RELATION OF FORESTS TO STREAM FLOW. 



287 



one-third of that from the forested catchment areas, and in June the 

 flow from the nonforested area had ceased altogether. 



DO FORESTS INCREASE THE RUK-OFF ? 



Owing to the very complex nature of the investigation involved in 

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

 able evidence does not admit a definite answer that will be of general 

 application. It is reasonably certain from present evidence 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 evenl}^ distrib- 

 uted over the year, and where the catchment area is sufficiently large 

 to permit the greater part of the seepage to enter the stream above 

 the point where it is gauged, the evidence accumulated 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 ver}^ materially decreases the total amount of 

 run-off. 



Annual rainfall and run-off on forested and nonforested catchment areas in the 

 San Bernardino Mountains, California. 



caiTment ^^^^^tion as to 

 basin, j ^°^^^- 



Precipita- 

 tion. 



T?„Ti r.ff Run-off in 

 ^- 1 cipitation. 



Sq. miles. 



0.70 



1.05 



1.47 



.5. 



Forested 



Inches. 

 46 



Acre-Jeet. 

 781 



Per cent. 

 28 



do ... 



46 756 



30 

 36 

 69 



do 



46 



904 



Nonforested 



33 



1,192 



