Modeling and comparative analysis of a flow and heat coupling model of the riparian zone based on thermal conductivity empirical models

Recently, the use of heat as a tracer to evaluate the process of hyporheic exchange in riparian zones has attracted wide attention. A more accurate flow-heat coupling model of riparian zones could help us understand the patterns of water flow and heat transfer in riparian zones and provide a scientific basis for the comprehensive management and efficient utilization of these zones. In this paper, a flow-heat coupling model of a riparian zone based on thermal conductivity empirical models (TCEMs) was built by customizing partial differential equations (PDEs) based on the simulation of soil water flow using porous media and the subsurface flow module in COMSOL. Combined with the data collected from the field heat tracer test in the riparian zone, the flow-heat coupling model of the riparian zone under 12 types of TCEMs was validated and compared. The results show that the use of the PDE module instead of the heat transfer in porous media (HTPM) module is very effective for modeling; the PDE module could basically replace the HTPM module in the heat transfer calculation. The performance of the flow-heat coupling model varies under different types of TCEMs, and the Chung and Horton (1987) model shows better simulation effects, with a root mean square error (RMSE), coefficient of determination (R-2), mean absolute error (MAE), and mean relative error (MRE) ranging from L37 to 2.48 degrees C, 0.73-0.94, 1.06-2.08 degrees C, and 11.94-15.79%, respectively. Therefore, this model could better reflect the dynamic temperature variations in the riparian zone. The sensitivity analysis results illustrate that the hydraulic conductivity (K-s), van Genuchten parameter (beta), volumetric heat capacity of dry solids (C-s), and porosity (n) of the flow-heat coupling model greatly influence riparian zone temperature variations, and the parameters beta, K-s, and residual water content (theta(r)) significantly affect the lateral hyporheic exchange rate in the riparian zone, of which beta has the largest effect.