Particle size and shape variation of Ultisol aggregates affected by abrasion under different transport distances in overland flow

Particle size variation or distribution is a recognized factor influencing crust characteristics, water infiltration and sediment transport during slope erosion. Limited studies have been conducted about particle size and shape variation resulting from the abrasion of soil aggregate in overland flow. This experiment was based on aggregate abrasion in a 3.8 m long flume with a fixed bed, and the particle size and shape variations were studied using dry sieving method and a Laser particle size and shape analyzer. Results indicated that the high abrasion rate of large particles (particles larger than 5 mm) caused the mass percentage of particles in intermediate size ranges (2 mm–1 mm, 1 mm–0.5 mm and 0.5 mm–0.25 mm) increased with transport distance. The D50 values of Average Feret Diameter of smaller abraded aggregates (0.0385 mm–0.25 mm) increased first and then decreased, and their shape became more regular and rounder with the transport distance. There were significant negative correlations between RMI (relative mechanical breakdown index) of initial aggregates with MWD (Mean weight diameter) for particles larger than 0.25 mm, and with Cirm (median value of Circularity Factor) for particles smaller than 0.25 mm in all the five transport distances (p < 0.05). These indicated that the initial aggregates had low susceptibility to mechanical breakdown, resulting in weak abrasion of particles larger than 0.25 mm and regular shape of particles smaller than 0.25 mm under certain transport distance in overland flow. A multiple regression equation was established, relating Cirm to RMI and transport distance (x) for estimating the particle shape of smaller abraded aggregates in overland flow. The information of the analysis of particle size and shape variation can be useful for the development of soil process-based erosion models.