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Show found in experiments comparing losses on bare ground to those under aspen. The loss was 19.9 inches under aspen, 15.7 inches under a shallow- rooted cover, and only 11.3 inches on bare groynd. Where it is desirable to increase stream flow, baring the ground may be a useful device. But it so greatly accelerates runoff and erosion as to make it a dangerous expedient. Reducing vegetal cover by patch-thinning of forest, or sub- stituting shallow-rooted plants for those with deeper penetration, may provide a useful increase in stream flow without unduly increasing erosion. Water Storage in the Soil The capacity of soil to store water is critical in the amount of runoff which occurs during a storm or thaw. When the amount of water in- filtrated exceeds the storage capacity, the excess generally passes through the soil mantle, reaches a zone of saturation, and eventually becomes stream flow. In some areas its appearance in streams is long delayed; in others it is so rapid as to be a part of flood runoff. The latter is likely to happen where shallow soils overlie rela- tively impervious layers. Increasing storage capacity can be accom- plished either by increasing the rate of evapo- transpiration between storms, so as to leave room for storm infiltration, or by increasing the mass of the soil mantle. The rate of evapo-transpiration may be in- creased by such practices as establishing vegeta- tion on bare areas and replacing shallow-rooted cover with plants which remove moisture to a greater depth. Increasing the mass of the soil mantle can be accomplished by measures which build up a layer of humus over the mineral soil, as in forest protection, or which make the soil more permeable, as with the planting of deep- rooted vegetation. These practices have some value in reducing flood runoff and lowering peak flows. But, as has already been stated, their primary value lies in their control of erosion. Ground Water Recharge Ground water may be derived from water which percolates the overlying soil, or from water which seeps into inclined structures, often some distance away. Where it is derived from infil- tration immediately above, the infiltration may be increased by improving the cover of vegeta- tion. Since improving the vegetal cover may also increase evapo-transpiration, ground water recharge depends upon choosing vegetation which provides relatively little evapo-transpira- tion. The following figures from the Forest Service show the difference in runoff, evapo-transpira- tion, infiltration, and percolation for a woodland chaparral cover and bare soil. Woodland chaparral Bare (inches) (inches) Rainfall......................... 41.5 41.5 Surface runoff................... 0 11.3 Infiltration...................... 39.3 30.2 Evapo-transpiration............... 16. 3 12.6 Percolation...................... 25.2 17.1 Increase in water stored in soil..... 0 0. 5 Runoff was high on the bare plot, while infil- tration, evapo-transpiration, and percolation were higher on the covered plot. While it is possible, in certain areas, to raise the ground water level by land management, this effect is generally incidental to runoff and erosion control. Flood Runoff For large watersheds drained by major rivers, effective flood control will require engineering structures such as dams, levees, and channel im- provements. Even in small watersheds, the an- nual flood loss may be large enough to justify the construction of major works. Good land man- agement is, of course, a desirable partner to large engineering works, to reduce sediment loads and lengthen the useful life of reservoirs. But there are many watersheds throughout the country where the most practicable means of flood relief is by watershed management. These are largely 129 |