A fast, accurate, physically-based, nonlinear and nonsteady method to obtain freshwater discharges, from hourly to climatic timescales, using water-level measurements in a tidal river

A nonlinear and nonsteady 0D river model is developed and calibrated by an inverse method to provide freshwater discharge rates of the St. Lawrence River (Canada) from water-level measurements at two adjacent tidal gauge stations separated by at most half a semi-diurnal tidal wavelength. The model is fast in that several decades of hourly discharges can be reproduced in few seconds on any modern desktop computer. The method is accurate in that, once calibrated against direct tidal current measurements, the method can reproduce the observed tidal discharge variability to within 3%. The method is simple and easy to manage in that the main function only contains a few lines of code. Here we present the details of the method and the results obtained at several cross-sections of the St. Lawrence River and we show how this model could be applied as a tool to support real-time water quality monitoring in highly unsteady tidal rivers as well as how it could be used to reconstruct historical river discharges, from tidal to climatic timescales.