Discussion 



Soil texture and other soil physical properties indicate that the Wasatch clay 

 soil used in this work is greatly different from the two granitic soils. As a 

 consequence, the soil variables used in these regression analyses covered a wide range 

 of values (table 3) . The effects of the soil variables were consistent across soil 

 types. During the analyses for the effect of soil variables, two soil-size fractions 

 were used: (1) greater than 2 mm. and (2) between 61 and 2,000 microns. From the 

 analyses, we were unable to distinguish between the well-aggregated, fine clay soil 

 and the poorly aggregated, coarse-grained granitic soils. Water-stable soil aggregates 

 of a given size class behaved in the same manner as nonaggregated soil particles of the 

 same size. 



Stripped of vegetation, all three of these soils exhibit little resistance to 

 erosive forces. High-intensity rainstorms over areas of sparse vegetal cover can be 

 expected to produce tremendous quantities of sediment. 



The regression models of soil erosion presented here are not intended for field 

 prediction purposes, but to characterize relationships between the variables observed 

 in the data. Rainfall intensity and slope steepness interact strongly to influence 

 soil erosion by overland flow. This relation, however, is modified by the proportion 

 of soil particles and water-stable aggregates greater than 2 mm. in diameter. This 

 latter factor is really a measure of soil coarseness. 



Raindrop-splash erosion is also affected to a large degree by an interaction 

 between rainfall intensity and slope steepness. The effect of soil particles and 

 aggregates between 61 and 2,000 microns is additive and directly related to the amounts 

 of soil splash. Apparently, sand-size material is especially susceptible to splash 

 erosion. During splash erosion, an interaction also takes place between slope steep- 

 ness and soil bulk density that is not completely understood. In fact, the statistical 

 model for soil splash (figs. 5A and SB) specifies this relationship imperfectly. On 

 the steep slope at low bulk density the model indicates a small decrease in splash 

 erosion as compared to the medium slope. While splash erosion probably does not in- 

 crease much between slopes of 18 and 32 percent with soil bulk density less than 1.00, 

 it is not expected to decrease. Ignoring the effect of bulk density, EQ(3) predicts 

 an 11 percent increase in downslope splash for 32-percent slope compared to 18- percent 

 slope. However, an increase in soil bulk density increases the amount of soil splash 

 erosion. On most forest and range soils bulk density exhibits a seasonal increase 

 from the spring to the fall (Laycock and Conrad 1967). Soil splash erosion can also 

 be expected to exhibit an increase from spring to fall. 



The strength of the interaction between rainfall intensity and slope steepness is 

 at least a full order of magnitude greater than that of any soil variable, and at least 

 four times as great as any combination of soil variables made in this study. Therefore, 

 it appears that before real expertise can be developed in soil erosion problems due to 

 storm rainfall, information must be assembled on the rainfall patterns and characteris- 

 tics as well as on topographic effects. Conversely, much of the work in the soil erosion 

 literature describing the effect of soil factors on soil erosion has been concerned with 

 explaining (at best) a small proportion of the variation associated with soil erosion. 

 Comparative erodibilities of soils have been made on the basis of soil factors; these 

 comparisons implicitly assume that rainfall characteristics and topographic effects are 

 equal. Under field conditions this assumption is very questionable. IVhile vegetation 

 was omitted from this study, any realistic evaluation of natural soil erosion must give 

 full recognition to the potentially overwhelming effects of vegetation. 



13 



