The mean particle size of the soil that was removed from the plot by overland flow 

 was consistently smaller than the mean particle size of the original soil. Based on 

 the MWD for all samples, the mean particle size of the original soil was 0.945 mm., 

 that of the runoff samples was 0.620 mm., a 38-percent reduction. 



SOIL SPLASH 



Splash erosion is the initial phase of the water erosion process. For the most 

 part, raindrops provide the detaching force prerequisite for transporting soil particles 

 by the sheet of surface detention water. Even on level areas where net erosion is small 

 (especially on bare soils) as much as 60 tons of soil per acre per hour may be detached 

 and splashed into the air. As slope steepness increases, discharge and velocity of 

 surface water also increases and, correlatively , the rate of soil removal. 



Soil splash is not difficult to measure in a laboratory study, but it is difficult 

 to interpret the results of such measurements. In plot studies, soil that is splashed 

 off the plot is "lost"; it is not subject to resplash or to further movement by the 

 sheet of surface detention water. In the field, soil splash is not "lost," but remains 

 available for further movement by splash or transport by the sheet of surface detention 

 water. No satisfactory method has been developed to handle this problem. 



On any inclined soil mass subject to raindrop impact, only a portion of the total 

 splash goes in a downhill direction; the balance is splashed laterally or uphill. 

 Neglecting wind effects, the proportion of downhill splash to total splash is largely a 

 function of the slope angle. Downhill raindrop splash and slope angle are directly 

 related, up to some critical slope angle. At that angle, virtually all splashed soil 

 goes downhill. 



The downhill component of raindrop splash erosion was not measured directly in 

 this study. The four sidepans on each side of the soil plot contained both the downhill 

 and uphill components of raindrop splash. However, the pan at the bottom of the plot 

 did contain only that soil material that was splashed in a downhill direction. But, 

 the amount of soil splashed into the bottom pan was less than the total downhill splash 

 because some downhill splash was caught in the sidepans. 



On the assumption that the same factors affect both bottom pan catch and total 

 downhill raindrop splash, the amount of soil material splashed into the bottom pan was 

 analyzed by regression methods. The weight of soil material splashed into the bottom 

 pan by raindrop action was used as the dependent variable. Rainfall intensity, slope 

 steepness, soil bulk density, and the percent of soil particles and water-stable aggre- 

 gates between 61 and 2,000 microns (sand-size soil material) were used as independent 

 variables. The regression model, illustrated in figures 3A and 3B, explains nearly 97 

 percent of the variance associated with splashed soil material that was caught in the 

 bottom pan (r2=0,966). 



Figures 3A and 3B show the effect of the strong interaction between slope steepness 

 and rainfall intensity on raindrop splash erosion. Note that raindrop-splash erosion 

 is quite small on the most shallow slope, even at the greatest rainfall intensity. This 

 does not mean that less soil was splashed on the shallow slope, but that less soil was 

 splashed and transported downhill on the shallow slope. Maximum amounts of soil are 

 eroded by raindrop splash where slope steepness and rainfall intensity are greatest. 

 Pretreatment soil bulk density and the amount of soil-splash erosion are directly re- 

 lated. However, the effect of bulk density on soil splash is more pronounced as slope 

 steepness increases; i.e., at a rainfall intensity of 3.1 inches per hour, the 

 difference in the weights of soil splash between bulk densities of 0.95 and 1.37 g. per 

 cm. ^ is only 10 g. at 3 percent slope, but 289 g. at 32 percent slope (fig. 3A) . 

 Each of the three regression surfaces in figures 3A and 3B represent a different value 

 of pretreatment soil bulk density, namely, 0.95, 1.16, or 1.37 g. per cm.^ The effect of 



6 



