A general unit hydrograph theory for water level and tidal range distributions in the Modaomen Estuary, China

Understanding the spatial distributions of river-tide dynamics in estuaries and their response to intensive human interventions is critical. However, the studies on the characteristics of the spatial distributions of water level and tidal range are insufficient, with inadequate direct established empirical formulas. In this study, we propose a general and analytical water level and tidal range distribution model based on the recently proposed General Unit Hydrograph (GUH) theory and apply it to the Modaomen Estuary in China. Due to the intensive human interventions, the evolution of river-tide dynamics in the Modaomen Estuary was divided into three distinct periods: Pre-human, Transitional and Post-human Periods. The results show that the water level increased at the SZ station, yet decreased at other stations, thereby reducing the maximum water level gradient by approximately 9.08 × 10⁻⁶ on average from the Pre-human to the Post-human Periods. The tidal range generally increased along the channel and the absolute value of the maximum tidal range gradient decreased by approximately 54 % on average. Correspondingly, the positions of both maximum water level gradient and minimum tidal range gradient generally move seaward by approximately 27.25 km and 0.34 km, respectively. Additionally, the spatial distributions of river-tide dynamics tended more linear after human interventions. The satisfactory correspondence of the model outputs with observations at the six gauging stations along the Modaomen Estuary (with maximum RMSE values being 0.05 m for the water level and 0.03 m for the tidal range, respectively) indicate that the newly proposed GUH model could serve as a simple tool to describe the spatial distributions of water level and tidal range in a specific channel together with their response to anthropogenic modifications, which is also particularly useful for the identification of regime shifts in river-tide dynamics and sustainable water resources management in other highly human-modified estuaries worldwide.