Changes in soil surface microrelief of purple soil under different slope gradients and its effects on soil erosion

Abstract: Soil surface microrelief is a key parameter which influences many processes of soil erosion and runoff. In order to clarify the effect of soil surface roughness on soil erosion in purple hilly areas of Sichuan, from the perspective of the surface roughness combined with the multifractal theory and method, the indoor artificial rainfall experiments were conducted to study the change of soil surface microrelief under different slope gradients. On that basis, the relationship between soil surface microrelief and soil erosion was explored. The results showed that: 1) Under the rainfall intensity of 1.5 mm/min and rainfall duration of 40 min, the variations of relative elevation were -11.66, -3.52 and -5.61 mm respectively for the slope gradients of 10°, 15° and 20°. After rainfall, there was not remarkable rill under the slope gradients of 10° and 15°, however, the initial low-depression parts of the soil surface formed the rill by runoff scouring under the slope gradient of 20°. After rainfall, soil surface roughness decreased in different slope positions (i.e. upslope, middle slope and downhill) under the slope gradients of 10° and 15°. However, soil surface roughness increased at the bottom of the slope when the slope gradient was 20°. For the whole slope surface, soil surface roughness decreased under different slope gradients after rainfall, and the proper order of the decreasing range was 15°>10°>20°. 2) The singular index span of soil surface microrelief increased by 2.81% and 12.90% respectively under the slope gradients of 10° and 15° in the course of the rainfall, which indicated that the spatial differentiation of soil surface microrelief increased. The singular index span of soil surface microrelief decreased by 3.57% under the slope gradient of 20° in the course of the rainfall, indicating that the spatial differentiation of soil surface microrelief decreased. Soil surface microrelief showed the certain multifractal characteristics. Under the different slope gradients, multifractal singular spectrum function showed a "left hooked" profile. The fractal dimension span, the singular index span and the difference in the numbers of minimum and maximum subsets of probability were sensitive indices to describe the space variation of soil surface microreilief. After rainfall, the multifractal parameters of soil surface microrelief increased, and the difference in space variation of the soil surface microrelief enhanced under the slope gradients of 10° and 15°, for under the two slope gradients, the relative elevation of soil surface was mainly increasing, and soil crust and deposition played a prominent role. Under the slope gradients of 20°, the relative elevation of soil surface was mainly decreasing, and rill was developed. The spatial heterogeneity of soil surface had been strengthened and soil surface became smooth and fruity under the slope gradients of 10°and 15°. However, the changes on the slope gradients of 20° were contrary to the slope gradients of 10°and 15°. 3) The runoff-yielding time under the slope gradient of 20° was the earliest, followed by 10°, and 15° was the latest. The surface runoff amount at first decreased and then increased with the increasing of the slope gradient, and the smaller the amplitude of soil surface roughness, the higher surface runoff volume. However, the sediment yield increased significantly with the increasing of the slope gradient. In the late rainfall, sediment yield rate under the slope gradients of 10° was relatively low but surface runoff rate was higher than those of the other slope gradients. Under different slope gradients, the sediment yield rate increased rapidly and then decreased, and reached a steady state finally. From this, the effect of soil surface microrelief on surface runoff volume was relatively strong, and slope gradient was the main factor of affecting sediment yield. This study provides a reference for revealing the essential characteristics and mechanism of soil surface microrelief in the process of water erosion.