parameter should be weighted more heavily by large-size fractions than by small-size 

 fractions. This was accomplished by using the mean weight-diameter (Van Bavel 1949). 



n _ 



MWD = Z Xi Wi 

 i=l 



where Xi = mean diameter of each size fraction in millimeters. 



Wi = proportion of the total sample weight in the corresponding 

 size fraction. 



In addition to the mean weight-diameter, the percent of particles and water-stable 

 aggregates greater than 2 mm. was measured. 



Results 



SOIL EROSION BY OVERLAND FLOW 



Cursory examination of table 1 reveals that the texture of the Wasatch clay soil 

 is very different from the two granitic soils. However, on the basis of preliminary 

 testing the effects of the soil variables appeared to be consistent across soil types. 

 Therefore, the results from all test runs on the three soils were grouped together for 

 regression analysis. A total of 18 test runs (three soils on three degrees of slope 

 steepness at two rainfall intensities) were analyzed. 



The weight of soil material washed off the plot was used as the dependent variable 

 in regression analysis. Several soil and nonsoil factors were used as independent 

 variables. The interaction model illustrated in figure 2 explains over 96 percent of 



Figure 2. — The relationship between soil 

 erosion by overland flow, rainfall in- 

 tensity ^ slope steepness 3 and the propor- 

 tion, of soil particles and aggregates 

 greater than 2 mm. Each of the three 

 regression surfaces represents a differ- 

 ent %■ value. The numbers at the comers 

 of each surface represent the amount of 

 soil erosion in grams (r'^=0 .96 ) . 



4 



