How Surface Roughness Affects Chemical Transfer from Soil to Surface Runoff?.

Soil surface roughness affects transport processes, e.g., runoff generation, infiltration, sediment detachment, etc., occurring on the surface. Nevertheless, how soil roughness affects chemical transport is less known. In this study, we partitioned roughness elements into mounds which diverge water flow and depressions which converge surface runoff and studied how these different roughness forms affect chemical loading to runoff water by calculating the mixing zone depth. Preliminary results using bromide (Br) as the chemical tracer showed that surface depressions provided temporary water storage and increased Br loading as much as four times as compared to flat surfaces and twice as compared to surfaces with mounds under 1 hour rainfall at 50 mm/h. The interaction between roughness and surface hydrologic conditions, i.e., seepage vs. drainage, on chemical loading was also studied. We evaluated the existing mixing zone model for chemical loading and compared the model concept with physical processes, i.e., diffusion, infiltration and erosion with the experimental data. We believe the current mixing zone model for runoff chemical loading needs to be modified based on surface microtopography and hydrologic conditions that affect chemical transport.