Coupled Water And Heat Transport And Numerical Simulation In The Hyporheic Zone Of Prairie Inland Rivers During Summer Flooding Season

The hyporheic zone is the region where surface water interacts with groundwater,and the hydrothermal exchange therein is important for the hydrological and biogeochemical processes in the river corridor.In this study,a typical prairie river was selected,and the hydrodynamic and temperature tracing methods combined with numerical simulations were used to quantitatively investigate the role of hyporheic exchange in the straight and meandering sections of the Xilin River during the summer floods in the year of 2020 and 2021 based on high-resolution water level and water temperature monitoring results.A 3-D hydrothermal coupling model of the hyporheic zone was constructed based on FEFLOW,and its hydrothermal exchange response to different hydrological events and channel morphology and its biogeochemical significance were analyzed.The results of the study are as follows.(1)During the summer flood season,the river water in the upper reaches of the Xilin River fluctuates frequently.The hydraulic gradient between the river water and groundwater changes significantly,indicating active exchange between the two waterbodies.The vertical hyporheic exchange rate calculated by the hydrodynamic method is slightly higher in the meandering section(-0.3?0.5 m/d)than in the straight section(-0.3?0.35 m/d).For the straight section,the lateral hyporheic exchange rate is higher on the right bank(-0.2?0.6 m/d)than on the left bank(-0.06?0.13 m/d)due to the influence of topography and conductivity.Influenced by the river morphology,the lateral hyporheic exchange is more intense in the meandering section,and the hyporheic exchange rate is between-0.5 and 0.5 m/d.The vertical hyporheic exchange rates of the straight section(-0.2 to 0.7 m/d)and the meandering section(-0.6 to 2 m/d)calculated by the temperature tracing method using VFLUX2 software are in the s ame order of magnitude as those calculated by hydrodynamics.Both methods are able to calculate the rate of hyporheic exchange under the influence of flood and rainstorm events accurately.The temperature tracing method based on temperature gradients is suitable for the calculation of hyporheic exchange in the depth of 0-50 cm below river where temperature amplitude and phase difference change significantly.(2)The hydrothermal coupling model based on FEFLOW can reasonably and accurately describe the water flow and heat transport processes in the hyporheic zone of prairie rivers.The simulated values of water level and water temperature can fit the measured values well,with the root mean square error(RMSE)of 0.038 and 0.4respectively,and the goodness of fit(R2)of 68.8% and 54% respectively.(3)Both measurement and simulation show that when the upstream flooding occurs,the straight section of the river recharges groundwater on both banks.The hyporheic flow on the right bank shifts from outflow(i.e.,river bank groundwater recharge into surface water)to inflow(i.e.,surface water recharge to river bank groundwater)due to the influence of permeability and river bank topography,while there is no significant change in the rate of hyporheic exchange on the left bank.The upstream river water in the meandering section directly crosses the right bank to recharge the downstream river water,forming a transboundary flow.The rainstorm process causes the intensification of groundwater recharge into river water on both banks or the weakness of river water recharge to both banks.Both river sections show a reversal of the exchange direction between river water and groundwater on river banks.(4)Flooding processes drive the recharge of river water into groundwater in the riverbed and river banks,leading to an increase of pore water temperature.Meanwhile,the transboundary flow in the inner part of the meandering river section prolongs the hydraulic residence time in the riparian zone,which facilitates the occurrence of biogeochemical reactions.Therefore,flooding period may be a hot moment in the hyporheic zone.This study provides a scientific basis for an accurate understanding of the hydrothermal transport characteristics and biogeochemical processes in inland prairie rivers.