The Importance of In-Stream Hydraulics in River Water Quality Models: Lessons from the Blackstone River

Water quality models of rivers often include three fundamental components: inflows and pollutant loads from point sources and watershed runoff, in-stream hydraulics, and in-stream water quality processes. In many cases, model development focuses mainly on the first and third components, while less attention is given to the in-stream hydraulics. An existing hydrologic model was used as a basis to develop a water quality model of the Blackstone River. Although the original model was calibrated to daily streamflow records, the hydraulics of the model proved to underestimate the travel time of the river. Initial attempts to calibrate the water quality model were unsuccessful since many of the parameter values governing in-stream processes were forced outside of acceptable ranges to compensate for the underestimated travel times. In order to develop a more robust water quality model, the mainstem of the river was resegmented into a greater number of reaches such that each reach represented a uniform hydraulic profile. The resegmented model resulted in a longer travel time more representative of the actual conditions of the river. Subsequent calibration of the water quality model yielded realistic parameters values, which were within an acceptable range and similar to those used for other rivers in the region. This work illustrates the importance of developing a good hydraulic, and not just hydrologic, representation of a river as the foundation for a water quality model.