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001509 necessary for periods of less than 30 minutes duration where small runoff areas are involved and adju stments are needed for convers ion of point- rainfall information to areal amounts and intens ities and where storms are known to be extremely limited in area and highly variable within the watershed. The adjustments for short-time intervals are shown on table 2. Soil i.~:',:~/':.:'" ,;. r.?~~i~:r-:~: ;":U~~;<:~ '?'.~:;'?::-:/ ~~l;~~:})t '::. ,~:.' ',.,:. <':~$:..;.~-I .:." ':':1 " Soil effects on the amount of runoff expected from an area can be quite striking. Differences in the amount of infiltration between two soil types under identical conditions of vegetation cover may be as great as a factor of 10 (ASCE, 1949, p. 48). In general, sandy soils and soils of open structure, such as some friable silt loams, may have infiltration capacities from O. 5 to 1 inch or more per hour; soils in the interm'ediate range of infiltrations typical of many loam soils in agricultural regions, have infiltration capacities of O. 1 to 0.5 inch per hour; and soils of the low infiltration group, characteristic of heavy loams and clay soils, may have infiltration rates as low as 0.01 to O. 10 inch per hour. More speCific information may be available from local soil infiltration and permeability measurements. When infiltration capacity is determined from local infil- trometer measurements rather than hydrograph analyses, the quantities must be used with caution since, in general, infiltrometers give relative rather than exact quantitative measures of infiltration. Diebold (1954) reports relationship of permeability ,of 215 soil series in'the Southwest to the silt content of those soils and'gives a basis for estimating permeability of a ser ies. Relative infiltration and perme- ability ar e given in the des cription of s oil series. More than 1,2,00 soil series have been described in the Southwest (U. S. SCS, 1961), and a' tabulation of 139 soil series, of this group in Arizona by the Soil ,Conserva- tion Service,' showed 50 percent having slow to' very slow infiltration "without the benefit of vegetative cover. " Vegetation Vegetative effects on infiltration and runoff are well known. Some generalizations were summarized by the Committee on Hydrology of the ASCE (1949). As an extreme example, forest cover in good condition has an infiltration capacity ranging from 3.0 to 7.5 inches per hour, and a high dens ity grass with greater than 80 percent cover has comparabLe infiltration. Stock ponds are seldom found in areas of this type. In con- trast, a poorgrass cover of less than 30 percent density which has been overgrazed or otherwise abused, may be expected to have an infiltration capacity in the neighborhood of 1. 0 inch per hour or less. More specific 18