Water Quality Model Calibration under Unknown Demands

Abstract

It has often been cited that a water distribution system (WDS) hydraulic model needs to be highly accurate before it may be used in combination with a water quality model (WQM) to simulate the dispersion and decay of a residual disinfectant. However, even a well-calibrated WDS hydraulic model may not have data relating to the specific water demands during a given period, which may impede WQM calibration. This study examines using residual disinfectant data to calibrate a WQM under unknown or uncertain demands by calibrating a residential demand multiplier pattern (DMP) in tandem with the WQM parameters. Two artificial scenarios and one real case study are investigated. The artificial scenarios are used to (1) verify the proposed methodology under ideal conditions and (2) validate the proposed methodology when the hydraulic model and calibration data contain realistic errors. The real case study uses residual chlorine data and a WDS model for which a hydraulic and WQM calibration had been performed previously. The estimated demands from the real case study are validated using tracer test data. Results from the artificial case studies may be summarized as follows: (1) the proposed methodology can estimate the demands and calibrate WQM parameters correctly, although increasing model and calibration data errors adversely affect calibration results; (2) the calibrated WDS models reproduce the true residual chlorine concentrations with very little error. Results from the real case study indicate that the original WQM calibration was performed using underestimated WDS demands. Tracer test data confirm that the calibrated DMP provides good hydraulic velocities. The calibrated WDS model from the real case study is in good agreement with measured residual chlorine concentrations. The mean absolute error between the simulated chlorine concentrations from the calibrated network model and the observed values is