RELATION OF FORESTS TO STREAM FLOW. 



285 



The forest, in extending the time during which the rain reaches the 

 soil, in its effect upon local topography, and in supplying a larger and 

 better absorbing medium, must necessarily have a profound influence 

 in increasing the seepage run-ofl', and in proportionately decreasing 

 the surface flow. 



COMPARISOX OF RUX-OFF FROM FORESTED AXD XOXFORESTED AREAS. 



There are so many complex conditions influencing the flow of 

 streams that it is extremely difiicult to determine the effect of forests 

 on run-off by the comparison of the discharge of streams on forested 

 and nonforested catchment areas. It is believed by many that stream 

 flow is so largely influenced by the amount, intensity, and character 

 of the precipitation, the configuration and area of the catchment basin, 

 the character of the absorbing medium and the underh'ing rocks, and 

 the general climate, -as well as the forest itself, that we shall probably 

 never be able to measure quantitatively^ the influence of forests on the 

 flow of streams b}^ the comparison of forested and nonforested regions. 

 Catchment areas differ so greatly in the features mentioned above that 

 our most conservative and able investigators have been forced to the 

 conclusion that ''in respect to run-off. each stream is a law unto itself." 

 Although the above is probably in the main true, 3^et, by the careful 

 selection of small catchment basins for comparison, it appears that the 

 influence of the forest in diminishing the surface run-off* can be deter- 

 mined with a fair degree of accuracv. When the catchment areas com- 

 pared are in the same region, are influenced by the same or nearl}' the 

 same climate and precipitation and by the same storms, have approxi- 

 mately the same configuration and area, and have a similar mineral 

 soil and underlying rocks, the effect of these various factors on the 

 run-off can be ignored, and the differences in the behavior of the stream 

 flow on the forested and nonforested areas can be assigned to the 

 influence of the forest. 



In a careful study of the behavior of the stream flow on several 

 small catchment areas in the San Bernardino Mountains, it has been 

 found that the effect of the forest in decreasing surface flow on small 

 catchment basins is enormous, as shown in the following tables, where 

 three well-timbered areas are compared with a nontimbered one: 



Precipitation and run-off during December, 1899. 



Area of \ 



catchment 



basin. ! 



Condition as to 

 cover. 



Run-ofE 



Run-off in 

 Precipita- , ^er'sQuare Percent- 

 tlon. I P^Lf^p^"^^ i age of pre- 

 °"^^- I cipitation. 



Sq. miles. 



0.70 



1.05 



1.47 



.53 



Forested 



do 



do 



Nonforested 



Inches. Acre-feet. 

 19- 



19+ 

 19+ 

 13^ 



73+ 



70- 

 312+ 



Per cent. 



40 



