A distributed-domain coupling model to explore river-aquifer hydrologic exchange and hydrograph tailing behavior

Hydrologic exchange flows (HEFs) in river-aquifer systems affect the water flow and solute transport dynamics. However, little is known about the influence of HEFs on hydrographs under varying hydrodynamic conditions, as well as the role of hydraulic conductivity (K) heterogeneity in shaping the hydrograph. In this study, a physical-based, distributed-domain coupled flow model is developed to investigate the effects of HEFs on hydrograph characteristics, including the peak discharge and tailing decay behavior. A series of numerical experiments were performed using a synthetic river case. The results indicate that the interaction between HEFs and river channel hydrodynamics could reduce the peak discharge and flood flow rates. With an increase in K, the peak discharge does not decrease monotonously but has a minimum value depending on the river channel characteristics. Additionally, a heterogeneous K field can generate multirate HEFs, which can shift the hydrograph tailing behavior from an exponential form to a power law form. The probability distribution of K has a notable impact on the tailing behavior and peak discharge of the hydrograph. Overall, this study addresses an important mechanism of hydrograph tailing behavior in flood events from a theoretical perspective, and the results provide insights into flood control and contaminant migration in river systems where HEFs are substantial.