| OCR Text |
Show Infiltration Evapo-transpiration The rate and amount of infiltration of water are governed primarily by the size and distri- bution of pores in the soil, the depth of the per- meable horizons, and the moisture content of the soil. Changes in permeability resulting from rain impact and increasing moisture content nor- mally lower the rate of infiltration during a storm. The total amount of infiltration depends on the length of time water stands on the sur- face, as well as the varying rates at which it enters during the storm. Dense vegetation promotes infiltration by pro- tecting the surface against sealing, as discussed above. It adds organic matter to the soil, thus forming larger pores. It makes the soil per- meable to a greater depth by more thorough pen- etration of the soil mass, permitting soluble or- ganic materials to move downward. And the duration of runoff is extended, permitting greater infiltration. A series of experiments by the Soil Conserva- tion Service on different soils showed that the differences in the rate of infiltration after a 5-hour rain ranged from one-twelfth to one-half of an inch an hour, depending on the type of soil cover. Differences in the amount of infiltration ranged from one-third of an inch to 4 inches. The data are shown below. TABLE 1.-Amount and rate of infiltration son Depth of surface soil Rate of infiltration on- Amount of infiltration on- Blue-grass pasture Corn land Difference due to land use Blue-grass pasture Corn land Difference due to land use Muscatine silt loam ___ Inches 20 13 10 7 6 Inches per hour 0.61 .77 .34 .29 .16 Inches per hour 0.11 .14 .12 .18 .08 Inches per hour 0.50 .63 .22 .11 .08 Inches 5.38 5.03 3.48 2.77 1.63 Inches 1.34 1.51 1.21 2.17 1.28 Inches 4.04 3.52 2.27 .60 .35 Tama Berwick__.. Clinton Viola Evapo-transpiration is the sum of water lost from a land area as a result of transpiration by vegetation and evaporation from soil, water, and other surfaces. Solar radiation supplies the necessary energy, but it is the differences in the moisture content of air, soil, and vegetation which show the greatest correlation with the rate and amount of evapo-transpiration. The capacity of the atmosphere to induce evaporation is influenced by temperature, humid- ity, and wind movement. Evaporation oppor- tunity depends upon the amount of water or snow upon the ground surface, the wetness of the soil, and the amount and type of vegetation. Watershed management has little effect on the condition of the atmosphere, but it can affect opportunity for evaporation by changing the amount of infiltration, the amount of water re- tained by the soil, the area, density and type of vegetation, and the extent to which plant roots can reach the water table. Transpiration can be modified by changing the vegetation covering the soil. Evaporation from the soil is reduced by protecting the surface from sun and wind. The following figures indi- cate how the type of cover used on an experi- mental plot of land affected runoff, percolation, and evapo-transpiration. TABLE 2.-Runoff, percolation, and evapo-transpiration under various types of cover Treatment Surface runofl Percolation into the soil Evapo-transpiration Fallow _______.........______ Inches 8.5 7.0 7.5 4.6 7.6 1.4 Inches 4.5 7.3 1.6 3.8 1.4 2.0 Inches 16.7 15.5 21.6 21.3 20.7 26.3 Fallow plus 16 tons per acre of Tnaniirft Corn Corn plus 16 tons per acre of manure-Corn plus 16 tons per acre of sweet clover ______ .______- Bluegrass _ -__ __________ Source: Soil Conservation Service. Source: The Soil Conservation Service secured these data by subjecting replicated plots to artificial rainfall. The plots were located upon actual farms in Peoria C ounty, 111. In most cases the grass was being grazed by livestock. The artificial rainfall was applied at a rate of 1.8 inches per hour. Denuding the ground substantially reduces evapo-transpiration losses. Further evidence is 128 |
