18 Canadian Forestry Journal. 



one of our winter storms ; or it may be part of a merely local air- 

 movement, such as our summer showers usually are; or it may 

 consist of a forced movement up the slope of a mountain-side. 

 The question is, to what extent may the forest modify any or all 

 of these movements, so as to affect rainfall? 



First, as to the cyclonic movement, even if evaporation 

 is increased over a forested area, the vapor is carried possibly 

 hundreds of miles by the great horizontal inovements of air in 

 this class of storms; hence, increased evaporation over a forest 

 does not increase the rainfall. Next, can the forest induce at- 

 mospheric conditions that will divert the storm-path? Theo- 

 retically, this seems impossible for great cyclonic areas, and 

 doubtful even for local storms. 



Secondly, as to storms of local origin, such as the thunder- 

 storms, vapor formed from any region may be deposited again 

 over that region. Hence, if evaporation is increased by the 

 forest, it seems likely that rainfall also in the summer time, when 

 local storms abound, may be increased. Referring now to the 

 table of evaporation given above, we may see that evaporation 

 from a forested region is less than that from sod or cereals, but 

 more than that from bare soil. The substitution by the farmer 

 of grass, root, and cereal crops for the forest in Ontario, would, 

 therefore, tend to increase evaporation from the cultivated areas, 

 and thus, whatever effect the forest may have upon local rain- 

 falls, would be intensified by deforestation and cultivation. 



As it does not appear probable that the forest on a mountain 

 side can intensify the efifect of the mountain in causing rainfall, 

 we are forced to the theoretical conclusion that the effect of the 

 forest in increasing rainfall can be but slight, and is probably in 

 most regions nil. This theoretical conclusion is, on the whole, 

 supported by such experimental data as have been obtained by 

 the various investigators. 



II. We come now to the consideration of the influence of the 

 forest upon the distribution of the water-flow. 



The forest controls the distribution of the water (precipita- 

 tion) that reaches it, in the following ways: 



(1) By intercepting part of the rainfall. 



(2) By diminishing evaporation, within its own borders and 

 beyond. 



(3) By transpiring large quantities of water, and thus dimin- 

 ishing the water-content of the soil. 



(4) By the influence of the forest-litter, (a) absorbing part 

 of the precipitation, (b) offering a mechanical obstruction to sur- 

 face flow and thus preventing run-off, (c) protecting the surface 



