 stabilizing and evening out the flow of water in the stream. 



This eft'ect of forest cover in stabilizing and making the streamflow 

 more uniform is due not directly to the forest cover but to the eft'ect 

 which it has upon the permeability and looseness of the soil and in 

 maintaining a uniform temperature and moisture of the air and soil. 

 The porosity and lightness of forest soil is brought about by the 

 protection furnisiied by the tree crowns, by the formation of leaf mold, 

 and by the presence of living and dead roots and an abundant soil 



