Abstract:

[Objective] This study aims to reveal the influence mechanisms of hydrothermal conditions on the spatiotemporal variability of hyporheic exchange and to develop more reliable estimation methods. We acknowledge the limitations of single methods and innovatively propose an estimation framework for hyporheic exchange that integrates hydraulic methods, environmental tracer methods, and numerical simulation technologies. The proposed method is expected to address the insufficient accuracy and scale mismatch in existing estimation methods and to enhance the capacity to quantify highly variable hyporheic exchange fluxes. [Methods] First, based on years of practical experiences, and combined with a systematic review and critical analysis of existing literature, we deeply analyze the intrinsic driving mechanisms of the spatiotemporal variability of hyporheic exchange from two core perspectives: hydraulics and thermodynamics. Second, we propose an integrated multi-method estimation framework to improve the accuracy and robustness of the estimation results. [Results] The mechanisms by which hydrothermal conditions drive the spatiotemporal variability of hyporheic exchange are summarized as follows.(1) Hydrological rhythm: the dynamic variations in river water level and discharge alter the hydraulic head difference between river water and groundwater, serving as the primary driver of the temporal changes in the rate and direction of hyporheic exchange.(2) Topography, geomorphology, and bed heterogeneity: local topographic features of rivers and lakes (such as sand bars, pools, and point bars) and the spatial heterogeneity of riverbed sediments shape the spatial distribution pattern of hydraulic head differences, which is the fundamental cause of significant spatial variations in hyporheic exchange.(3) Temperature variation: strong daily temperature differences can generate significant thermal gradients within riverbed sediments, inducing rapid flows and shaping the diurnal variation patterns of hyporheic exchange.(4) Seasonal freezing and thawing processes substantially alter the spatial structural characteristics of riverbed permeability, profoundly affecting both the intensity and spatial extent of hyporheic exchange at seasonal and spatial scales. These driving factors are often in a state of nonstationary variations and exhibit complex couplings. Collectively, their combined effects make the spatiotemporal variation patterns of the hyporheic exchange difficult to be accurately captured or predicted by simple methods. [Conclusion] This study systematically elucidates the mechanisms by which hydrothermal conditions jointly influence the complex spatiotemporal variations of hyporheic exchange through hydraulic and thermodynamic processes. It deepens the understanding of surface water-groundwater interactions, providing a theoretical basis and practical guidance for developing more accurate watershed hydrological models, assessing the health of river ecosystems, and formulating science-based ecological restoration strategies for rivers and lakes.

Key words: river, hyporheic exchange, spatiotemporal variability, ecological function, response mechanism