OCR Text |
Show Meteorological and Physical Factors Affecting Water Supply Water resource management is restricted to water in its liquid state; in its vapor form it is not subject to control. Its management is feasible only where facilities for collection, channeling and storage are present on the land surface or in the geological structures of the earth's crust. All manageable water originates as precipitation condensed from atmospheric vapor and is deposited on the earth in liquid or solid forms. Even before reaching the earth, however, the process of its reconversion to vapor begins; and after a brief period most, and in some cases all, is returned to the atmosphere as vapor. Of the total precipitation of the United States, a residual, approximately 30 percent runs off to the oceans; but the percentage varies widely in different parts of the country. Water makes its greatest contribution to human welfare as it is utilized by plants in the process of photosynthesis. Most vaporization of water as it now occurs on land surfaces is either wasteful or of low economic significance. The primary technique of water resource management therefore is to reduce wasteful and uneconomic vaporization and to channel water thus salvaged to useful evapotranspiration and other high order consumptive uses. The stages in the hydrologic cycle are each governed by their own set of physical forces. During the process of precipitation a portion of the water crop is reconverted to vapor before reaching the ground. The proportion lost is determined by the intensity and duration of the storm, the temperature and humidity of the atmosphere, and the density and character of the vegetative cover. On reaching the earth's surface, the reconversion of the water crop to vapor begins immediately. The rate and proportion vary directly with temperature, the directness of the sun's rays, the velocity of the wind and turbulence of air movements, and inversely with the degree of saturation of the surrounding atmosphere. Especially heavy rates of vaporization proceed from open water surfaces, swamps and marshes, and dense stands of heavy water- consuming plants. As it enters the soil mantle, a portion of the water crop, sometimes all of it, is quickly diffused and disassociated into minute water particles occupying the smaller inter- spaces and forming thin films on soil particles. Within the range of the capacity of the soil to hold such moisture, further movement of such water is restricted; except through roots of plants and there may be no surplus available for water management. The amount of water thus held in place by the soil against further movement or withdrawal as water, depends upon the thickness of the surface soil mantle, its texture, and the volume of the spaces between the soil particles. This amount, termed the " field capacity," varies with the type of soil; from a low of 8 percent in some sandy soils, to 20 to 40 percent in clay loam. Water entering the soil will be progressively entrapped, to the full extent the precipitated amount can provide the full field capacity of each successive layer; while lower layers remain unaffected. Only when this has been attained for all layers of the soil mantle will water be available for collection and control. The ratio of field capacity attained is the resultant of the volume and character of the storm, the degree and ease of infiltration, and the thickness and texture of the soil mantle. For each storm this proceeds at rates which vary directly with the water absorbing capabilities of the soil mantle covering and inversely with the slope 39 |