Changes of soil microrelief and its effect on soil erosion under different rainfall patterns in a laboratory experiment

Soil microrelief has been found to influence processes of water infiltration, runoff and erosion. However, its mathematical description remains poorly understood. The primary objective of this study was to explore changes in soil microrelief and its effect on surface runoff and sediment yield. The associated experiments were based on multifractal analysis (MFA) under a simulated rainfall series in a laboratory located in the purple hilly area of Sichuan, China. A total of 12 rainfall simulation experiments were conducted in two 1 m by 2 m boxes under 2 rainfall series, including an increased rainfall series (60 mm·h− 1, 90 mm·h− 1 and 120 mm·h− 1) and a decreased rainfall series (120 mm·h− 1, 90 mm·h− 1 and 60 mm·h− 1) on a 15° slope. The results indicated that decentralized depressions were formed in the middle and lower slopes after the first rainfall event, while rills were gradually formed under the increased rainfall series. Under the decreased rainfall series, obvious rill erosion appeared following the first rainfall event. Under increasing rainfall intensity, the change of soil roughness index (R) is in the range of 75.99-79.29, and the roughness index (R) of the middle and lower slopes exhibited a gradually increasing trend, and the R of the lower slopes was noticeably higher than the middle slopes. Under the decreased rainfall series, the change of R was in the range of 77.65-79.80, the R of the middle and down slopes did not exhibit an obvious change during rainfall period. The soil microrelief indicated multifractal characteristics under the different rainfall series. Under an increased rainfall series, the fractal dimension span (ΔD) and singular index span (Δα) showed a gradually increasing trend, while an opposite trend was exhibited under the decreased rainfall series. The R and singular index span (Δα) were positively correlated with ΔD. Compared with R, Δα reflected the changes in soil microrelief for the entire spatial structure. The surface runoff volume of the increased rainfall series was only 5.82% higher than that of the decreased rainfall series, and the total sediment yield in the former was 35.83% higher than that of the latter. The use of multifractal parameters of soil microrelief to forecast sediment yield was deemed to be feasible.