A CRITICAL APPROACH TO THE CALIBRATION OF A WATERSHED MODEL

A complex watershed-scale water quality simulation model, the Hydrolog ical Simulation Program-FORTRAN (HSPF) model, was calibrated for a 16 km(2) catchment. The simulation step size was 0.33 hours with predicte d and recorded hydrologic flows compared on an annual and monthly basi s during a total calibration period of four years. Unguided numerical optimization when applied alone did not yield a model parameter set wi th acceptable predictive capability; instead, it was necessary to appl y a critical process that included sensitivity analysis, numerical opt imization, and testing of derived model parameter sets to evaluate the ir performance for periods other than those for which they were determ ined. Using this critical calibration process, the model was proven to have significant predictive capability Numerical optimization is an a id for model calibration, but it must not be used blindly.