Evaluating and understanding tide-river interactions based on both physical models and data analysis

A precise and thorough understanding of tide-river interactions holds practical significance for estuarine management and the sustainable utilization of water resources, particularly in the context of saltwater intrusion. Previous research on tide-river interactions has been limited by a lack of measured discharge stations in the upper and lower reaches of the river, especially in the tide-river transitional zone, leading to assumptions that streamflow from upstream river is representative of streamflow in the transition zone. In addition, the differences between simulated and measured discharges in revealing tide-river interactions are poorly understood. To address this knowledge gap, we employed the hydrodynamic model MIKE21 to simulate downstream river discharge as well as the cross wavelet transform (XWT) and stage-discharge rating curves to reveal the internal relationship between the simulated discharge (SD), measured water level, and tidal range. We applied this approach to the Yangtze River Estuary (YRE), a large tide river, and found compelling results. Notably, near tideriver transitional zone, where the river discharge may differ significantly from the measured upstream discharge. For instance, in Jiangyin, the maximum error of the SD was equal to approximately 15% of the measured Datong discharge (MDD), which may be caused by industrial and agricultural water use. Additionally, the use of XWT and the stage-discharge rating curve allowed us to accurately identify the positions of the river-dominated region, tide-dominated region, and tide-river transitional zone. Particularly in the upstream and downstream of Jiangyin, the R2 of the stage-discharge rating curve varied significantly. The R2 in the upstream of Jiangyin is above 0.9, while the R2 in the downstream of Jiangyin is generally below 0.4. XWT results indicated that there is a strong significant resonance period (SRP) between the SD and the tidal range and water level in the downstream of Jiangyin, while the SRP in the upstream is weaker. Our findings underscore the necessity of conducting discharge measurements in the transitional zone and tide-dominated region to gain precise insights into tideriver interactions. This work may serve as a useful reference for further investigations into tide-river interactions in estuaries across the globe.