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This study combined a reach-scale field survey and numerical modelling analysis to reveal the pattern of transient hyporheic exchange driven by rainfall events in the Zhongtian River, Southeast China. Field observations revealed hydrodynamic properties and flux variations in surface water (SW)/ groundwater (GW), suggesting that the regional groundwater recharged the study reach. A two-step numerical modelling procedure, including a hydraulic surface flow model and a groundwater flow model, was then used to interpret the transient hyporheic flow system. The hyporheic exchange exhibited strong temporal evolution in the study reach, as indicated by the rainfall event-driven hyporheic exchange. The reversal of the hydraulic gradient and transient hyporheic exchange were simulated using numerical simulation. Anisotropic hydraulic conductivity is the key to generating transient hyporheic exchange. A revised conceptual model was used to interpret the observed temporal patterns in hyporheic exchange. The seasonal rainfall events generate transient hyporheic exchange, and the pattern of transient hyporheic exchange indicates that transient hyporheic exchange appears only after an increased phase of the river stage but does not last long. The temporal pattern of hyporheic exchange can significantly affect the hydrodynamic exchange and the evolution of hydrology in the hyporheic zone for a gaining stream, and these results have important guiding significance for the comprehensive management of surface water and groundwater quantity and quality. [ABSTRACT FROM AUTHOR]

The fractional Brownian motion (fBm) and fractional Lévy motion (fLm) can easily describe the geometry and the statistical structure of hydraulic conductivity (K) for real‐world. However, the fBm and fLm models have not been systematically evaluated when building the K field for a low‐permeability site. In this study, both the fBm and fLm are used to simulate the low‐K field at NingCheGu (NCG), Tianjin, China. Groundwater flow and solute transport are then computed using MODFLOW and MT3DMS, respectively, and the influence of the fBm/fLm models for K on groundwater flow and solute transport is discussed. Results show that the fLm fits better the statistics of the low‐K medium than fBm, and the random logarithmic K (LnK) field generated by fLm is more stable because the resultant LnK field captures more of the measured properties at the field site than that generated by fBm. In contrast, the LnK generated by fBm is more likely to form both high‐K channels and low‐K barriers. The fBm therefore predicts more extreme behaviours in flow and transport, including the preferential flow, low‐concentration blocks and solute retention. The overall groundwater renewal period and solute travel time for the fLm simulation are slightly shorter than those for fBm. The impacts of the fLm and fBm models on the statistics of the resultant LnK fields and the dynamics of groundwater flow and solute transport revealed by this study shed light on the selection and evaluation of the fractional probability distribution models in capturing the K fields for low‐K media. [ABSTRACT FROM AUTHOR]

• K is negatively exponential correlated with soil EC. • A one-dimensional sand column apparatus was developed to establish the relation. • Impacts of compaction, grain diameters and solution EC were tested. • Two groups of empirical models were proposed and found to be applicable. Soil hydraulic conductivity (K) significantly affects subsurface flow and solute transport, but convenient measurement of K remains a challenge. Electrical conductivity (EC), which can be measured conveniently and accurately, can be quantitatively related to K because of the similarity between current movement and groundwater flow, motivating us to estimate K using EC. A soil column apparatus was built to explore the relation between EC and K. Fifty-four experiments were conducted under various degrees of soil compaction and grain diameters, including six different particle sizes, three compaction levels, and six solution concentrations. The overall (negative) correlation between K and EC can be fitted reasonably well by an exponential function with a large determination coefficient (>0.81) for each solute concentration, while a power-law function better fits the correlation when K is larger than 10 m/d. These experiments led to several empirical models (for specific solution concentrations) and one comprehensive model (with variable parameters) to estimate K given EC. In the field, if the EC of pore water can be measured, the empirical models under the specific solution concentrations can be applied to greatly improve the K estimation. Comparison showed that the proposed models, which provides a quick estimate of K using easily available EC , performed better than most of the classical grain size-based formulas. Although there is uncertainty in the K estimation likely due to the limited control factors considered in this study, the new method proposed above can provide a convenient estimate for K for the field site with strong spatial heterogeneity. [ABSTRACT FROM AUTHOR]

Abstract: Interactions of surface water and groundwater (SW–GW) play an important role in the physical, chemical, and ecological processes of riparian zones. The main objective of this study was to describe the two‐dimensional characteristics of riverbank SW–GW interactions and to quantify their influence factors. The SW–GW exchange fluxes for six sections (S1 to S6) of the Qinhuai River, China, were estimated using a heat tracing method, and field hydrogeological and thermodynamic parameters were obtained via inverse modelling. Global sensitivity analysis was performed to compare the effects of layered heterogeneity of hydraulic conductivity and river stage variation on SW–GW exchange. Under the condition of varied river stage, only the lateral exchange fluxes at S1 apparently decreased during the monitoring period, probably resulting from its relatively higher hydraulic conductivity. Meanwhile, the SW–GW exchanges for the other five sections were quite stable over time. The lateral exchange fluxes were higher than the vertical ones. The riverbank groundwater flow showed different spatial variation characteristics for the six sections, but most of the higher exchange fluxes occurred in the lower area of a section. The section with larger hydraulic conductivity has an apparent dynamic response to surface water and groundwater level differences, whereas lower permeabilities severely reduced the response of groundwater flow. The influence of boundary conditions on SW–GW interactions was restricted to a limited extent, and the impact extent will expand with the increase of peak water level and hydraulic conductivity. The SW–GW head difference was the main influence factors in SW–GW interactions, and the influence of both SW–GW head difference and hydraulic conductivity decreased with an increase of the distance from the surface water boundary. For each layer of riverbank sediment, its hydraulic conductivity had greater influence on its groundwater flow than the other layers, whereas it had negligible effects on its overlying/underlying layers. Consequently, the variations in river stage and hydraulic conductivity were the main factors influencing the spatial and temporal characteristics of riverbank groundwater flow, respectively. [ABSTRACT FROM AUTHOR]

• The weight of fine-grained material decreased with distance from the main channel. • Riverbank sediments with smaller K values had weak heterogeneity. • An experimental framework showing how to quantify sediment properties is proposed. A detailed measurement of floodplain riverbank sediment hydraulic conductivity (K) is the first step in analyzing the hydraulic relationship between streams and aquifers. Poyang Lake is the largest freshwater lake in China and has historically been a flood-prone area. A detailed field experiment was conducted in the Xiushui River, China, before and after the flood season. The grain-size distribution analysis method was used to determine the hydraulic conductivity of the riverbank. A total of 116 sediment samples were collected from the test site. An in situ permeameter test was used to confirm the consistency of the spatial distribution trend of the estimated hydraulic conductivity based on the grain-size distribution analysis. A Kruskal–Wallis test of the riverbank sediment K values obtained before and after the flood season showed that they belong to different populations. The probability distribution illustrated the strong heterogeneity of the riverbank sediments before and after the flood season. Previous studies have confirmed the presence of strong hyporheic exchange and biological processes at this study site. Following the flood season, the K values had a higher mean, median, and coefficient of variation, confirming that the flood process enhanced the heterogeneity of the riverbank sediments. Overall, the hydraulic conductivity of the three areas changed with distance from the river; the closer the area was to the river, the smaller the hydraulic conductivity and weaker the heterogeneity. An experimental framework of spatio-temporal variability in riverbank sediments is proposed, providing recommendations for researchers on conducting an integrated experiment. [ABSTRACT FROM AUTHOR]

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• The magnitude of the increase in the matrix hydraulic conductivity affects the karstic water storage variation estimation accuracy. • The impact of the variation path of matrix specific yield on the karstic water storage variation estimation accuracy varies, specifically manifested in the amplitude of estimation accuracy. • The findings could provide greater insights for estimating the karstic water storage variation error in karst areas under different parameter changes and offer scientific support for water resource management and utilization. Karst water plays a crucial role as a freshwater source, which provides drinking water for millions of people worldwide. Accurate assessment of karst water resources and prediction of the impact of rainfall events on karst systems require a comprehensive understanding of medium parameters, such as the karst matrix hydraulic conductivity and specific yield. Therefore, in this paper, we investigated the effects of karst matrix hydrogeological parameters on the estimation accuracy of karstic water storage variation using a numerical modelling approach. Specifically, we considered matrix–conduit coupled karst systems and analysed the spring flow recession hydrograph following rainfall events. The objective of this study was to explore the vertical and horizontal variations in karst matrix parameters and their effects on the estimation accuracy of karstic water storage variation and to provide insights into the role of the hydraulic conductivity and specific yield in controlling groundwater dynamics. The results of this study have important implications for karst water resource management, highlighting the necessity for improved characterization of karst aquifer medium properties. [ABSTRACT FROM AUTHOR]

• Hyporheic mixing (inherit ensemble average concepts) is a scale-dependent phenomenon. • Flux variance at different scales is a proper metric to assess the hyporheic mixing. • Stream velocity, groundwater are the most dominant factors for hyporheic mixing. Hyporheic mixing plays a vital role in physical, biogeochemical, and thermal processes in river networks. While many studies have emphasized the role of streambed heterogeneity in hyporheic mixing processes, this study articulates the scale issues inherent in concentration, mixing, heterogeneity, and modeling approaches. It subsequently conducted simulations of hyporheic mixing in synthetic, heterogeneous, 2-D cross-sectional river beds based on prescribed hydraulic conductivity defined at the local-scale. It then investigated the flux variation distribution and the mixing zone under different degrees of heterogeneity and flow scenarios. Since the characterization of the heterogeneities in detail at the local scale is practically impossible, Monte Carlo simulation based on stochastic theory was used to demonstrate the hyporheic mixing under the large-scale control volume (macromixing). Afterward, a first-order stochastic analysis was undertaken to explore the relationship between local-scale flux variance distribution and the mixing zone under different heterogeneity and flow scenarios. The results of this study show that the flux variance in the streambed is an appropriate metric for assessing the magnitude of hyporheic mixing at all scales. Further, surface water velocity and upwelling groundwater are found to be the dominant controlling factors of the flux variance and in turn, the mixing process, followed by the streambed heterogeneity. In addition, it demonstrates that the hyporheic mixing process is significantly affected by the complex surface and groundwater circulation regimes and the stagnation zone under steady-state flow conditions. [ABSTRACT FROM AUTHOR]