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Show with the necessity for maximum stream flow to irrigate the fertile valleys and flat lands. Regulation of flood waters is a separate func- tion, in which management of the land reservoir is useful, but a less valuable tool than the con- struction of large reservoirs, levees, channel and other improvements. In some areas, watershed management can help to lower flood peaks, and to some extent make use of flood waters by slow- ing down runoff and permitting maximum infil- tration. It is less useful in severe floods than in the more frequent ones of lower intensity. The chief usefulness of watershed management in connection with flood control in many areas is that it may reduce flood damage by controlling erosion, thus lowering the sediment load of streams. Also, it protects the major flood con- trol structures against filling up too rapidly with sediment. For all these reasons, watershed management is an integral part of the formulation and opera- tion of multiple-purpose development programs and projects. The pace at which it should go forward in basin development depends on the objectives of the basin program, the state of the land and water resources, and the extent to which the watershed can and should be stabilized through the encouragement of vegetation and other conservation practices. Once the decision is made as to the relative importance of agricultural, industrial, municipal, and other uses of water, and the extent to which water should be held where it falls or channeled quickly to some lower point where it may be more valuable, the function of watershed management will be clear. At that point, the basin commis- sion can lay out a series of techniques either to hold the water on the land or to channel it into the streams- By this means, the land and water together will be used to their full capacity. In either case the plan will be designed to conserve the soil. How the Watershed Works As rain falls to earth, some is stored on the leaves and tranches of vegetation, some on the soil surface, some in the soil mantle, and some in the cracks and fissures of bedrock, in the under- ground reservoirs, and in valley fill. Then this water is released in different ways. A part evap- orates from the vegetation, litter, and soil surface. Some is extracted from within the mantle by evaporation and transpiration. Some may never enter the mantle but move over the land surface and reach stream channels directly as overland flow. Some may sink through the mantle by gravity to emerge as springs, in stream channels, or remain in ground water. Some plants form a canopy over the land reser- voir surface. This cover may be tall and thick, as in many forests, or short and thin, as on some desert ranges. Tall or short, thick or thin, plants affect the reservoir functions of the land in many ways. Plants deepen the soil at the surface by the addition of organic material. They deepen the soil beneath by the prying down of their roots and by an endless chemical contribution to the disintegration of the underlying rock. Thus they increase the water storage capacity of the soil; for 3 inches of good topsoil are capable of holding half an inch of water. To disrupt or destroy the plant robe may dis- astrously reduce the water-storing capacity of soil. Destruction by fire, for example, of a 6-inch layer of litter on a forest floor can mean an immediate loss of as much as 2 inches of soil storage; and as long as 80 years may be needed to build back the same amount of litter and storage capacity. Storage can likewise be lost by the removal of the soil mass through erosion. Eroded soil means not only soil lost for all practical purposes, but reduced water-storage capacity and a lessened control of water. Watershed Management Results Plants, the characteristics of the soil, and sup- plementary man-made structures all affect the ways in which the land reservoir gets and gives up its water. Hence they affect the quality and amount of water and the rate at which it is ab- sorbed into or drained off the surface. 126 |
