In many watersheds, including the Great Lakes and Lake Tahoe Basins, two basins where the land cover is dominated by forests, the pollutants of concern are fine sediments and phosphorus. Forest runoff is generally low in nitrogen, and coarse sediment does not adversely impact the quality of lake waters. Predictive tools are needed to estimate not simply sediment, but fine sediment (<10 μm) and phosphorus delivery from forested hillslopes. We have been developing methods for making such predictions with the Water Erosion Prediction Project (WEPP) model. WEPP is a physically-based hydrology and erosion model that runs on a daily time step, with sub-daily runoff, erosion and sediment delivery predictions. The fine sediment delivery for forested hillslopes is relatively easy to estimate because WEPP provides a breakdown of primary particles (clay, silt and sand) and aggregates (silt size aggregates (30 μm) and sand size aggregates (300 μm)). The size distribution of eroded sediment is disaggregated to determine the amount of fine sediment below a user-specified size in each of the particle classes. Phosphorus transport is complex, as research has shown that in steeper forested watersheds, the dominant hydrologic flow paths are lateral flow and base flow. Surface runoff and sediment delivery are generally minimal unless the site has been disturbed by logging or fire, or the soil layer is thin (shallow to impermeable bedrock). Thus, in an undisturbed forest, the main phosphorus pathway will likely be in subsurface lateral flow as soluble reactive phosphorus (SRP), whereas in a disturbed forest, the dominant pathway may be in surface runoff as SRP, or as particulate phosphorus adsorbed to eroded sediment. Prediction is further complicated as research has shown that the SRP concentration in the soil water may be higher in undisturbed forests than in burned or harvested forests. Delivered sediment also is complicated in that preferential particle size sorting may occur, increasing the content of clay and organic matter in delivered sediment and thereby increasing the phosphorus concentration in delivered sediment above that in the forest. We have developed a way to use the current predictions within the WEPP technology to estimate not only the surface runoff and sediment delivery, but also delivery of fine sediment below a user-specified threshold, and phosphorus through both surface and subsurface lateral flow pathways.
