| OCR Text |
Show of the land surfaces. It will be highest with heavy plant covering, thick organic soil accumulations, loose unconsolidated soil texture and level land surfaces. The higher elevations of the Pacific Northwest and all regions east of the Mississippi River are generally classiffied as areas of water surplus. These humid areas have by inference a water supply in excess of the needs of farm crops and other plants. So much of the water precipitated, however, is unavailable to crops that water deficiencies are often serious even though the annual precipitation is far above the amount that can be consumed by plants. To secure best results, water must be available in amounts which harmonize with the pattern of needs. Generally these needs are nonexistent in the dormant winter season, small during the early spring, reach their maximum during midsummer and subside gradually to zero in the late fall and winter. Water defi cits may occur even when large annual surplus amounts are precipi- tated. Heavy storms may be only fractionally available to growing plants, and light storms are often of small consequence. Regions of matching water supply and water needs do not exist except in restricted areas and then only during years with specially favorable patterns of precipitation. Deep, favorable soils are usually necessary to absorb and make water available in harmony with the variable plant requirements in any area, making a close approach to harmony of water s upply and requirements for effective photosynthesis. Large over- supplies often impair the total products as much as water deficits. Nowhere, however, is water supplied by nature in the amounts and at the times which permit the maximum harvestable results from photosynthesis. Many plants have necessarily developed specially designed root systems and structures to compensate for shortcomings in rainfall distribution and in the water- holding qualities of the soil. Water requirements of plants are determined primarily by meteorlogical forces which not only vary with the seasons but also have a distinct daily pattern which follows closely the variable intensity of the sun's light. No area in the entire country provides water at the times and in the amounts that conform exactly to these patterns and seldom is maximum photosynthesis achieved. Accelerated rates of production are possible with each successive adjustment of water supply to more nearly meet exact evapotranspiration requirements. These highly remunerative returns from improved applications of water contrast with the diminishing returns usually secured by other methods of increasing crop production, such as more intensive cultivation and the adjustment of other factors. From these fundamental principles, several important conclusions follow: 1. Except for restricted areas, water is scarce in all parts of the country. Even in humid sections, water is generally inadequate for uses which require a controlled supply. 2. The greatest productive gains can be secured only when water is under such effective control that it can be made available in harmony with the pattern of uses, especially to meet the exact evapotranspiration requirements of plants. 3. The most productive use of water can usually be obtained by applying water to compensate for deficiencies when natural supplies closely approach adequacy. 40 |
