WATERSHEDS AND HOW TO CARE FOR THEM

GEORGE W. CRADDOCK, CHARLES R. HURSH.

A watershed is a concave or trough-shaped land area in which the runoff from rain and snow drains toward a single channel. A watershed may cover less than an acre, or it may be a complex of many watersheds. Our entire land surface is made up of watershed units. On them we depend for our supply of water.

Never before has our interest been greater than now in water for irrigation, power, industry, navigation, domestic use, and recreation. Most of the water for those purposes has its source on the forest and range lands, which comprise two-thirds of the land area in the United States. Stream flow is a natural product of most of those lands, but the usefulness of the runoff from them hinges on their management.

Watershed management is a system of handling land resources within a drainage primarily to achieve usable runoff. This generally involves the same methods of husbandry that are employed in good forest and range management, but the objectives go beyond the attainment of sustained timer and forage production. Watershed management aims to keep the land in such condition that there will be maximum yields of high-quality water.

Because watersheds have been inexpertly handled, the water problems are critical in all parts of the country. In the past 100 years, while population increased from 17 million to 140 million, the demands for water increased manyfold. Industrial development and municipal expansion are now restricted in many places because of insufficient water. The extent of destructive floods is increasing. Sediment eroded from the land is filling reservoirs, stream channels, and harbors. Those problems Will become more serious as our populations and business expand.

Through research on watersheds we are finding out how different types of land use affect runoff and water quality, how to avoid past mistakes, and how to restore and maintain our water resources in the future. Some 40 years ago two experimental watersheds near Wagon Wheel Gap in Colorado were equipped to measure the effect of clear cutting of timber on stream flow. A few years later, a pair of range watersheds near Ephraim, Utah, were similarly equipped to determine the effects of grazing herbaceous plant cover on summer storm flow and erosion. More recently, additional forest and range watershed laboratories have been established in the Rocky Mountains of Colorado and in the mountains of Idaho, Utah, Arizona, and California. The Forest Service has developed an outdoor hydrologic laboratory on the Coweeta Experimental Forest in western North Carolina. Research on runoff and erosion problems of farm lands also has expanded greatly.

"The watershed with good plant cover, litter, and humus (made up of the decayed and decaying litter) functions like a blotter. It soaks up the water from rain or melting snow Some of this water goes back into the air later through evaporation from the ground and plants. Some enters and is stored in the soil. Part of the stored water is held in the soil for plant use; the rest slowly moves downward to feed the streams by underground flow. When very heavy and long rains occur, the soil may be unable to take in all the water that falls. The excess water then runs off over the surface, but at a slow rate." (From Know Your Watersheds, U. S. D. A. Agricultural Information Series 67.)

EVERY ACRE of land in a drainage basin receives and disposes of precipitation and thus functions as an integral part of a whole watershed. On each acre, the plant cover and soil mantle control the reception and disposition of precipitation. The control varies from place to place, resulting in different degrees of balance between the destructive forces of the weather and the developmental processes of soil formation and plant succession.

Before man started to move soil around, the developmental processes of soil formation and plant succession were stronger than the forces of degradation on much of the forest and range lands. That is, soil had been formed on most of those lands faster than it had eroded. The naturally adjusted balances between land and weather that had been in the process of development for thousands of years, however, were disrupted by land clearing, devastation logging, overgrazing of livestock and game, and fire.

Changes took place at the ground surface that altered the manner in which precipitation entered the soil. The storage capacity of the soil was also altered. Those changes threw the original control of water and of soil stability out of balance. The result has been widespread accelerated erosion, sediment in the streams, erratic stream flow, and damaging floods. Nature's original controls were maintained by vegetation. Today, better land-management practices must be inaugurated to restore a more favorable plant cover and soil structure if we wish to maintain land and stream conditions to serve our present and future needs for usable water.

THE SOIL and the underlying rock mantle is the key to understanding the control of water on the land. Soil is capable of storing water. Some of this water is retained by the soil just as water is held behind a dam. But the soil also releases water when the mantle is filled to capacity.

Soils on forest and range lands can absorb and retain against the force of gravity from 1 to 3 inches of water per foot of mantle depth. Fine-textured soils with a high content of organic matter have a greater retention-storage capacity than coarse soils a dry soil mantle 4 feet deep can absorb and hold from 4 to 12 inches of rain or water from melted snow without yielding a drop of runoff. This retention-storage function is the same as that performed by a dam. Removal of the soil by erosion, or otherwise, reduces the capacity of a site to retain water and so increases the chances for greater runoff and flood discharges in the same way as would the lowering of a dam.

Retention storage is only one of the storage functions of the watershed mantle. After a soil mantle is wet to its capacity to hold water against the force of gravity, it is not yet saturated. Air space still remains between the wet soil and rock particles. This additional storage space may be equivalent to as much as 2 inches a foot of mantle depth. Water that enters these spaces is not retained by the mantle but moves downward to the subsurface aquifers, where it may replenish the groundwater levels, or may emerge in channels or at springs to sustain stream flow.

The percolation of the free water through the soil and rock mantle of a watershed takes time much longer than the escape of water over the spillway of a dam. The slowness of the percolation process is attested by the fact that streams continue to flow for periods as long as a year after free water disappears from the soil mantle.

The delayed yield of water is one of the most important and valuable functions of watershed lands. Communities and industries pay millions for a sustained yield of water and one of the major purposes of billions of dollars worth of dams is to catch spring floods and make them useful in the autumn droughts. The same functions are performed by the soil on many millions of acres of forest and range watershed lands. These natural and beneficial functions of the soil must be maintained through good management.

PLANTS herbs and shrubs, as well as trees are important in maintaining an efficient watershed mantle.

All who have sought shelter under a tree during a rainstorm do appreciate that vegetation intercepts precipitation in its descent to the earth. In a 40-inch rainfall belt, an old-growth hardwood forest will prevent 6 or 7 inches of rain from reaching the ground during the course of a year. This means that insofar as the soil under the forest is concerned there is really only about 34 inches of rain instead of 40. During individual storms the plant canopy may intercept up to 50 percent of the precipitation. The plant canopy, in other words, is an integral part of the watershed reservoir with the special function of intercepting and dissipating a part of the precipitation before it reaches the soil mantle.

Plants and the plant debris on the ground surface protect the soil from the direct impact of dashing raindrops. Big drops are broken into little drops that have less force. Tree trunks, the stems of shrubs and herbs, and dead twigs, leaves, and other trash on the ground keep surface water spread out and moving at low velocities, thus reducing the capacity to erode the soil and retarding movement toward channels. This favors infiltration of precipitation into the soil and rock mantle, and the subsequent yields of water as seepage, rather than overland runoff.

Roots of plants also help in the process. They provide channels for the percolation of water. They bind the surface soil against the scouring effect of storm runoff and anchor the soil mantle on steep slopes to the bedrock.