Development of a Mechanistically Based, Basin-Scale Stream Temperature Model: Applications to Cumulative Effects Modeling

We describe a mechanistically-based stream model, BasinTemp, which assumes that direct shortwave radiation moderated by riparian and topographic shading, controls stream temperatures during the hottest part of the year. The model was developed to support a temperature TMDL for the South Fork Eel basin in Northern California and couples a GIS and a 1-D energy balance model. Spatially varying insolation is calculated in the GIS and heat and mass transfer processes are modeled using a simple steady-state scheme integrated with an optimization procedure which improves model predictions. BasinTemp can be applied to basins of varying sizes and requires minimal measured input data. Model predictions for three sub basins in the South Fork Eel yielded RMSE statistics ranging from 0.25 oC to 0.30 oC. The model also performed well using pooled data for all three sub basins, yielding an RMSE of 0.36 °C. BasinTemp has been used to assess local and downstream stream heating effects after modifying riparian shade. Model predictions for the three sub basins illustrate the importance of riparian shade provision on low order channels and show the shifts in the quality and quantity of potential coho habitat following different shade prescriptions.