Enhancing physically based and distributed hydrological model calibration through internal state variable constraints

Abstract. This study investigates the effectiveness of various calibration approaches within the Water Balance Simulation Model (WaSiM) to enhance the representation of hydrological variables. We assess the impact of three distinct configurations: Baseline (BL), Physical Groundwater Model (GW), and Physical Groundwater with Recharge Calibration (GW-RC) on the representation of hydrological variables. The analysis demonstrates that while traditional calibration primarily enhances streamflow prediction, integrating recharge and groundwater dynamics significantly refines the model’s ability to depict subsurface processes. The GW-RC configuration, with minimal emphasis on recharge in the objective function, shows a marked improvement in representing both the spatial and seasonal variability of groundwater recharge, suggesting that even small and targeted calibration adjustments can significantly enhance the accuracy and realism of model outputs. Although this approach may reduce the model’s flexibility in mirroring observed streamflow, it enhances the precision with which other hydrological processes are represented, providing a more accurate reflection of watershed dynamics. Our findings underscore the importance of multi-variable calibration frameworks, which incorporate both streamflow and internal hydrological variables, in developing robust models capable of adapting to anticipated hydrological shifts due to climate change. This approach provides a more accurate reflection of watershed dynamics and offers valuable insights for calibration strategies in hydrological modelling, water resource management and climate adaptation strategies.