OCR Text |
Show Watershed management can increase or de- crease the amount of water yield by creating plant cover and soil conditions conducive to high or low evaporation and transpiration losses. It can affect the yield of ground water by stepping up or holding down the amount of surface runoff. The extent of increase or decrease in yield may be accompanied by increases or decreases in crop productivity, as well as in timber yield. In humid areas, these changes may be even more important than water yield as such. In arid re- gions, the water yield may be the most important factor to be considered. No data are available as to the specific effects of watershed management on a national scale. Experiments have been performed, under con- trolled conditions and in specific areas, to deter- mine the effect of management on raindrop splash, infiltration, evapo-transpiration, retention of infiltrated water, ground water recharge, flood runoff, total runoff, erosion, and the sediment load of streams. The results of these experiments indicate the effectiveness, and the limitations, of watershed management practices. Raindrops Bombard the Soil Each falling raindrop that strikes bare ground "explodes" and splashes soil particles into the air at its point of impact. These explosive actions appear somewhat like miniature bombs; during a violent rainstorm they may blast into the air more than 100 tons of soil per acre. On slop- ing lands, this splashed soil migrates downhill, a type of erosion coming to be known as splash erosion. On flat lands, where actual soil loss is light, the soil particles are often splashed loose from the surface and churned into suspension in the run- off. Tests have shown that on some soils the surface water, even though standing on level ground, may contain more than 20 percent of soil stirred up by the raindrops during short periods of violent rainfall. When water containing such a charge begins to flow, it is certain to transport some soil, even over almost level lands. Splash erosion is most effectively checked through shielding the soil with growing crops or mulches. Where crops only are employed, with- out mulch or litter, short grasses and vines are more effective than tall plants whose stems and leaves growing high above the surface drip in- tercepted rainfall on the soil. The drip of intercepted rainfall can damage the soil substantially. The effect of shielding material placed at different heights from the ground has been tested by the Forest Service and indicates great differences in the amount of both erosion and infiltration. In the first test, use of a mulch to deliver the rain to the soil without impact resulted in no splash erosion, and infiltration of 2 inches of water in a 20-minute period. In the second test, the shielding material was placed 2 inches above the soil. There was only a trace of soil splash, but even this minor disturbance reduced the 20- minute infiltration to 1.6 inches. In the third test, the shielding material was placed 7 inches above the soil. Soil splash was about 3 tons per acre, and infiltration was only 0.66 inch. For the final test, the shielding ma- terial was lifted to 7 feet. Soil splash of 70 tons per acre resulted, and infiltration totaled only 0.23 inch. The reduction in infiltration resulted from changes in both soil and water. Impact from the raindrops broke down surface clods and crumbs and puddled them into mud, which is less permeable. In addition to this puddling, the infiltration of surface water, made muddy by the splashes, tended to plug the soil filter in much the same way that muddy water plugs a sand filter. Over the years the plugging and sealing effects of rainstorms extend deeply into the soil. On cultivated lands this deeper sealing helps to form a layer of impermeable material at plow depth. Soils that are exposed become puddled and hard, and their structure tends to deteriorate through- out. Splash erosion has been found to be very sensitive to the differences in protection afforded by different cover. 127 |