Transformation rates of pollutants in rivers for water quality modelling under unsteady state: A calibration method

•Temporal variation of kinetic rates affects simulations of pollutant’s transport.•The proposed method overcome common model calibration challenges.•Variability of input conditions is related to pollution attenuation over time/space. While advection, dispersion, and their hydraulic background are broadly explored in water quality modelling, conversion processes (reaction and decay rates) usually lack of attention. They represent physical, chemical and biological processes that reactive substances go through in contact with water, typically assumed as calibration parameters. This study introduces a method for model calibration considering a temporal aspect of transformation coefficients: TRATS – Transformation Rates Time Series. Daily decay rates are generated as result of: (i) attributes of the river station, (ii) reference values from literature, and (iii) random variability. These transformation parameters are then applied in water quality simulations under unsteady state in a river, for biochemical oxygen demand (BOD), organic nitrogen (N-org) and dissolved oxygen (DO). The proposed method for integrated modelling suggests that temporal variation of kinetic processes may play an import role in transport and fate of pollutants in terms of overall variability and attenuation over time and space, and this aspect can be included in the calibration phase.