Function formula for first flush analysis in mixed watersheds: A comparison of power and polynomial methods

► Third-polynomial is more powerful in first flush analysis at mixed watersheds. ► First flush is proportional to impervious area and rainfall intensity. ► First flush has no obvious relationship with watershed area. ► Land use distribution can play a key role in first flush in mixed watersheds. In first flush (FF) phenomenon analysis, the dimensionless cumulative curve appears as a nonlinear form which can be expressed in the form of function formulas. In this paper, a comparison between the power and polynomial functions for the FF phenomenon analysis in mixed watersheds was performed. The results indicated that both power and polynomial functions were appropriate with satisfactory fitting of the dimensionless cumulative curve. The three-polynomial function could give more descriptive parameters and was most applicable if most of the pollutant load was transported in the mid part of the runoff. The FF phenomenon was obvious at high paved watersheds during intense storm events. No apparent relationship was found between FF phenomenon and watershed area. The spatial distribution of land use plays a key role in FF phenomenon in mixed watersheds.