Samples of the surface 6 inches of mineral soil were taken at 168 places in northern California and analyzed for the physical characteristics which index erodibility of the soil. The samples were selected in the major soil-geologic types of California, under standard conditions of slope (west, 20 per cent), at 1000-foot intervals of elevation (1000 to 4000 feet), by vegetation types (forest, brush, and grass), and in three separate zones (North Coast, Central Coast, and Sierra). A multiple regression analysis related the ‘surface-aggregation’ and ‘dispersion’ ratios, as the indexes of erodibility, to geologic type, vegetation type, zone, and elevation and to their interactions. The surface-aggregation ratio was somewhat more significantly related to soil erodibility than was the dispersion ratio. Soil developed from acid igneous rock was about 2 ½ times as erodible as soil developed on basalt. Erodibility was highest for soils under brush, next under trees, and least under grass. No clear-cut relation of erodibility to elevation was found. The interaction of zone and geologic rock type showed significant variation in erodibility. The prediction equation explains 52 per cent of the variability in erodibility in soils. By combining predicted erodibility from this equation with chemical base status, for a sample of 20 of the soils, the explained variance was improved. Application of these relationships in studies of sedimentation from watersheds is illustrated.
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