We present a method for forecasting water levels at river open boundaries in coastal ocean models, where tidal signals are influenced by variable river discharge. The method uses non-stationary tidal analysis (NSTIDE) to generate future water levels based on a deterministic tidal signal and extrapolations of upstream hydrological predictors. This allows the river boundary to be placed within the tidal propagation zone without modelling the full tidal extent or coupling a separate river model. Case studies for the Fraser and Saint John rivers demonstrate that the method reproduces water levels and river outflow skilfully under a range of seasonal flow conditions. Forecast accuracy is comparable to gauge-driven configurations in most downstream areas, with root mean square values of model data using gauge data or NSTIDE for river water level differing by no more than five centimetres. Performance gradually declines upstream. This novel approach in modelling tidal estuaries reduces computational cost, avoids the complexity of coupled river modelling, and is well suited to operational forecasting systems requiring short-range river boundary forcing.
Dunphy et al. (Sun,) studied this question.