Method for Determining Lateral Groundwater Inflow and Initial Conditions of 1D Hydrodynamic Models for Large and Medium-Sized Rivers Based on Steady Flow Calibration

Abstract To enable the direct and efficient determination of lateral groundwater inflow and initial hydrodynamic conditions in one-dimensional (1D) hydrodynamic models for large and medium-sized rivers, we propose a steady flow calibration–based method to simultaneously determine these two key parameters. Taking preflood measured discharge as the benchmark, a hydrodynamic presimulation model with lateral groundwater inflow as the sole recharge is constructed. Numerical iteration drives the flow to steady state while lateral groundwater inflow is adjusted until the simulated discharge matches the benchmark. The lateral groundwater inflow thus determined serves as the constant input for the flood process, and the corresponding spatial distributions of water level and velocity under this steady state serve as the initial hydrodynamic conditions for flood simulation. Source-term calculations and stability conditions are optimized in the presimulation to improve accuracy and stability. The method is validated using historical flood data from the Tuhai River, China. For the typical events, the mean absolute error, root mean square error, and mean absolute percentage error remain low, demonstrating satisfactory accuracy. This method enables simultaneous quantitative determination of the two key parameters, significantly reducing parameter uncertainty, subjective biases, and numerical oscillations associated with their manual calibration. It improves simulation efficiency while maintaining numerical stability and simulation accuracy, offering an effective data-lean alternative for large and medium-sized rivers with stable preflood baseflow.