Investigating raindrop effects on transport of sediment and non-sorbed chemicals from soil to surface runoff

A simple modeling and laboratory investigation was carried out to investigate the raindrop effects on both sediment detachment and chemical transport from soil-water into runoff. Solute movement between soil-water and runoff is usually modeled as either a mixing model or as a diffusion-like process, both of which ignore the important roles of raindrop impact on the transport process. We hypothesized a process-based chemical transport model that incorporated both rain-drop induced mixing and diffusion and tested it using a small-scale experiment in which simulated rainfall fell on soil, pre-saturated with chloride (Cl−) laden water. We simultaneously observed sediment and Cl− runoff concentrations trends and the evolution of the 'shield' layer composed of relatively heavy particles that resettle after each raindrop-impact. Using recently published and directly measured parameters, the model results generally agreed very well with measured concentrations. The exception was for the early (<5 min) Cl− transport, which was faster than the model predicted, suggesting that an additional process needs to be added to our model. Even with this deficiency, the model developed here described our experimental results better than popular 'mixing-layer' and 'diffusion' models. This study provides a new approach to chemical transport modeling by linking the rain-controlled processes with similar soil erosion processes.