Methodology for analyzing ranges of uncertain model parameters and their impact on total maximum daily load process

A methodology for analyzing uncertain parameter ranges prior to model calibration or uncertainty analysis is presented. The method considers parameters that exist in complex models and are typically difficult to set using site-specific data (i.e., parameters that have suggested ranges, national average ranges, or ranges set with land characteristic data). The method applies Monte Carlo runs and an interval-spaced sensitivity analysis to determine the parts of parameter ranges that will most likely cause unrealistic model results. An application of the method is presented using the Soil and Water Assessment Tool model as applied to the Cannonsville Reservoir system watershed for hydrology and sediment simulations. Results indicate that after parameter range reduction, the model output range was reduced by an order of magnitude, thereby reducing the uncertainty of the model and aiding the calibration effort. Sediment transport is difficult to monitor and model in its many stages of transport so significant uncertainty in the sediment erosion and transport parameters for this model still exist. This uncertainty will impact the application of the model for Total Maximum Daily Load development and management decisions.