Partial least squares regression for linking aggregate pore characteristics to the detachment of undisturbed soil by simulating concentrated flow in Ultisols (subtropical China)

Abstract

Soil pores are known as an effective medium for water, air, and chemical movement. However, quantifying pore structures and their relationship to the detachment of undisturbed soil by concentrated flow is not fully understood. The objectives of this study were to quantify 3D aggregate microstructures using synchrotron-based high-resolution X-ray micro-computed tomography (SR-μCT) under wetting and drying cycles and to investigate the effects of pore characteristics on the detachment of undisturbed soil by simulating concentrated flow. The detachment rates (Dr) of undisturbed topsoil samples, which were subjected to a pretreatment of zero to eleven wetting and drying cycles, were measured in a 3.8-m-long, 0.2-m-wide hydraulic flume under unitary flow shear stresses (τ = 14.49 Pa); 3–5 mm aggregates were collected from the pretreatment soils, which were subjected to zero, five, and eleven wetting and drying cycles and were scanned at a 3.7 μm voxel-resolution for reconstructing three-dimensional microtomography images. The relationships among pore characteristics and Dr were analyzed using partial least squares regression (PLSR). The results indicated that Dr and the aggregate microstructure changed significantly after the wetting and drying cycles. Dr was closely associated with the pore characteristics. The pore characteristics, such as a >100 μm porosity, total porosity, fraction of regular pores, 30–75 μm porosity, fraction of elongated pores, and fraction of irregular pores, were identified as the primary parameters that control the Dr. The pore characteristics and clay content could account for as much as 68.2% of the variation in Dr. Dr was significantly and positively related to a >100 μm porosity, total porosity, and fraction of elongated pores. Thus, soil fragmentation depended on microcracks that formed during the wetting and drying cycles and on the geometrical characteristics of the pores or cracks. The results from this study provide a quantitative evaluation of the relationships between pore characteristics and the detachment of undisturbed soil by simulated concentrated flow. A further understanding of the effect of topsoil microstructure on water erosion and quantitative information for assessing and modeling erosion processes was obtained.