Many models have been developed which simulate the quality of runoff from urban impervious areas. Common to these models is the use of exponential accumulation and washoff equations. The two equations have often been calibrated separately by trial and error. However, field data of nonpoint source investigations are usually collected at the watershed outlet or at a few points on impervious surfaces, both of which reflect the combining effects of accumulation in dry weathers and washoff in wet weathers. It is therefore preferable to estimate these parameters simultaneously to obtain the global optimal values. In this study, the accumulation and washoff equations have been formulated into one model. And optimization techniques for estimating the best fit values of the parameters in the model have been developed using adaptive step size random search and Marquardt method. Bounds of these parameters have been set during optimal search to make sure that the calibrated values are physically meaningful. The method is demonstrated in Kyoto catchment in Japan. Results show that the simulated and measured data match quite well and the calibration approach can generate accurate estimates efficiently.

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