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<br />I <br />,. <br /> <br />- 4 - <br /> <br />N <br />(lC <br />-.) <br />~ <br /> <br />Climate and livin~ organisms acting on parent material, as conditioned by <br />relief or topography over a period of time, are the essential factors for <br />soil development. Anyone of these factors may overshadow or depress <br />another in a given area and cause a difference in soil formation. For in- <br />stance, climate determines what type of vegetation and animal eopulatkon <br />will be present in an area, and chis will have a definite influence or deter- <br />mine the tyee of soil that evolv~s. As an example, soils developing under <br />a forest canopy are much different from soils developing in a grassland <br />conununity. <br /> <br />The raw, shaley type areas (marine shales) of che Pacific Southwest have <br />very little, if any, soil deve'lopment. Colluvial-alluvial fan type areas <br />are usually presenC at the lower extremities of the steeper sloping shale <br />areas. Infiltration and percolation are usually minimal on chese areas due <br />to the fine textured nature of Che soil material. This mate.rial is easily <br />dispersed and probably has a high shrink-swell capacity. Vegetation is <br />generally sparse, and consists of a salt desert shrub. type. <br /> <br />There are areas Chat contain soils with definite profile development and <br />also stony' soils that contain few fines which constitutes an improved <br />physical condition for infiltration and plant growth over the fine textured <br />shaley areas. These areas usually occur at higher and more mois.t elevations <br />where bare. hat:li. crystalline rocks provide che soil parent material;; Vege- <br />tation and other ground cover, under these circumstances,. provide ~dequate <br />protection against the erosive forces and thus low sediment yield results. <br /> <br />In arid and semi-arid areas an accumulation of rock fragments (desert pave- <br />ment) or calcareous material (caliche) is not uncommon. These layers can <br />offer substantial resistance Co erosion processes. <br /> <br />The two extreme conditions of sediment yield areas have been described. <br />Intermediate situaCions would contain some features of the two excremes. <br />One such situation might be an area of predominately good soil development <br />thaC contains small areas of badlands. This combination would .possibly <br />result in an intermediate classification. <br /> <br />Climate and Runoff <br /> <br />Climatic factors are paramount in soil and vegetal development and determine <br />the quantity and discharge rate of runoff. The same factors constitute the <br />forces that cause erosion and resulcant sediment yield. Likewkse, temperature, <br />precipitation, and particularly the distribution of precipitation during the <br />growing season, affect the quantity and quality of the ground cover as well <br />as soil development. The Quantity and intensity of precipitation determine <br />the .amount 'and dischar~e rates of runoff and resultant detachment of soil and <br />the transport media for sediment yield. The intensity of prevailing and <br />seasonal wknds affects precipitation pattern, snow accumulation and evapor- <br />ation rate. <br />