Timber Cutting and Water Yields 



amount of loss from this source by thin- 

 ning the forest and reducing the over- 

 all density of the canopy on a given 

 area. Similar results might be accom- 

 plished by encouraging the growth of 

 hardwood vegetation like aspen in- 

 stead of conifers like spruce or fir. 



After precipitation passes through 

 the forest canopy, what is left piles up 

 on the ground, if it occurred as snow, 

 and remains there until warm weather 

 makes it melt. When this happens, or 

 if it fell as rain in the first place, it be- 

 gins to move toward the nearest stream. 

 In a forest it does this ordinarily by 

 entering the litter and humus on the 

 soil surface, then going down through 

 porous layers of soil toward the ground- 

 water table. If the soil was dry when 

 melting began or the rain occurred, 

 some of the water is held by the soil 

 itself and may not reach the ground- 

 water table at all. But after the soil 

 reservoir is filled to capacity, any addi- 

 tional water from the rain or melting 

 snow reaches the water table and moves 

 through it to the stream. Because the 

 soil is a complex body with varied 

 layering and structures, the movement 

 of water may be quite complicated, but 

 this is its general course. 



When water has entered the soil it 

 is exposed to another influence of the 

 forest on water yields: During warm 

 weather a part of the soil water is 

 drawn out by the trees. This draft, to- 

 gether with evaporation directly from 

 the soil rather than through the roots 

 and crowns, is what has made the soil 

 under the forest relatively dry before 

 rain comes or winter snows are stored 

 on the ground. In a heavy forest, tran- 

 spiration is likely to be high because of 

 the large volume of tree crowns that 

 are exposed to air and heat. Direct 

 evaporation from the soil or from 

 stored snow, on the other hand, is likely 

 to be low because the ground is shaded, 

 and soil evaporation is further reduced 

 where the ground is covered with litter. 



When the forest is thinned or 

 removed entirely, transpiration is re- 

 duced or even eliminated. But evap- 

 oration goes up as the ground becomes 



595 



more and more exposed to the sun 

 through the removal of the shading 

 canopy and the loss of organic litter. 

 So, evaporation and transpiration tend 

 to offset each other to some extent 

 if one is reduced, the other is increased. 

 On the whole, however, it is believed 

 that losses of water from the soil are 

 decreased by thinning or removing 

 forest vegetation. And when savings 

 due to reduced interception are in- 

 cluded, the increase in potential stream 

 flow is likely to be substantial. 



But there is a real flaw in the story 

 as told up to this point. 



It seems generally agreed that the 

 removal of forest vegetation almost al- 

 ways results in some deterioration of 

 the soil and site, and that the rate at 

 which the soil can take in and trans- 

 mit water may be lessened to a greater 

 or smaller extent and for a varying 

 length of time. Then the water may 

 not all go through the soil; some of it 

 may run off over the land surface. If 

 it does so in any appreciable volume, 

 it moves much faster than it can 

 through the pores of the soil. Unless 

 the ground is protected by a heavy sod 

 or a deep layer of humus and litter, 

 this water picks up soil particles as it 

 travels. The result is a quick accumu- 

 lation of muddy water in the streams 

 we see it in our valleys in the form of 

 flashy, destructive floods and clouded 

 rivers. In contrast, water that moves 

 through the soil does so more slowly 

 and stays free of sediment; we see the 

 result in clear, well-regulated brooks 

 and rivers. 



Whether or not surface runoff is 

 actually produced depends, of course, 

 on more than just the reduction of the 

 infiltration capacity of the soil. It re- 

 quires also a rate of water application 

 in excess of this capacity. If the peak 

 rainfall intensity, for instance, is rela- 

 tively high (say, 3 to 4 inches an hour) , 

 any reduction in the capacity of even 

 a porous soil to take in water may re- 

 sult in excessive surface runoff. If, on 

 the other hand, rates of snow-melt or 

 rainfall intensity are quite low, reason- 

 able reductions in infiltration capacity 



