QUANTIFYING MODEL UNCERTAINTY FROM ALTERNATE STREAM HYDRAULIC REPRESENTATIONS: A Case Study of the Truckee River

HSPF is used frequently for TMDL studies, and is a component of the USEPA BASINS software. One enhancement the BASINS framework provides is the automatic creation of stream reach hydraulic characteristics (i.e. HSPF F-tables). The method used by BASINS to estimate reach characteristics is based on work by Leopold (1953), Allen (1994), and Muttiah (1997). Using BASINS to estimate stream hydraulic characteristics is a quick, simple, and inexpensive method when compared to more intensive field measurement options. However, the potential error in TMDLs using this method has not been quantified.Other methods of estimating stream hydraulics include physically measuring stream width, depth, flow, and travel time (Berris, 1996; Nowlin, 1984). These methods can be expensive and time consuming. For a robust estimate of stream hydraulic characteristics, multiple measurements at various locations and flows may be required. Costs for these studies can be quite high for the collection and analysis of the data, and take several months to insure a proper range of flow conditions have occurred. Thus, alternate methods that accurately estimate hydraulic characteristics of stream reaches, such as used by BASINS are useful.An understanding of the potential uncertainty in TMDL predictions resulting from different hydraulic representations would be useful when determining the amount of resources that should be invested in this part of model development. This study quantifies the uncertainty of model predictions due to different hydraulic representations for the Truckee River.This paper compares model results for a conservative and non-conservative pollutant using an HSPF model configured with three distinct hydraulic representations. The hydraulic representations were estimated using the following methods:The method implemented in BASINS;Hydraulic characteristics estimated from multiple stream cross section measurements and;Hydraulic characteristics estimated from width, flow and travel time measurements taken at various flows and locations.This exercise demonstrates the potential uncertainty in model results caused by uncertainty in reach hydraulics. Quantifying the potential uncertainty in this model parameter will help future modeling projects focus limited resources on collecting data most useful for developing a robust water quality model, demonstrate utility of the method implemented in BASINS, and explore the impact that stream hydraulics can have on model calibration.Results of the analysis demonstrate that the three hydraulic representations developed for the Truckee River have a significant impact on predicted traveltime, depth, and width. These hydraulic properties effect the amount of pollutant decay, reaeration, and algal growth predicted by the model. The differences between model predictions impact model calibration, verification, and TMDL estimation.