t 
DISCUSSION : FORESTS, RESERVOIRS, AND STREAM FLOW 385 
as in mountain regions, in consequence of the surface run-off of rain Mr. willis. 
and snow water from the slopes, a factor is introduced through which 
wooded mountain slopes receive a much larger amount of water than 
bare unwooded water-sheds.” 
Any one who will examine the data, derived from 30 years of 
observation and experiment, on which Ebermayer bases his conclusion, 
will hardly think that “post hoc, ergo propter hoc is the argumentative 
process relied upon.” 
The question of regulation of floods from mountain valleys by 
forests cannot be allowed to pass without a discussion of Colonel 
Chittenden’s argument. In assuming “an inclined-plane surface, practi- 
cally impervious to water,” he introduces a special condition that may 
arise under a forest, but only as a limiting case, namely, when the 
forest has stored all the water possible and can do no more. On slopes 
which facilitate run-off, no other agent can do as much. 
Contrasting the division of rainfall on forest slopes with that on 
bare slopes, there is, in each case, water which is lost to use, a minus 
quantity, and. water which is gained for use, a plus quantity. The 
loss on the forest slope is: (1) That which stays in the tree tops; 
(2) that which is held by litter and evaporated from it and from the 
soil; (3) that which is transpired through roots and leaves. The loss 
from a bare slope is evaporation plus run-off. The gain in either case 
is through percolation and ground storage for a longer or shorter 
period. 
During a period of little or no precipitation, the chief loss from 
the forest is by transpiration; on the level, as determined by observa- 
tion in Russia, France, and Germany, it is greater than that by 
evaporation from a bare surface, and lowers the water table relatively 
to that in an adjoining field; on mountain slopes, it no doubt has a 
similar effect, which is, however, in considering the losses from 
forested slopes, to be balanced against quick run-off from bare hills. 
At the end of a dry period the ground reservoir beneath the forest, on 
plain or mountain, is in a condition to receive more water than that 
beneath a bare slope. 
During a period of precipitation the chief loss from the forest 
is in the tree tops. As already stated, it varies greatly in amount, 
but, whatever it comes to, it is, in the limiting case of an excessive 
rain, a beneficial because a minus quantity. 
At the end of a dry period, the layer of leaves, rotten wood, and 
humus beneath a forest has a capacity to absorb as much as 50% of 
its volume of water. When rains come, this amount is retained from 
run-off or ground storage and later evaporated. So retentive is the 
humus that, while it serves as a sponge to prevent run-off and transmit 
water to the ground reservoir, it does not give up much to storage, 
and yields its water-content only slowly to the moderate influences of 
