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<br />7 <br /> <br />C~ <br />~,~ <br />I <br /> <br /> <br /> <br />LI <br />• <br /> <br /> <br /> <br />~~ <br />,~ <br />• <br />removed by turbulence in the overland flow above it, further washing <br />in of fine soil particles to the lower layer of the crust is prevented. <br />Seasonal changes in the soil can cause seasonal changes in infil- <br />tration capacity, Frost action during the winter on a soil studied by <br />Schumm and Lusby (1963) increased the infiltration capacity, while <br />raindrop impact during spring and summer caused compaction of the soil <br />with a corresponding decrease in infiltration capacity. Probably the <br />compaction is similar to the crust formation on bare soil described <br />by McIntyre. <br />The relationship between compaction or higher bulk density, and <br />decreased hydraulic conductivity has been demonstrated experimentally <br />by Jackson (1963), Warkentin (1971), and Diebold (1954). Jackson found <br />that two soils with a high percentage of fine particles (less than 2 <br />microns) showed large decreases in hydraulic conductivity when com- <br />pacted, while the same compaction of a coarser sail showed little <br />decrease in hydraulic conductivity. Clay soils are not easily compacted, <br />according to Warkentin, because swelling which occurs during wetting <br />decreases bulk density. ~Aarkentin theorized that compaction is most <br />easily achieved with soils consisting of different sizes of grains <br />where smaller soil particles can move into voids between the larger <br />particles. Diebold, while studying medium textured soils in the south- <br />west, found soils containing less than 40 percent silt had hydraulic <br />conductivities 1-1/2 times greater than soils containing more than <br />40 percent silt. for compacted layers with bulk densities approaching <br />1.5 g/cm3 the hydraulic conductivities of the lov;er silt content soils <br />were twice as high. Diebold assumed the silt acts as a clogging material <br />