6 FORESTS, WOODS, AND TREES 



and the water reservoirs below to impound the flood waters 

 of the stream. The development of the former is the 

 immediate and feasible task of the forester, the construction 

 of the latter is the work of the engineer. Prof. Church's 

 investigations open up a new field for planting trees at high 

 altitudes with a view of lessening the volume and frequency 

 of disastrous floods, which are due to rapid melting of snow 

 in the mountains from which they take their source. 



4. The Influence of Forests on Water Supply, on Floods, 

 and on Soil Erosion. — The most important action of the forest 

 probably is its influence on water supply or stream-flow. 

 We must distinguish the great difference of this influence 

 in the plain and in the mountain. 



Only a part of the precipitation is available for water 

 supply. Of the total rainfall over an area, 



(1) a part is intercepted by the foliage of the trees and 



evaporated ; 



(2) a part is evaporated from the soil ; 



(3) a part is absorbed by the trees and other plants, 



and is partly transpired by them ; 



(4) a part flows directly off the ground, joining im- 



mediately a stream or river ; this is known as the 

 run-off ; 



(5) the remainder of the rainfall sinks into the ground, 



and joins the stratum of underground water, 

 ultimately going to supply springs and streams. 

 On level land the total loss of water from these various 

 causes, interception, transpiration, and evaporation, is some- 

 what greater from the forest than from any other cover, and 

 is less from bare soil, where it is about 50 per cent of the 

 rainfall. The surface run-off in level country is negligible, 

 and a bare soil would retain the greatest amount of water 

 for wells, etc. As a net result, in the plain, the forest acts 

 as a drainer of the soil. Hence, the action of plantations 

 of woods and trees in draining marshy ground is often of 

 considerable hygienic value. The total effect of forests or 

 woods on the springs and water supply in the level country 

 may, however, be looked on as of little importance. 



