Impact of rainfall on the distribution of soil aggregate fractions caused by shallow runoff

Abstract

Exploring the fragmentation and loss of soil aggregates is of great significance for revealing the mechanism of water erosion. To determine the effect of raindrop size on the distribution of soil aggregate fractions induced by shallow runoff, a variable-needle raindrop-generating device and a slope runoff scouring device (length 90 cm × width 10 cm × height 10 cm) were employed in the study. Five raindrop sizes (2.67 to 4.05 mm) and three runoff depths (3.82, 4.86, and 7.98 mm) were selected to conduct the runoff alone (F) test and the rainfall + runoff (RF) test. The results showed that for the RF test under the raindrop size >3.05 mm, the soil loss rate was the highest at runoff depth 4.86 mm and was 104.30 to 421.52, 69.86 to 169.81, and 26.80 to 89.28 times higher than that of the F test under the three runoff depths, respectively. The rainfall disturbance coefficient (the ratio of soil loss caused by the RF test to that caused by the F test) was closely related to the raindrop size and runoff depth and had a significant power function relationship with both. Under the F test condition, the soil loss rate increased with the decreasing aggregate fraction distribution, and the loss rate of each aggregate fraction increased with increasing runoff depth. Similarly, for each RF test, the soil loss rate also increased with the decrease in the aggregate fraction distribution. The maximum soil loss rate was observed in the <0.053 mm aggregate fraction and was 5.32 to 191.89, 2.75 to 79.33, and 1.51 to 41.33 times higher than that in the F test for the three runoff depths, respectively. Moreover, at the raindrop sizes of 2.67 and 3.05 mm, the rainfall mainly promoted the transportation of the >1 mm aggregate fraction by shallow runoff. However, rainfall mainly enhanced the transportation of 1 to 0.053 mm aggregate fractions as the raindrop size increased to greater than 3.05 mm.