Figure 1. — Rainfall simulator 

 in operation. The plot is 

 inclined to approximately 32 

 percent slope. Each of the 

 interlocking splash trays on 

 the floor is 9 inches wide. 

 The nozzle rack is 12 feet 

 above the floor. 



12 feet above the concrete floor and the hole in the end disc of each nozzle was reduced 

 from 0.125 to 0.104 inch (fig. 1). These modifications greatly increased the fall 

 distance of the drops and reduced the average drop size. The range of drop sizes for 

 both the 3- and 7-inch per hour rainfall intensities varied from less than 0.5 mm. to 

 slightly more than 5.0 mm. At 7 inches per hour intensity, the average drop size was 

 1.87 mm. and the D50 drop size,-"- 3.55 mm. For the 3-inch-per-hour intensity, the average 

 drop size was 1.91 mm. and the D50 size, 3.09 mm. 



All surface runoff of both water and soil particles was collected and weighed 

 continuously throughout each run. One-pint runoff samples were collected periodically 

 during each run to determine the concentration of soil material in the runoff. 



Soil splash was collected all around the perimeter of the plot. Four concentric 

 interlocking trays, each of which was 9 inches wide, were placed around each side and 

 the upper end of the plot. Soil splash at the foot of the plot was collected in one 

 large, unsegmented pan (fig. 1). 



After each 30-minute run, a new plot was prepared. In all, 18 runs were made using 

 three soil types, each on three different slopes at two rainfall intensities. 



The size distribution for both particles and water-stable aggregates was determined 

 for all soil samples by using a wash bottle to gently wash the soil sample through a 

 set of nested 3-inch sieves. 



We desired to use a single parameter to express the aggregate- and particle-size 

 distributions. Since erosion is basically a work process, the size distribution 



^The D50 drop size is that size at which 50 percent of the water comes in drop 

 sizes larger than the D50 and 50 percent in drop sizes smaller than the D50. 



3 



