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Show OBJECTIVE OF CATCHMENT AREA MANAGEMENT 1. REDUCE THE DISSIPATION OF THE WATER CROP THROUGH VAPORIZATION ALONG ITS COURSE FROM THE ATMOSPHERE TO THE EARTH'S SURFACE 2. PREVENT OR REDUCE DISPERSION OF THE WATER CROP INTO THE INTERSPACES OF THE SOIL MANTLE AND AS FILMS COVERING SOIL PARTICLES 3. PREVENT OR REDUCE LOSSES FROM RUNOFF TO FORMATIONS FROM WHICH IT CANNOT BE RECOVERED Basic Principles of Catchment Area Management to Maximize the Water Yield Available for Control /. Reduce the dissipation of the water crop through vaporization along its course from the atmosphere to the earth's surface. Powerful meteorological forces are required to liquify the vapor of the atmos- phere for its transfer to the earth's surface as rain, snow or dew, but other powerful forces begin at once the process of reconversion to atmospheric vapor. In areas of high temperatures, high wind velocities and arid atmosphere, a large proportion of precipitation never reaches the ground or at least the root zones in the soil. The extent of this vaporization is also influenced by the height and density of the plant cover. Any such reconversion during the process of precipitation constitutes a net subtraction from the water crop. Opportunities for minimizing such losses are restricted to feasible methods of controlling the density and character of the plant cover. The extent of this vaporization would be influenced primarily by the height of the vegetation, its density, and the pattern of distribution of branches and foilage. Such plants should be brought under control wherever the economic or other values of a plant cover are less than the alternative values of water that can be salvaged for other uses. 2. Prevent or reduce dispersion of the water crop into the inter- spaces of the soil mantle and as films covering soil particles. A portion and sometimes all precipitation is diffused through the surface soil mantle where, within the range of the " field capacity" of each soil, it is tenaciously held and does not again emerge as surface or subsurface streamflow. The soil mantle serves essentially as a sponge and facilitates the dispersion of water into numerous minute particles and as thin films. Its recovery to a liquid, manageable form is difficult and generally impossible. The amounts so held do not re- enter the liquid supplies and are lost from the manageable water crop. Beneficial uses of any water thus diffused throughout the soil mantle are restricted to consumption in place by plants with mechanisms equipped to withdraw this water for their purposes. The extent of this loss to the manageable water supply depends upon the texture, thickness, and substance of which the soil is composed. These determine the " field capacity" of each soil - the residual held in place after surpluses have drained away. All such amounts are removed from the realm of possible water management. 3. Prevent or reduce losses from runoff to formations from which it cannot be recovered. That portion of the water crop not entrapped and held in soil mantle moves in surface and subsurface channels to lower elevations. A portion of such water may enter formations from which it cannot be recovered, and is lost to further use. For any catchment area the determination of such losses and the methods of avoiding them can best be approached by systematic mapping of all geological formations and such other studies as may be necessary to determine the amounts and course of movement of such water. 37 |