Managing and Protecting Forest Water Resources

Dale S. Nichols, principal soil scientist, North Central Forest Experiment Station, Forest Service, Grand Rapids, MN.

One of the most important functions of forest land is the delivery of fresh water for drinking and sanitation, for agriculture and industry, and for recreation and wildlife. Where water is scarce and the demand is high, water may be the single most valuable product of forest land. Managing and protecting forest water resources is becoming an ever more difficult, more complex, and more important task.

Forests cover about 30 percent of the total land area of the United States, but an estimated 75 percent of the Nation's water supply originates on this land. In the Western States, high-elevation, largely forested watersheds produce some 90 percent of the usable water. With growing populations, especially in the West, the demand for water is increasing.

Besides water supply, forests are used for timber production, mining, grazing, waste disposal, recreation, and numerous other purposes. These activities may adversely affect forest water resources. The water resources of many forest lands are already impaired. U.S. timber demand in the year 2030 is predicted to be double that of 1976. Increased demands for other forest resources are anticipated as well. In addition, the total forest area is expected to decrease as forested land is converted to other uses.

Researchers have an interesting challenge to provide the new information and technology necessary to protect and manage water in the face of ever-increasing pressure on all the forest's resources.

Public concern for water was responsible, in part, for the establishment of our National Forest System. Some 154 National Forests contain about 190 million acres of federally owned forest land that were reserved from the public domain or purchased for the protection of critical timber and water resources. In addition to it; many other uses, this land has serve( for more than 50 years as a vast open-air laboratory for the study of forests and water. The research branch of the U.S. Department of Agriculture's Forest Service maintains for research purposes about 90 designated experimental forests on National Forest lands.

Water Supply

While much of the United States has abundant water, water supply in some areas is far less than the demand and is a limiting factor in resource development and economic growth. Water-supply projections indicate that by the year 2000, people living in 17 major river basins in 11 Southwest and Midwest States and in some localized areas in the Northeast and East will suffer serious water shortages. Demands for water for nonconsumptive purposes such as fish, wildlife, and recreation also will increase.

Watershed scientists are studying ways to increase the production of water from forest lands. It was once thought that forests conserved water, making more water available for streamflow. Researchers have found that this is not true. In general, trees need and use more water than do other types of vegetation. Forested lands yield water simply because forests tend to develop on those lands where water is most available.

Maintaining high-quality water for fish and wildlife is an important aspect of forest management. North Central Forest Experiment Station technician measures dissolved oxygen and temperature in an experimental wildlife impoundment.

A large share of the water falling onto a forest as precipitation is returned to the atmosphere as water vapor. Trees take up large quantities of moisture from the soil that is then lost to the air by transpiration through the leaves. In addition, water evaporates directly from the soil, and some rain and snow is intercepted by the tree crowns and evaporates before it ever reaches the ground. The water that is left over flows from the forest in streams or as ground water and is available for other uses. The water leaving an area as streamflow is called runoff and is measured in units of depth. Ten inches of runoff, for example, means that if all the water leaving as streamflow were spread evenly over the area from which it came, it would be 10 inches deep.

Runoff from forest land has been measured on dozens of experimental watershed areas in the United States. Where precipitation is barely enough to support tree growth, such as along the boundary between the eastern forests and the prairies, and in many places in the West, annual runoff may be as low as 1 or 2 inches. Average runoff from the eastern forests and from the high-elevation forests in the Rocky Mountains is about 10 to 20 inches a year. In the Appalachian Mountains and in the coastal mountains in the Pacific Northwest, this figure is 20 to 50 inches or more.

In 1956 Rocky Mountain Forest and Range Experiment Station scientists began a long-term study of the effect of forest-cutting practices on water yields by cutting a steep slope in a pattern of strips and patches. Snow accumulates in the openings, and the water yield from cutover areas is higher than from uncut control areas.