Soil aggregate loss affected by raindrop impact and runoff under surface hydrologic conditions within contour ridge systems

Detachment and transport of particles by raindrop impact and runoff determine the characteristics of soil aggregate loss, which are largely related to surface hydrologic conditions. The water ponding and sediment deposition in furrow areas within contour ridge systems complicate the effects of raindrop impact and runoff on soil aggregate loss responding to surface hydrologic conditions. The objective of this study was to determine the effect of raindrop impact and runoff to macro-aggregate and micro-aggregate losses under surface hydrologic conditions within contour ridge systems. The losses of 16 size aggregates were measured under three surface hydrologic conditions (free drainage, soil saturation, and seepage) and two soil surface conditions (with and without raindrop impact) through simulated rainfalls. Results showed that raindrop impact was the dominant factor controlling soil aggregate loss under free drainage condition, while soil aggregate loss was mostly affected by runoff under soil saturation and seepage conditions. Under free drainage condition, raindrop impact accounted for 44.6 %–100 % of the loss of each size aggregate, with maximum losses from 0.5 to 1 mm and 20–50 μm aggregates. Raindrop impact mainly caused the breakdown of >0.5 mm and 50–250 μm aggregates, and enhanced the transport of 20–50 μm aggregates by saltation. Under soil saturation and seepage conditions, runoff contributed 25.5 %–247.6 % to the loss of each size aggregate, and most dramatically to the losses of 2–5 mm and 10–50 μm aggregates. The role of runoff was mainly characterized by the breakdown of 0.25–1 mm and 50–250 μm aggregates, exhibiting a propensity toward transporting 2–5 mm aggregates by rolling, following 10–50 μm aggregates by suspension/saltation. This study highlights the need to understand the effects of raindrop impact and runoff on erosion processes associated with surface hydrologic conditions for better erosion modeling, and provides guidance for soil erosion control within contour ridge systems.