A Subbasin Element represents a drainage area where precipitation induces surface runoff, influenced by burn or unburn conditions. Within this catchment, erosion occurs due to various physical processes, notably in post-fire scenarios. Raindrops initiate erosion by impacting the ground, dislodging soil particles, which are carried by overland flow. This flow also imparts erosive energy to the terrain, potentially further disrupting the topsoil layer. As overland flow intensifies, it becomes channeled into rills, concentrating erosive energy and exacerbating surface erosion. The extent of erosion closely correlates with precipitation rate, land surface slope, and surface condition. Occasionally, soil eroded from higher up in the catchment may settle before reaching the subbasin outlet.
All Surface Erosion Methods for the subbasin element share certain simulation features. Each method calculates the total sediment load transported out of the subbasin during a storm, repeating this process for each storm within the simulation time window. The computed sediment load is then distributed into a time-series of sediment discharge from the subbasin. Each method was developed with distinct purposes and applications, as outlined below:
Another common feature among these methods is the treatment of Grain Size Distribution. Initially, all methods compute the overall Sediment Discharge, encompassing all grain sizes. Subsequently, a Gradation Curve specifies the proportion of the total sediment discharge allocated to each grain size class or subclass. Users define and select a gradation curve for each subbasin, allowing for distinctions in erosion, deposition, and resuspension processes within each subbasin. These processes are often collectively represented by an Enrichment Ratio.