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<br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br />I <br />I <br /> <br />I <br />I <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />001413 <br /> <br /> <br />a single storm event produced approximately one-fourth of the annual <br />values presented in this table varying from a low of 23 percent to a <br />maximum of 33 percent. <br /> <br />Soil-Erodibility Factor (K) <br /> <br />Soils developed under semi-arid environments such as on Walnut Gulch <br />are undoubtedly quite different from the soils listed in Agricultural <br />Handbook 282. The soil properties listed as influencing erodibility <br />by water are: "(1) those that affect the infiltration rate, permeability, <br />and total water capacity, and (2) those that resist the dispersion, <br />splashing, abrasion, and transporting forces of the rainfall and runoff." <br />Both elements are quite important in the soils encountered in this test <br />evaluation where the surface contains an erosion pavement covering <br />30 percent of the area unprotected by vegetation. <br /> <br />The soils of the watersheds used in this test evaluation are Rillito- <br />Laveen gravelly loams. These soils are found on gently and moderately <br />sloping ridges formed by the deep dissection of old alluvial fans and <br />valley plains. The Rillito series (forming about 75 percent of the <br />mapping unit) consists of deep, well-drained, medium and moderately <br />coarse textured gravelly soils formed in calcareous old alluvium. <br />The surface layer, dominated by an erosion pavement, is light brownish <br />gray gravelly loam 4 to 7 inches thick. The subsoil is light brownish <br />gray or pinkish gray gravelly loam to a depth of 40 inches or more. <br />Laveen soil is a well drained sandy to gravelly loam which is found on <br />level terraces and alluvial fans above flood plains. The texture of <br />the A horizon varies with 1 to 15 percent fine gr~vel on the surface. <br />The numbers of soft lime masses and modules in the Cca horizons range <br />from few to many with 20 to 35 percent carbonates. <br /> <br />The erosion pavement present on the soils of the watersheds used for <br />this evaluation greatly reduces the splash erosion and provides additional <br />roughness to reduce overland flow. However, due to very high rainfall <br />rates associated with the short duration storms, high runoff rates are <br />still encountered. <br /> <br />Slope Length (L) and Gradient (S) <br /> <br />In field practice, slope length and gradient are generally considered <br />as one term. Agricultural Handbook 282 presents a graph for determination <br />of the LS value to use in the prediction equation. The narrative discussion <br />also states that when convex and concave slopes are involved, the erosion <br />is not that of the average slope. Rather, when the lower end of the slope <br />is steeper than the upper end, the gradient of the steeper segment should <br />be used with the overall slope length to enter the slope-effect chart. <br />This procedure was used for the two watersheds used in the analysis. <br /> <br />11 <br />