If the independent soil variables are dropped from the regression model illustrated 

 in figures 5A and 5B,the resulting is 0.75. Therefore, while the slope steepness- 

 rainfall intensity interaction accounts for 75 percent of the variance associated with 

 calculated total downslope soil splash, the two soil variables account for an additional 

 17 percent of the variance. This result is very similar to that of the bottom pan 

 model (figs. 3A and 3B) . 



Five soil variables--percent of soil particles and aggregates greater than 2 mm., 

 percent of soil particles and aggregates between j6L and 2,000 microns, silt plus clay 

 divided by the mean weight diameter, silt plus clay, and soil bulk density--were used 

 in multiple regression analysis with the calculated downslope splash as the dependent 

 variable. The resulting regression equation gave an of only 0.31. The failure of 

 soil variables (in the absence of nonsoil variables) to explain an acceptable proportion 

 of the variance associated with soil erosion has been consistent throughout this study. 

 In order to explain as much as one-third of the variance associated with any type of 

 soil erosion on the plots, the slope steepness and rainfall intensity factors must be 

 accounted for. Soil physical factors increase the strength of the regression, but are 

 unable to produce an acceptable regression equation by themselves. On the other hand, 

 the interaction between slope steepness and rainfall intensity explained at least 75 

 percent of the variance associated with soil erosion. 



The distance that soil was splashed off the plot was assessed by the average 

 weighted distance computed for the side splash trays only: 



4 



Average weighted distance = Z Xi Zi 



i=l 



~4 



Z Zi 

 i = l 



where : 



Xi = distance from the edge of the plot to the center of a splash tray 

 in centimeters 



Zi = weight of splashed soil in a given splash tray in grams. 



The average weighted distance of splashed soil material did not vary greatly 

 between the three soil types. It was 25.18 cm., 25.31 cm., and 26.42 cm. for the low- 

 and high-elevation granitics and the Wasatch clay, respectively. At a rainfall intensity 

 of 3 inches an hour the average weighted distance was 24.82 cm.; at 7 inches an hour, 

 26.45 cm. The distance increased more noticeably with slope. It was 22.77 cm., 

 25.15 cm., and 28.98 cm. for 2^-, 18-, and 32-percent slopes, respectively. These 

 splash distances are for soil material splashed only a single time. Splash distance 

 was not measured directly downslope; so the downslope vector may not be equal to the 

 distances indicated above. However, on steep slopes with multiple splashes, surface 

 soil material could be moved considerable distances downslope irrespective of transport 

 by overland flow. 



The size (diameter) of splashed soil material varied inversely with the distance 

 that it was splashed (table 2). As was expected, most of the splashed soil material was 

 in the sand and silt fractions. However, both gravel and clay fractions were found in 

 the splashed soil. Soil material in the clay sizes ordinarily was splashed as aggre- 

 gates (Wasatch clay) or as clay particles adhering to sand and gravel (Idaho granitics). 



10 



