Effect of microrelief on water erosion and their changes during rainfall

Authors: ZHAO, LONGSHAN - Northwest Agricultural & Forestry University; Huang, Chi Hua; WU, FAQI - Northwest Agricultural & Forestry University

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/17/2015
Publication Date: 10/14/2015
Publication URL: https://handle.nal.usda.gov/10113/6563707
Citation: Zhao, L., Huang, C., Wu, F. 2015. Effect of microrelief on water erosion and their changes during rainfall. Earth Surface Processes and Landforms. 41:579-586.

Interpretive Summary: Soil roughness affects many processes occurring on the surface, such as water infiltration, runoff, sediment detachment and transport. In order to quantify the roughness effect, we partitioned the roughness element into two basic roughness forms: mounds and depressions. During rainfall events, water moves away from mounds into depressions or areas between mounds, causing flow concentration at local areas. A rainfall experiment was conducted on soil surfaces with artificially created surface mounds and depressions and the results on runoff and sediment productions were compared with those produced from smooth surfaces. The results showed that rough surfaces, either from mounds or depressions, delayed runoff initiation while the surface with mounds produced slightly higher sediment and surfaces with depressions produced lower sediment as compared to those form the smooth surface. During the rainstorm, mounds and depressions changed their shapes differently due to different processes. Mounds were lowered due to raindrop impact and erosion processes occurred directly on them while the depressions were filled by runoff water and sediment from areas above each depression, therefore, with a much larger contributing area. The process-level understanding on how surface roughness elements affect runoff and sediment processes helps to improve the erosion science and a better quantification on the overall soil roughness effects.

Technical Abstract: Soil surface roughness contains two elementary forms, i.e., depressions and mounds, which affect water flow on the surface differently. While depressions serve as temporary water storage, mounds diverge water away from their local summits. Although roughness effects on runoff and sediment production has been studied, there were no specific studies designed to quantify the changes in depressions and mounds and how they affect runoff generation and sediment delivery. The objectives of this study were to analyze how different surface forms affect runoff and sediment delivery and to measure the changes in surface depressions and mounds during rainfall events. A smooth surface was used to be the control. Both mounds and depressions delayed the runoff initiating time, but at different levels, and slightly reduced surface runoff when compared to the runoff process from the smooth surface. Surface mounds significantly increased the sediment delivery, while depressions provided surface storage and hence reduced sediment delivery. However, as rainfall continued and rain intensity increased, the depression effect on runoff and erosion gradually decreased and even produced higher sediment than smooth surface. Furthermore, depression and mound affect the particle size distribution of the discharged sediments. Much more sand-sized particles were transported from the surface with mounds than with depressions. The morphology of mounds and depressions changed significantly by the rainfall, but to different extents. The difference in change shows the spatial scale effect, i.e., erosion from each mound contributed to its own morphological change while sediments deposited in a depression came from a runoff contributing area above the depression, hence a much greater source area than a single mound. The results provide a mechanistic understanding on how soil roughness affects runoff and sediment production.