64 



The above table clearly shows the importance of forests in sus- 

 taining the flow of mountain streams. The three forested catch- 

 ment areas, which, during December, experienced a run-off of but 

 5 per cent, of the heavy precipitation for that month, and which 

 during January, February, and March of the following year had a 

 run-off of approximately 37 per cent, of the total precipitation, 

 experienced a well sustained stream flow three months after the 

 close of the rainy season. The nonforested catchment area, 

 which during December, experienced a run-off of 40 per cent, of 

 the rainfall, and which during the three following months had a 

 run-off of 95 per cent, of the precipitation, experienced a run-off 

 in April (per square mile) of less than one-third of that from the 

 forested catchment areas, and in June the flow from the nonfor- 

 ested area had ceased altogether. 



DO FORESTS INCREASE THE RUN-OFF } 



Owing to the very complex nature of the investigation involved 

 in determining the effect of forests on the amount of run-off, the 

 available evidence does not admit a definite answer that will be 

 of general application. It is reasonably certain from present evi- 

 dence that in some regions the effect of the forest is materially 

 to increase the run-off. It appears equally certain, however, that 

 in other regions, and on certain classes of catchment areas, the 

 effect of the forest is to materially decrease the stream flow 



Mr. Rafter, in his recent publication, " Relation of rainfall to 

 run-off," makes this statement: "With similar rainfalls, two 

 streams, one in a region having dense primeval forests, the other 

 in a region wholly or partially deforested, will show different 

 run-off. The one with the dense forest will show a larger run-off 

 than the stream in the deforested area." This author concludes, 

 from the careful study of a large number of catchment areas in 

 the State of New York, that the effect of the forest on at least a 

 portion of the area studied is to increase the run-off to an amount 

 equal to from 5 to 6 inches in depth over the entire catchment 

 area. 



In humid regions, where the precipitation is fairly evenly dis- 

 tributed over the year, and where the catchment area is suffi- 

 ciently large to permit the greater part of the seepage to enter 

 the stream above the point where it is guaged, the evidence accu- 

 mulated to date indicates that stream flow is materially increased 

 by the presence of forests. 



In regions characterized by a short wet season and a long dry 

 one, as in southern California and many other portions of the 

 West, present evidence indicates, at least on small mountainous 

 catchment areas, that the forest very materially decreases the 

 total amount of run-off. 



