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Show rural watersheds, where flood damage is extensive, but relatively small in any particular spot. The extent of flood reduction that can be brought about by land management has not been measured for large areas, but experimental data are available. In the Forest Service's White Hollow Watershed Investigation in northeastern Tennessee, for example, 1,700 acres had been al- lowed to return to nature. In 1934, the poor, steep soil had a thin woods cover on about two- thirds of the area, while most of the remainder was abandoned to wild grasses. Today, after 15 years of protection from fire and grazing, and with planting and gully control work in the worst spots, peak flood rates have been greatly reduced and the runoff delayed. The water yield, or total runoff, remains un- changed. Jn the summer season, the peak dis- charge from a typical storm has been reduced to about 10 percent of its original rate. Originally, about 90 percent of the runoff passed the gaging station in 10 hours; now more than 24 hours are required, at much lower rates. The Copper Basin of Tennessee had been prac- tically devoid of vegetation for more than 50 years. Land management experiments by the Tennessee Valley Authority reduced surface run- off in one area by about one-half, while erosion, as measured by sediment loads, was reduced to about 5 percent of the initial rates. A study of 1,300 square miles on the Upper French Broad River, also within the jurisdiction of the TVA j points to the following conclusions: 1. Improved land use management and prac- tices will produce beneficial effects in flood height reduction and water conservation. 2. The effect is greater on ordinary floods of common frequency than on great floods of rare occurrence. 3. Improved land use in many watersheds can substantially reduce surface runoff during the growing season of an average rainfall year. 4. Benefits from agricultural cover improve- ments, where these are suited to the land, are achieved more rapidly than from forestation. 5. The effect of forest cover is more uniform throughout the year than that of agricultural crops, but the difference is reduced where a satis- factory winter cover crop growth is provided. 6. Watershed management is of positive value in enhancing the usefulness of engineering works for flood protection. As it is extended, the use- ful life of the works will be increased. Management and Total Runoff The quantity of stream flow is a matter of serious concern in many areas. Stream flow in most basins is equal to the difference between precipitation and evapo-transpiration. A small quantity of water passes out of the basin below the surface of the land, but the amount is gen- erally inconsequential. Thus, land management practices that increase evapo-transpiration will reduce stream flow and vice versa. Increasing infiltration makes for vegetation of greater vigor and density, and hence may in- crease evapo-transpiration. Where this hap- pens, total runoff is diminished, but the reduction is less than the increase in evapo-transpiration because evaporation is reduced by the improved soil cover. Evapo-transpiration may be reduced and total runoff increased by removing vegetation, by sub - stituting shallow-rooted for deep-rooted vegeta- tion, or by cutting timber in such a way as to reduce the amount of snow intercepted by and evaporated from the tree crowns. Where the value of water is high, certain watershed manage- ment programs which increase total runoff may be useful; data on the value of the increased runoff and the risk of increased erosion and sedi- mentation are inadequate for an evaluation of this device. Further study will be needed in the particular areas where problems arise. Erosion Erosion is due primarily to the splash action of rain and the scouring effect of running water on erodible soils. These effects may be limited by the presence of dense low-growing vegetation, 130 |