Forests and Water

Above: A watershed like this, a burned-over area in the Rockies, requires a careful use.

TIMBER CUTTING AND WATER YIELDS

H. G. WILM.

THE VALUE of forest vegetation in protecting watershed land has been so clearly demonstrated that its development and maintenance are recognized more and more as a powerful tool to control erosion and floods.

Forests are useful for this purpose in wide areas of the United States in the Ohio River Valley, the southern Mississippi River Valley, and the Appalachian Mountains; on the rolling countrysides of New England and the rough, steep slopes of the Rocky Mountain Front Range. In all of those places and in many more, deterioration of watershed conditions due to the removal of forest cover has led to flashy, destructive stream flow and greatly increased soil erosion and sediment production. Wherever such conditions exist on land that is adapted to o growing forest vegetation, the logical solution is to restore the forest by any available means.

But it is less widely recognized that the same virtues of the forest that make it valuable in the control of erosion and floods may become disadvantages in other areas, where such problems are small but water shortages are acute.

Water troubles with this different aspect can be found on long belts of high-altitude watershed land along the big backbone of the Rockies and in the upper areas of other mountain ranges: Land that produces large volumes of water and sends it down the mountain canyons to spreading plains and and valleys below, places where almost every available drop is consumed by cities, towns, and irrigated farms.

For many years the people of dry areas like these throughout the West have worried about their water supplies. Water there is so precious that it is bought and sold by the acre-foot or even by the gallon; it is so greatly in demand that in some places existing legally established water rights exceed the highest recorded annual flow in the streams.

Under such intense pressure for water, naturally these people look to the mountain watersheds that are the source of their water, and they wonder whether they are getting all that might be made available. Knowing, for instance, that plants of every kind use water in considerable quantities to maintain life and produce vegetable matter, they speculate as to whether any water might be saved if watershed vegetation could be thinned or even completely removed.

This conjecture has given rise to long-standing arguments, first brought to a peak by Col. H. M. Chittenden in an article which was published in 1909. Rather significantly, Chittenden had been studying western water problems for some time and was engaged in the development of water resources for the general region east of the Rocky Mountains. He argued that forests diminish total runoff through evaporation and transpiration, and that they are not so valuable in reducing floods as had been believed. This contention was raised by others repeatedly in ensuing years and gained support by engineers and others interested in the development of water resources.

Partly as a result of such arguments, people have learned much about the true value of forest cover in watershed protection since those early days, and realize that it serves an extremely important function in stabilizing soil and in reducing floods.

But on and western lands the need for water is sometimes so great that people are still willing to take the risk of floods and erosion in order to get it. In the unusually dry but highly developed valleys of the Southwest, men have been heard to say: "Give us the water, clean or dirty we'll take care of the mud somehow!"

Obviously that kind of talk shows an incomplete understanding of the disastrous consequences of soil depletion and erosion or perhaps it shows a loss of perspective, resulting from the extreme need for water in those areas. Anyway, such statements give a picture of how acute water-supply problems can be in the West and how important it is for watershed managers to get every drop that can be produced safely on mountain watershed lands. We dare not overlook the chance that Chittenden and others may have had a strong basis for their contentions that there may be areas where vegetation can safely be removed and water yields thereby increased.

At first glance it does look like a hard problem. For any given area we have to learn whether it is necessary to maintain a complete forest cover and accept the resulting water consumption in the interest of protection, or to what extent we can relax this requirement in order to reduce water losses. In working on this problem we must also remember that, wherever a watershed contains merchantable timber, protection may mean depriving people of badly needed lumber and other products. We want to insure reasonable use rather than unnecessary protection.

WHAT HAPPENS TO WATER IN THE FOREST is the basis of forest management for maximum water yields under safe conditions.

When snow or rain falls on a forest, some of it is intercepted by the tree crowns and is stored for the time being on leaves and twigs. A large part reaches the ground by dripping from branches or running down the trunk, but a part of it remains on the crowns, where it is lost by evaporation after the storm is over.

If the yearly precipitation on any area is made up of small storms separated by periods of clear weather, this evaporation from crowns is high as much as 35 to 50 percent of the yearly total. Where storms are larger and much cloudy weather occurs, the relative amount of crown interception and loss is smaller. Interception varies also with the kind and the density of the crowns: Thick spruces catch and hold more water than thin-crowned pines, while leafless cottonwoods and aspen intercept much less winter precipitation than any of the conifers. Hence it should be feasible to cut down the amount of loss from this source by thinning the forest and reducing the overall density of the canopy on a given area. Similar results might be accomplished by encouraging the growth of hardwood vegetation like aspen instead 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 begins 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 groundwater 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 groundwater table at all. But after the soil reservoir is filled to capacity, any additional 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, together 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, transpiration 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 reduced or even eliminated. But evaporation goes up as the ground becomes 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 included, 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.