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• Hydrogeological model using Equivalent Porous Media and Conduit Network approaches. • Discrete features tool application to represent karst conduits with turbulent flow. • Using tracer test results to calibrate the flow rate of karst springs. The current study consisted of the development of a hydrogeological numerical model in the Lagoa Santa Karst Environmental Protection region, in order to represent the conduit-flow karst springs in the most well-studied karst system in Brazil. Once carbonate aquifers are characterized by a complex 3D pattern with a combination of a double porosity of matrix and conduits, two different modelling approaches were used: Equivalent Porous Medium (EPM), based on Darcy's law, to represent laminar flow in the fractured massif and Conduit Network (CN), using Manning-Stricker Law and the discrete feature tool, to represent the karst conduits with turbulent flow. Modeling was calibrated in a steady-state and simulations of groundwater flow and an evaluation of water exchanges between the karstic aquifer and the other hydrogeological units, were conducted. The model was developed based on data available for many studies carried out in the area, including tracer-derived route data. The calibration results for spring discharges, water levels and mass balance were satisfactory and aligned with modeling good practices. In turn, the hydraulic conductivities and recharge rates obtained by back-analysis were in agreement with literature references. The regional groundwater flow was well represented and indicated how external water entrances can be relevant to explain the higher discharge rates observed in some karstic springs. In addition, the representation of conduits allowed for the simulation of tracer pathways, enabling the inference of potential flow rates to the karstic springs from the particle simulations. [ABSTRACT FROM AUTHOR]

• Analytical solutions for defining the riparian lens extent and saltwater discharge in stratified aquifers are developed. • The applicability of the proposed analytical solution is validated through numerical simulations and sand-tank experiments. • The impact of aquifer stratification on riparian lenses depends on inland boundary conditions. Analytical solutions for defining stable freshwater lenses within saline riparian aquifers ("riparian lenses") are largely limited to homogenous aquifers. However, real-world riparian aquifers typically consist of a surficial layer of low-permeability soils or deposits, the impact of which on the occurrence of riparian lenses has not been investigated. This study develops steady-state, sharp-interface analytical solutions for riparian lens shape and saltwater discharge in a two-layered riparian aquifer. The developed analytical solutions are verified by sand tank experiments and numerical simulations, showing good agreement. The sensitivity analysis based on the proposed analytical solutions suggests that aquifer stratification (i.e., considering a low-permeability layer overlying a high-permeability layer) leads to larger lenses in head-controlled systems (i.e., with the inland constant-head boundary) but smaller lenses in flux-controlled systems (i.e., with the inland constant-flux boundary), compared to the scenario of a homogenous aquifer. The surficial layer with hydraulic conductivity only an order of magnitude lower than the underlying high-permeability layer would have a pronounced effect on lens extent and volume. Importantly, the results suggest that the presence of the surficial low-permeability layer may lead to the outcropping of the groundwater table, favoring the occurrence of saline groundwater seepage near the edge of the river valley (i.e., a phenomenon reported by the field survey of the Murray River floodplains in South Australia). The analytical solution presented in this study can be used to provide a preliminary assessment of riparian lenses in saline stratified aquifers, expanding the applicability of analytical methods to more realistic riparian settings. [ABSTRACT FROM AUTHOR]

• Hydrogeological parameters are main uncertainty source of physical methods • Lag effect of the thick vadose zone limits WTF and dating-based methods • Ignoring the mixing and dispersion may overestimate recharge by 36–48 % for CFC • Aquifer heterogeneity reduces the potential of 14C for recharge estimation • CMB is recommended as the optimal method for GR estimation in the loess area Groundwater recharge (GR) can be estimated with physical, tracer techniques and numerical modelling; however, the uncertainties in these methods have not been fully interpreted. This study selected six methods for actual GR estimation in the saturated zones with different data requirements and method assumptions, including two physical methods (i.e., water table fluctuation WTF and Darcy's law DL) and four tracer techniques (i.e., chloride mass balance CMB, tritium renewal rate TRR, chlorofluorocarbons CFC and 14C dating method CBD). Taking the loess tablelands of China as an example study area, we first analyzed the uncertainties in these methods and then recommended the appropriate methods for GR estimation in regions with thick vadose zones. Overall, the recharge rates from WTF, CMB and CFC (88 ± 24, 98 ± 33 and 53 ± 9 mm/year) were much greater than those from DL, TRR and CBD (10 ± 3, 14 ± 3 and 7 ± 5 mm/year). The differences in estimated GR are closely related to the method parameters, assumptions and temporal scales. Specifically, the model parameters (e.g., specific yield, hydraulic conductivity) exhibit large ranges due to spatiotemporal heterogeneity, and the physical methods are more sensitive to hydrogeological parameters compared with the tracer techniques. The uncertainties related to model assumptions originate from the simplified water/solute movement processes. The lag effect of the thick vadose zone largely limits WTF and TRR, but contributes to CMB. Ignoring the mixing and dispersion effect may overestimate recharge by 36–48 % for CFC, and the heterogeneity of the aquifer reduces the potential of CBD for recharge estimation. Overall, CMB is considered as the most optimal method for estimating GR in the thick loess deposits. This study deepens the understanding of the uncertainties in GR assessment methods in regions with thick vadose zones. [ABSTRACT FROM AUTHOR]

• The recession curves of all features followed the triple permeability model. • The percentages of water capacity in the three aquifer media were calculated. • The average values of EC in fissure, fracture and conduit water were calculated. • A hydrogeological conceptual model was constructed for each karst feature. Understanding of the hydrological behaviors in different karst features is crucial for evaluating the hydrological storage and transfer functions of a karst system. In this study, the Maocun karst underground river system, characterized by typical karst features, was selected as the research area in the southwest region of China. The main objectives of this research are to analyze the hydrogeological structure characteristics of each karst feature and assess water storage capacity in different aquifer media. Frequency distribution curve (FDC) of electrical conductivity (EC) values revealed that recharge sources in the five karst features differed and varied according to the amount of precipitation. All hydrographs of water levels in five karst features were divided into three periods due to the presence of triple aquifer media in the basin. The spatial structure of each feature was interpreted by analyzing the variation characteristics of recession exponential coefficients (λ). Additionally, the ratios of fissure water, fracture water, and conduit water sources were calculated based on the recession curves of water levels. Results showed that fast water flow had the highest proportion in the relatively open subsystem located upstream, while fracture water flow had the lowest proportion. The slow water flow was the main pattern of water storage in the basin. Values of EC in water decreased with a well-connected spatial structure and high hydraulic conductivity. There were significant differences in the values of EC among conduit water, fracture water, and matrix water. These values were calculated using mathematical relationships between EC and water level hydrographs. The results indicated that the EC values in matrix water exceeded 200 μS/cm, while EC values in conduit water/channel water were below 100 μS/cm. Finally, hydrogeological conceptual models were constructed for five karst features based on the depletion characteristics of the aquifer media during each recession period. These characteristics were interpreted using variation curves of EC and hydrographs of water levels. These findings not only elucidated the characteristics of hydrogeological structures in multiple karst features, but also had significant implications for the assessment of karst water sources. [ABSTRACT FROM AUTHOR]

• Conductivity is employed in flow models upon averaged Monte Carlo fields. • A Monte Carlo volume of 10 % of that required for full numerical stability. • A 10 % volume returns stable outputs, i.e. heads and residuals at wells' locations. The knowledge of aquifer systems, their geological setting, their structure, and subsequent modeling is highly uncertain and is usually faced through Monte Carlo-based methods in hydrogeology. One of the most important uncertainty sources for groundwater models is represented by input hydraulic conductivities, related to the aquifer's structure. There are no specific rules when simulating hydraulic conductivity fields within Monte Carlo frameworks to instruct numerical models, and information about employed conductivity fields and their numerical convergence is often not given. This technical work aims to fill this gap by investigating the impact of employing conductivity information upon different volumes of Monte Carlo simulations applied to a real case study. Thus, this work estimates the minimum volumes of Monte Carlo hydraulic conductivity fields to be employed in groundwater flow models for achieving numerically stable (i) boundary conditions, (ii) global model performances, and (iii) local ones such as simulated hydraulic heads. The present results aim to be indicative of similar hydrogeological settings and will serve as a basis for more complex ones and for investigating transport problems. [ABSTRACT FROM AUTHOR]

• Van Genuchten-Mualem soil's steady flux analyzed; analytical solutions developed. • First solution for flow equation without prior assumptions on pressure head's form. • Solution derived using double Maclaurin series and multinomial theorem. • Analytically identify hydraulic parameters, avoiding numerical inverse procedures. Various analytical solutions for steady-state vertical flux through unsaturated soils exist in the literature. These solutions often assume specific hydraulic conductivity forms such as exponential form, rational power form, or power-law form such as the Brooks-Corey model. The van Genuchten-Mualem model is widely used in practical applications of flow through unsaturated soils. It is among the most complicated hydraulic conductivity forms for which no analytical solutions are available. This paper deals with the development of an analytical solution for steady-state vertical flux using van Genuchten-Mualem model. The analytical solution is obtained using Maclaurin series expansions and the multinomial theorem. The general form expresses the depth as a function of pressure head. The analytical solution can be used to model both infiltration (positive flux rate) and evaporation (negative flux rate). The derived analytical solution is compared with numerical results and shows excellent agreement. It can also be used to identify the hydraulic parameters analytically, eliminating the need of a numerical inverse procedure. As far as the author's knowledge, the proposed solution is the first analytical solution dealing with van Genuchten-Mualem model, which is directly derived from the flow equation without any predefined mathematical form of the pressure head. [ABSTRACT FROM AUTHOR]

• A novel T -form hydraulic conductivity (k w) function of frozen soils (HCFF) is proposed. • The proposed T -form HCFF is extended to consider vapor flow and bimodality. • HCFF prediction is supported by statistical k w models and thermodynamic equilibrium. • HCFF prediction is clarified as 3 routes: single SWCC, single SFCC and combining two. Hydraulic conductivity function for frozen soils (HCFF) is crucial for accurately simulating the water transfer process in cold regions, impacting hydrological states and frost heave diseases. However, the HCFF model capturing the relation between hydraulic conductivity and unfrozen water content (θ) or temperature (T) remains an open question. This study delves into HCFF encompassing both empirical and prediction models. This study begins with introducing θ -form and innovative T -form empirical HCFF models, showcasing their solid performance in fitting measured hydraulic conductivity data. Furthermore, the extension of the T -form empirical model to incorporate vapor flow and bimodal soils, through the introduction of maximum vapor conductivity and weighting factors, demonstrates a closer alignment with physical mechanisms and observed trends. In terms of predicting HCFF, the study provides a theoretical foundation through statistical hydraulic conductivity models and thermodynamic equilibrium in frozen soils. Additionally, it clarifies three routes for HCFF prediction—single Soil Freeze Characteristic Curve (SFCC), single Soil Water Characteristic Curve (SWCC), and a combination of the two—and validates their effectiveness through test results and comparison of HCFF prediction outcomes using SWCC and SFCC data from specific soil samples. In practice, the empirical and prediction models are recommended for available hydraulic conductivity data and available SFCC or SWCC data, respectively. Overall, this study not only lays the theoretical groundwork for most prediction models but also presents a valid empirical equation for modeling hydraulic conductivity function tailored specifically for frozen soils. [ABSTRACT FROM AUTHOR]

• A three-dimensional coupled model is used to quantify seepage from lined canals. • The effects of cracks on the hydraulic conductivity of lining materials are evaluated. • A quantitative relationship between hydraulic conductivity and its factors is formed. • The accuracy of empirical formula and numerical model for canal seepage is improved. The equivalent saturated hydraulic conductivity of concrete lining (K sce) is a key parameter to estimate seepage loss, while few studies have quantified the effect of cracks on K sce , potentially leading to significant errors in canal seepage estimation. A three-dimensional numerical model is employed to simulate seepage loss from canals lined with concrete, and then to obtain the value of K sce by an equivalent method. A total of 58,400 scenarios are simulated for different concrete lining and crack conditions (e.g., crack length, crack width, crack location, hydraulic conductivity of concrete (K sc), and underlying soil (K soil)), and the corresponding K sce values are estimated. Results show that the K sce value determined by the equivalent method is reliable in calculating seepage loss from concrete lined canals. K sce increases linearly with crack length with a slope C k , and C k exhibits a power function relationship with crack width, reaching a maximum value when the crack width surpasses the critical threshold, which ranges from 0.424 to 0.435 mm. When the crack width follows a log-normal distribution, the logarithmic value of C k increases linearly with the mean value of the logarithmic crack width. The influence of crack location and K sc on K sce can be negligible. The maximum increment of K sce of a crack is proportional to K soil. Considering the variation of K sce with crack parameters and K soil , equations to determine the value of K sce are established by adding the increment of K sce caused by cracks to K sc. This study establishes a quantitative relationship between K sce and crack parameters, thereby enhancing the accuracy of seepage loss calculations in lined canal under different damage status. [ABSTRACT FROM AUTHOR]

[Display omitted] • Streambed temperatures monitored for 22 months in an international watercourse. • Six methods for deriving seepage flux from temperature time series are benchmarked. • Hydraulic gradient data are utilized to evaluate the consistency of each method. The importance of the interaction between groundwater and surface water is increasingly being recognized for both understanding and managing water systems. Many efforts have been made to characterize and quantify groundwater–surface water exchange. In particular, temperature–based methods have quickly established themselves given their monetary and practical advantages. In the last 15 years, several methods for interpreting passive temperature time series measured in the streambed have been developed. Still, the benchmarking of these methods has only been carried out in specific and distinct hydrological conditions. This article aims to fill this research gap by benchmarking the performance of six commonly used methods for deriving seepage fluxes using a two-year-long temperature time series covering various meteorological and thermal conditions. This work compares three analytical methods that calculate seepage flux using the amplitude and/or phase of the temperature signals, and three numerical methods that use different schemes to inversely solve the one–dimensional heat transport equation in the streambed. The temperature measurements were made in the context of an international dispute between Chile and Bolivia over the status and use of the waters of the Silala River, Northern Chile. Flux estimations are tested against Darcy's flux derived from measured hydraulic gradient and hydraulic conductivity. Flux estimations from the benchmarked methods ranged from −0.5 to 3.5 m/d (with positive fluxes directed downwards), whereas fluxes estimated using Darcy's law ranged from 0.5 to 6 m/d. Results show that the amplitude method is the best–performing method. This method is best suited for estimating the direction of the fluxes, while the method using both the thermal amplitude and phase is best suited for monthly flux trends, and the combination of a Local Polynomial (LP) method and a Maximum Likelihood Estimator (MLE) method (LPMLEn) is appropriate for estimating flux in transient conditions. The use of heat as a tracer proved to be an effective tool for monitoring groundwater–surface water exchange in a river reach for two years, and yielded exchange flux estimates with lower point-scale variability than Darcy's law. [ABSTRACT FROM AUTHOR]

• Transport model guided conditional generative adversarial networks are proposed. • The new networks improve the characterization of groundwater contaminations. • The networks can also help to accurately identify the contaminant sources. Accurate characterization of sources and spatial distribution of contamination remains crucial in groundwater survey and remediation efforts. Nevertheless, conventional geo-statistical methods, like kriging, usually produce overly smooth concentration fields that violate the transport physics. Furthermore, the identification of contaminant sources based on kriging methods may be misleading. As a deep learning approach, Generative adversarial networks (GANs) is being increasingly employed to extract patterns and insights from intricate geospatial data. The conditional variant of GANs (CGANs), can reconstruct complete spatial data from sparse point observations. In this study, we propose a deep learning framework with CGANs for the purpose of groundwater contamination characterization and source identification. In ensure compliance with the principles of transport physics, we use a numerical transport model as the underlying prior model for training the CGANs. Through the adaptation of network architecture, the generator transforms low-dimensional random variables and observation data into concentration images, while the discriminator is responsible for extracting source information from these images. The developed framework is tested through two numerical cases with channelized random hydraulic conductivity fields, which pose a challenge due to the complexity of the concentration fields. The results demonstrate that the proposed method can give accurate results for both contaminant characterization and source identification in groundwater. CGANs excel in reconstructing concentration fields compared to traditional interpolation methods such as Kriging and inverse distance weighting (IDW). Additionally, CGANs offer the capability to estimate uncertainty and exhibit a certain level of error robustness. [ABSTRACT FROM AUTHOR]

• Hydraulic properties of tuff breccia bedrock cores were measured in the laboratory. • Relationships between hydraulic properties and weathering degree were examined. • Core shape and in situ saturated hydraulic conductivity were also determined. • The contributions of matrix and fractures to total water flow are discussed. Although recent studies have emphasized the importance of water flow through bedrock to hydrological processes in headwater catchments, information on the hydraulic properties of bedrock, required for physically characterizing water flow processes, is lacking. In this study, the saturated hydraulic conductivity, K s , and water retention curve of tuff breccia bedrock core were determined in samples collected from boreholes in a headwater catchment in northern Japan. These core-scale properties, together with core shape and in situ K s values, were used to define two groups of bedrock: C M - and C L -class (weakly to moderately weathered) and D-class (highly weathered). Little change in the volumetric water content θ of C M - and C L -class bedrock was observed in the dry range at a pressure head ψ < –200 cm, but slight changes occurred in the wet range (ψ > –200 cm), resulting in low core-scale K s values of 10–8–10–7 cm s−1. Core shape and in situ K s values much larger than the core-scale values suggested that in situ water flow is restricted to fractures, with little contribution of the matrix to total flow. In D-class bedrock, on the other hand, the change in θ with ψ was somewhat larger than in C M - and C L -class bedrock, especially in the wet range, resulting in moderate core-scale K s values of 10–5 cm s−1. These results indicated that flow in D-class bedrock is better represented by matrix flow than by preferential flow through fractures. The hydraulic properties presented herein provide a basis for analyzing subsurface water flow, including through bedrock layers. Those analyses will provide a better understanding of water flow processes in headwater catchments. [ABSTRACT FROM AUTHOR]

• GALDIT inspired, z-number based modeling applied for GW quality assessment. • Method applied for aquifers around Urmia Lake where is facing environmental problems. • Classic GALDIT, Fuzzy logic (FL) and proposed Z-number based methods were compared. • GALDIT parameters and EC considered as inputs and output for FL and Z-number models. • Z-number outperformed GALDIT, FL methods by 85% and 35%, respectively based on HSS. Groundwater (GW) quality assessment is an essential issue, especially in arid and semi-arid regions, due to its critical role in obtaining requirements for freshwater. The GALDIT method is widely recognized as a practical approach for assessing GW susceptibility, which considers six key factors, including the occurrence of groundwater (G), hydraulic conductivity of the aquifer (A), elevation of groundwater above sea level (L), distance from the shoreline (D), impact of existing seawater intrusion (I), and thickness of the aquifer (T). The GALDIT method, being an unsupervised model, has a drawback in that it depends on expert opinion for parameter ranking, leading to an increase in uncertainty. To address this uncertainty and evaluate the susceptibility of the aquifers of Urmia lake basin to saltwater intrusion, this study employed the Z-number which is a novel adaptation of Fuzzy Logic (FL). In contrast to classic FL that does not account for data reliability, the Z-number approach considers both constraints and data reliability, making it a more effective method to manage data uncertainty compared to classic fuzzy models. To illustrate this methodology, the GALDIT parameters (inputs) and Electrical Conductivity (EC) values (outputs) were employed to determine the vulnerability of aquifers. Additionally, the GALDIT method was utilized as a benchmark model to evaluate the inherent vulnerability of aquifers, and its outcomes were compared with those of the proposed model. Upon analyzing the results, it was found that the Z-number Based Modeling (ZBM) not only outperformed the GALDIT method but also enhanced the quality of outcomes by 85% and 35% based on Heidke Skill Score (HSS) and Total Accuracy (TA) criteria compared to the classic FL. [ABSTRACT FROM AUTHOR]

• Critical rainfall for recharging flat sandy lands is 15 mm in hyper-arid desert. • Lateral gravity water flow and surface flow on sandy slopes were identified. • Water flow on sandy slope improves the recharge rate of deep soil from rainfall. • Local rainfall has the capacity to sustain the development of Badain Jaran Lake. • Extreme rainfall does not play a decisive role in the sustaining of desert lake. To elucidate the slope hydrology and their influence on the regional water balance and desert lake development, water migration and soil water conditions were studied in the megadunes of Badain Jaran Desert. Simulated rainfall of 15–60 mm with infiltration depths of ∼ 0.3–0.5 m, the lateral water migration ratios of ∼ 52.9 %-86.2 % and the differential distribution of tracers with depth in the flat sand lands indicate that 15 mm rainfall is the critical rainfall for effective recharge of deep soil water and that there is a shift from piston flow to preferential flow of water during infiltration. Soil micro-bedding and 4.2 % soil gravity water from locally wet surface soils and 6 mm of natural rainfall resulted in lateral migration of the tracer ∼ 1.2 m and 0.4 m in the slope surface and soil respectively, indicate surface flow and lateral gravity water flow resulting from differences in hydraulic conductivity and water-holding capacity of the soil micro-bedding. Soil water storage at 0–4 m depth showed that the bottom of the slope was ∼ 30 mm higher than the middle of the slope after the rainy season and ∼ 9 mm higher before the rainy season; in general the post-rainy season was ∼ 30–40 mm higher than the pre-rainy season. Surface flow and lateral gravity water flow in unsaturated soils increase the intensity of lateral water movement on the slope and improve the effective conversion rate of rainfall to deep soil water, especially below 15 mm. Annual recharge of deep soil water, assessed using soil water change and critical rainfall, will not be less than 8.5 mm and may even reach an upper limit of ∼ 30–40 mm. The megadune slopes respond strongly to local precipitation, which is sufficient to maintain the lake, but extreme precipitation does not play a decisive role. The methods and results provided in this work can advance our understanding of the desert hydrological cycle and water resources. [ABSTRACT FROM AUTHOR]

• Natural internal redistribution reduces overall soil evaporation following surface wetting. • A new analytical framework quantifies surface evaporation rates with concurrent redistribution. • Experiments reveal nonlinear relations between evaporative losses and initial water content. Under natural conditions, infiltrated rainwater redistributed into the soil profile, becomes partially sheltered from surface evaporative losses relative to undrained wet surfaces. Surface evaporation rates and internal drainage dynamics are both sensitive to soil hydraulic properties and initial soil hydration state. We report a novel analytical framework for quantifying surface evaporation rates with concurrent internal drainage, considering soil properties and dynamic surface evaporation resistance. Interplay among soil hydraulic properties, boundary conditions, and atmospheric demand result in nonlinear relationships between cumulative evaporative losses and initial water content. The new analytical solution shows good agreement with laboratory column experiments and literature data. Results highlight the role of soil texture on evaporative losses and the nonlinear effects of initial wetness on evaporation. The largest evaporative losses per rainfall amount are predicted for small rainfall events over initially dry loamy soils due to the detention of infiltrated rainwater near the surface for soils with large porosity and low hydraulic conductivity. The analytical framework uses readily available parameters hence can be implemented in land surface models to explicitly consider how soil properties and rainfall event size affect surface evaporation dynamics. [ABSTRACT FROM AUTHOR]

• Hydraulic and pneumatic tomography studies in fractured media are reviewed. • Field, laboratory, and synthetic studies concerning fractured media are summarized. • Continuum and discrete fracture network modeling approaches are compared. This work reviews various studies of hydraulic and pneumatic tomography for estimation of flow properties of fractured geologic media with hydraulic and pneumatic tomography. The underlying conceptual inversion models can be broadly classified as continuum and discrete fracture network models and deterministic and stochastic approaches. A heterogeneous continuum method applies porous media parameters, while a DFN approach utilizes structural and hydraulic properties of fractures. An overview of field, laboratory, and synthetic studies with applications of hydraulic, pneumatic, or tracer tomography for the characterization of fractured geologic media shows that most studies rely on a heterogeneous continuum conceptual model and geostatistical methods to achieve a solution to the inverse problem. The application of a heterogeneous continuum model results in hydraulic properties that are representative of both fracture and matrix. Therefore, this approach may be more operationally useful for large scale sites with a non-negligible hydraulic conductivity of the rock matrix and high fracture intensity. The flow properties of single fractures can be estimated by applying a discrete fracture network (DFN) model. However, assumptions concerning fracture patterns and corresponding flow properties can lead to an oversimplified geological model. Possibilities for future research include integrating additional data and results from other inversion methods, the application of neural networks for inversion, the implementation of inversion results for the prediction and optimization of processes according to the planned application at the site, and opportunities for real-time inversion. [ABSTRACT FROM AUTHOR]

• A low dimensional lateral subsurface flow model in hillslopes. • Development of a kinematic wave model describing percolation effect. • The proposed model can reproduce percolation effect calculated by the Richards equation. Kinematic wave (KW) models are commonly used for flood forecasting in large river basins, but have limitations in their ability to simulate percolation effects. Here, we suggest an extended KW model wherein the vertical pressure head is under non-hydrostatic conditions. The proposed model uses the Brooks Corey and Mualem models to estimate the water retention curve and unsaturated hydraulic conductivity and adopts the pressure head gradient as a new variable to calculate runoff based on changing water content distributions over time due to percolation. The increase in the pressure head gradient with rainfall infiltration is determined from the relationship between water storage and pressure head distribution, while the decrease in pressure head gradient due to percolation is modeled as exponential decay with time. A comparison of the proposed KW model and a numerical solution of the Richards equation shows that the proposed KW model can reproduce the storage effect caused by percolation, delaying runoff and reducing peak discharge. The agreement between the proposed KW model and the Richards equation is high for soils with high water retention. Conversely, the agreement becomes lower for soil with low water retention or deep soil layer thickness, where the complexity of water content distribution requires further refinement. The proposed model can easily be incorporated into distributed models while low computational cost with keeping the important percolation effects. [ABSTRACT FROM AUTHOR]

• Roles of recharge rate and flow regime on conduit flow are quantified. • Roles of conduit properties, density and morphology on spring discharge. • The percentage of conduit flow over spring discharge is quantified for Silver Springs. The physical controls on the contribution of conduit to spring discharge, quantified as the ratio of conduit flow to spring discharge (Q c / Q), are evaluated through numerical simulations using MODFLOW-Conduit Flow Process. The controlling factors include the ratio of conduit surface roughness to conduit diameter (ε / D), the hydraulic conductivity ratio between the conduit wall and the porous matrix (K c / K), the ratio of groundwater recharge to the hydraulic conductivity of the porous matrix, and conduit density (C d) defined as the ratio of total conduit length to springshed area. For a single conduit, Q c / Q increases with K c / K when flow in the conduit is laminar. When flow is turbulent, Q c / Q is stable and then decreases with ε / D , increases with K c / K , and decreases with R / K due to the limited exchange capacity between matrix and conduit (i.e., small K c / K). The results of hypothetical conduit networks show that Q c / Q increases with C d but is not sensitive to the two sets of network structure evaluated. The sensitivity analysis for the Silver Springs in Florida shows that the spatial heterogeneity of conduit diameter causes higher Q c / Q , and Q c / Q is most sensitive to the conduit density, followed by R / K , and then K c / K. [ABSTRACT FROM AUTHOR]

• A modelling approach to study the fate of infiltrated stormwater. • Centralized infiltration systems increase stream baseflow. • Distributed infiltration systems bring microclimate benefits to urban landscapes. • Important implications to design and spatial arrangement of infiltration systems. Urbanization inevitably involves the replacement of native soils with impervious areas. Doing so reduces infiltration and thus the contribution of groundwater to stream baseflows. The use of infiltration-based Stormwater Control Measures (SCMs) is increasingly common to restore lost baseflows in urban areas, although there remains considerable uncertainty regarding the optimal arrangement of such measures in the landscape, along with the influence of site conditions on the path and fate of infiltrated stormwater. This study aimed to investigate how site conditions and arrangement of infiltration-based SCMs influence the water cycle and stream baseflows. To undertake this study, we used a modelling approach which combined the outputs from a stormwater model (MUSIC) with a 3D groundwater flow model (MIKE SHE). We simulated a range of plausible site conditions and spatial arrangements of SCMs for a hypothetical hillslope situated in two different locations in Australia—Gold Coast (sub-tropical climate) and Melbourne (temperate climate). The modelling results predicted that placing the SCMs in a centralized way at the bottom of the hillslope (End of the pipe approach) could deliver more stream baseflows compared with a distributed network of systems. Placing the SCMs in a distributed manner throughout the landscape was predicted to deliver local benefits such as enhanced evapotranspiration and augmented soil moisture. In addition, we found that soil hydraulic conductivity played a major role in the fate of infiltrated stormwater for all scenarios. Our results highlight the importance of placing infiltration-based SCMs appropriately in the landscape if the goal is baseflow restoration. They also point to the importance of understanding local site conditions before designing and siting stormwater infiltration systems. A mix of different SCM strategies (e.g. rainwater tanks with a dedicated baseflow outlet) will be required for sites with very low hydraulic conductivities. [ABSTRACT FROM AUTHOR]

Space-temporal distribution of the contaminant plumes and aquifer properties is critical for groundwater management. However, most previous studies have focused on point source identification, barely exploring the identification of non-point sources. Xu et al. (2022) proposed to identify non-point sources but did not consider uncertainties in aquifer properties and release mass loading. In this work, we have implemented an application of the localized ensemble smoother with multiple data assimilation (LES-MDA) for the simultaneous identification of Gaussian hydraulic conductivities and non-point source parameters including Gaussian release mass-loading by assimilating both piezometric head and concentration observations in a synthetic confined aquifer. The results prove that the LES-MDA is not only capable of providing accurate identification of the spatial architecture of non-point contaminant sources and related release parameters (such as initial release time, and release duration) but also spatially heterogeneous release mass-loading and hydraulic conductivities. • Localized ES-MDA jointly identifies non-point source and conductivity field. • Uncertainty in the spatial distribution of field and mass-loading is considered. • Three scenarios with two different techniques test the joint identification. • Combining localization reduces filter inbreeding and spurious correlation. [ABSTRACT FROM AUTHOR]

This study applies two hybrid inversion frameworks, GAN-ILUES and GAN-OANW-ILUES, to accurately estimate the line contaminant source information and hydraulic conductivity in non-Gaussian groundwater fields. These frameworks combine data assimilation and deep learning methods, using WGAN-GP as a generator model for non-Gaussian hydraulic conductivity field and a CNN-based surrogate model, OANW, for surrogating the original forward groundwater model. The effectiveness and feasibility of WGAN-GP and OANW are verified separately. Then, the frameworks are verified on a 64 m by 64 m channelized aquifer with a linear contaminant source using measurement data from 20 observation wells. Results show that both frameworks can estimate model parameters and reproduce the contaminant concentration field. Compared with GAN-ILUES, GAN-OANW-ILUES with OANW surrogate model significantly improves simulation efficiency at a price of slightly larger model deviations. The inversion time for GAN-OANW-ILUES is less than 5% of that for GAN-ILUES. This study demonstrates the potential of combining data assimilation and deep learning methods for improving the performance of traditional inversion methods on tracing line contaminants in non-Gaussian fields. • Two deep-learning methods are integrated with data assimilation for inverse modeling of groundwater pollution. • The proposed modeling framework successfully traces the line contaminant source in non-Gaussian conductivity fields. • Model error slightly increases when the conductivity changes drastically. • The combined inversion framework significantly enhances inversion efficiency. [ABSTRACT FROM AUTHOR]

Current groundwater models face significant challenges in their implementation due to heavy computational burdens. To overcome this, our work proposes a cost-effective emulator that efficiently and accurately forecasts the impact of abstraction in an aquifer. Our approach uses a deep neural operator (DeepONet) framework to learn operators that map between infinite-dimensional function spaces via deep neural networks. The goal is to infer the distribution of hydraulic heads in a confined aquifer in the presence of a pumping well. We successfully tested the DeepONet framework on multiple problems, including forward time-dependent problems, an inverse analysis, and a nonlinear system. Additionally, we propose a novel extension of the DeepONet-based architecture to generate accurate predictions for varied hydraulic conductivity fields and pumping well locations that are unseen during training. Our emulator's predictions match the target data with excellent performance, demonstrating that the proposed model can act as an efficient and fast tool to support a range of tasks that require repetitive forward numerical simulations or inverse simulations of groundwater flow problems. Overall, our work provides a promising avenue for developing cost-effective and accurate groundwater models. • We propose, for the first time, a neural operator (DeepONet) as a surrogate model to efficiently and accurately emulate the impact of abstraction in the distribution of hydraulic heads in confined, heterogeneous aquifers. • The proposed method can efficiently learn solutions of both the forward and inverse operator. • By proposing a modification to the original formulation of DeepONet, we have achieved precise predictions for varied groundwater head distributions, accommodating the random positioning of wells in the heterogeneous aquifer. • Our experiments demonstrate DeepONet's capability in tackling tasks demanding repetitive forward numerical simulations, even in time-dependent and nonlinear setups. [ABSTRACT FROM AUTHOR]

• High-permeability and water-rich zones were identified by the hydraulic tomography. • Strong groundwater runoff paths were inferred for the water source area. • The hydraulic tomography was improved with various prior information. • THT can better reveal larger-scale heterogeneity of aquifers compared to SSHT. • Hydraulic tomography holds valuable applications in groundwater exploitation plans. Determining the spatial hydraulic parameters of aquifers is of paramount importance for the development and utilization of groundwater in water source areas. However, most methods are unable to accurately characterize the heterogeneity of aquifers because of the influences of complex geology, hydrogeology, structure and insufficient information. Hydraulic tomography (HT) is an effective and robust method for hydraulic parameter inversion to characterize heterogeneity. In this study, a two-dimensional HT model was established to simulate hydraulic parameter fields for a fractured karst aquifer in the Zhangji water source area (Xuzhou city, China). Steady-state hydraulic tomography (SSHT) and transient hydraulic tomography (THT) with different prior information conditions were applied to determine the high-permeability area and water-rich zones, and to infer possible locations of karst conduits and strong runoff paths in the water source area. The results indicate that: (1) THT can better reveal larger-scale heterogeneous characteristics of hydraulic properties compared to SSHT. The hydraulic conductivity distribution estimated by THT correlates well with the geological conditions of the area, especially in identifying water-rich zones formed by faults and other formations. (2) When combined with geological conditions, HT possesses the ability to identify strong runoff paths in fractured karst aquifers. (3) Accurate prior information, which is more relevant to the head response information, is conducive to obtaining more acceptable HT results. (4) THT can simultaneously identify high-permeability and high-storage zones, and the analysis of the overlap of these two regions has the potential to predict underground water reservoirs, which is of the utmost importance for the development and utilization of groundwater resources in water source areas. [ABSTRACT FROM AUTHOR]

• Green-Ampt model under muddy water conditions was developed. • A model describing the variation of hydraulic conductivity of infiltration in saturated soil columns under muddy water conditions was developed. • The established Green-Ampt model under muddy water conditions was simplified and validation tests showed it to be very reliable. The Green-Ampt infiltration model has been widely used because of its simple structure and clear physical meaning of the parameters. The model parameters were improved using the equivalent saturated hydraulic conductivity to develop the Green-Ampt infiltration model for muddy water (MWGA). A saturated soil column infiltration test with muddy water was conducted to determine the influence of the soil texture (sandy soil and silt loam) and the sand content (S = 3 %, 6 %, 9 %, and 12 %) on the hydraulic conductivity of saturated soil columns. The equivalent saturated hydraulic conductivity was derived based on several assumptions and under different conditions. Eight one-dimensional vertical infiltration tests were conducted under the same conditions for verification. The MWGA was simplified according to the results, and four tests with different sand contents (S = 4 % and 10 %) were conducted to validate the MWGA for both soils (the simplified infiltration model was used for the sandy soil). The results indicated that the hydraulic conductivity trend of the saturated soil column during the infiltration of muddy water into sandy soil was in accordance with the physical model. The hydraulic conductivity of the silt loam was only influenced by the sand content and showed a significant linear relationship with the sand content. The root mean square error (S RMSE) obtained from the validation test under the same conditions was close to 0, and the absolute value of the relative error (S RE) was less than 15 %, indicating a good fit. The results of the validation tests for different sand contents showed that the simulated and observed values were close to the 1:1 line, demonstrating high model accuracy. Our results provide new insights into the infiltration of muddy water and the mechanism of reducing the infiltration rate due to the dense layer formed on the soil surface. [ABSTRACT FROM AUTHOR]

• We study pumping optimization for both fixed well cases and moving well cases. • For fixed well cases, we derive an analytical solution for the critical pumping rate. • For moving well cases, Bayesian optimization is employed. • The critical pumping rate of a well network is independent of hydraulic conductivity. • In moving well cases, different solutions can achieve the same maximum pumping rate. For residents living on small islands, groundwater serves as their primary source of freshwater, which exists in the form of a freshwater lens floating above denser seawater. Previous research on optimizing pumping strategies in freshwater lenses focused on pumping from fixed wells using numerical models, and optimal well locations have not been explored. In this study, we analyze pumping optimization for both fixed well cases and moving well cases in a rectangular island using an analytical approach. This approach allows for a clear understanding of the relationship between hydrogeological parameters and optimization results. For fixed well cases, we provide an analytical solution for the critical pumping rate of each well. For moving well cases, Bayesian optimization is employed to efficiently search the extensive space of possible well locations and identify the critical pumping rate. Our results reveal that the well location has a substantial impact on the critical pumping rate in the fixed well cases, indicating that investigating the optimization of well placement is essential. In the discussion of the moving well cases, the maximum total pumping rate of a pumping well network is independent of hydraulic conductivity, and the critical pumping percentage relative to total recharge is independent of both hydraulic conductivity and recharge. The distribution of optimized well locations for islands with a fixed aspect ratio but different sizes follows the same pattern. In the moving well cases, different solutions can achieve the same maximum pumping rate, especially when multiple wells are involved. In addition, our approach is applicable to scenarios where new wells are added to an existing well system. Our solutions and findings significantly contribute to a deeper understanding of the hydraulic performance of pumping activities from freshwater lenses and offer valuable insights for developing more sustainable and robust groundwater management strategies in small islands. [ABSTRACT FROM AUTHOR]

• The hyporheic exchange is positively correlated with hydraulic conductivity, with K near the dune trough matters the most. • With limited sampling of K , the high cross-correlation zone is the preference of sampling location of K. • The optimal sampling location of hydraulic conductivity can be estimated by a linear formula on height and stoss length of dunes. The hyporheic exchange is not evenly influenced by heterogeneous hydraulic conductivity (K) values throughout the streambed. Few studies have investigated the impact of K heterogeneity in different regions of the streambed on the hyporheic exchange. This study employed a first-order stochastic analysis and a dislocation superposition method to examine the cross-correlation between hyporheic exchange flux (HEF) and K anywhere in synthetic, heterogeneous, 2-D cross-sectional streambed models. Results show that hyporheic exchange flux positively correlates with K heterogeneity in every part of the streambeds. This relationship provides a mechanistic explanation for the impact of high / low K zones on the HEF in the hyporheic zone. The effect of K impact on the HEF intensifies in streambeds when 1) upwelling groundwater flux increases, 2) the dunes are steep and asymmetric, or 3) the streambed is stratified. This study develops an empirical linear formula based on dune height and stoss length for estimating the high cross-correlation zone (HCZ) locations, potentially guiding the sampling scheme of hydraulic conductivities for evaluating the HEF in practical fieldwork. [ABSTRACT FROM AUTHOR]

• 3D pathways of IGF are identified with an assessment on flow rates. • IGF contributes 36%∼59% to the baseflow and 32%∼52% to the streamflow in the studied river. • Infiltration recharge and aquifer properties affect the development of IGF. Inter-basin groundwater flow (IGF), defined as groundwater flows across the topographic divides, occurs in nature as a common phenomenon, and seriously affect water balance and solute transport in a basin. Although the existence of IGF has been checked in drainage basins with indirect methods, such as using information of hydrogeochemistry, few studies are conducted to directly identify and quantify the IGF in the three-dimensional (3D) space using numerical modeling of groundwater flow. In this study, we identify, partition and quantify IGF based on a regional-scale 3D groundwater flow model covering several traditional surface water basins. The investigation is focused on the Hailiutu River, a groundwater-fed stream in the Ordos Plateau, China. It is the first time that the source zones and 3D paths of IGF as well as its contribution to the streamflow are fully captured for a groundwater-fed stream. Results indicate that the IGF contributes 36%∼59% to the baseflow and 32%∼52% to the streamflow of the Hailiutu River, through different groups of circulation paths with the maximum depth up to ∼ 1000 m. Uncertainty analyses of the model show that the contribution of IGF to the streamflow is positively correlated to the infiltration recharge and hydraulic conductivities of the aquifer media. Several indirect methods are also checked for comparison, but they could only be used to assess the potential contribution of IGF, not the IGF paths. This study will facilitate in deeply understanding the evolution mechanism of the inter-basin groundwater circulation and the interaction between surface water and groundwater. [ABSTRACT FROM AUTHOR]

• Analytical solution for water release of a nonlinear consolidated aquitard is derived. • A new method was developed to estimate the hydraulic parameters of the aquitard. • The proposed method was applied to aquitards in Changzhou and Shanghai. Estimation of hydraulic parameters and groundwater depletion of aquitards is necessary to evaluate the groundwater resources and land subsidence. In this study, an analytical solution for water release from a nonlinear consolidated aquitard was derived by approximating the drawdown history as a continuous piecewise linear drawdown function (PLD) with several stages, while the water level in adjacent aquifers varies arbitrarily over time. According to dimensionless and parameter sensitivity analyses, groundwater depletion of an aquitard increases with an increase in compression index, aquitard thickness, and water drawdown rate in adjacent aquifers, decreases with an increase in initial void ratio and effective stress, and is not affected by the consolidation coefficient. In addition, we proposed a type-curve fitting method to calculate the hydraulic parameters of the aquitard under the condition of PLD in adjacent aquifers. The proposed method was applied to estimate the hydraulic parameters of aquitards at two field sites in the Yangtze River Delta in China, i.e., Changzhou and Shanghai. Results show that the compression index of the aquitards in Changzhou and Shanghai are 0.074 and 0.24, respectively, while the consolidation coefficients are 7.62 × 10-7 m2/s and 2.16 × 10-6 m2/s, respectively. The hydraulic conductivity and specific storage estimated by the proposed method are in good agreement with previous studies and the results of the geological method of Konikow and Neuzil (2007). The proposed analytical method for estimating hydraulic parameters and groundwater depletion of aquitards will contribute to the sustainable management of groundwater resources in multi-layer aquifer systems. [ABSTRACT FROM AUTHOR]

• Impacts of wall porosity on fracture flow were systematically investigated at pore scale. • Mechanisms governing direction and magnitude of fluid exchange at the fracture-matrix interface were elucidated. • Flux contributions of porous walls to fracture outlet and fracture-matrix interface were functionalized by wall porosity. • Influence of porous walls cannot be equivalently represented by applying slip boundary conditions. This study investigates the influence of porous walls on fluid flow behavior in single rough-walled fractures. A series of direct numerical simulations (DNS) were conducted to examine fluid flow through rough-walled fractures surrounded by porous walls under various hydraulic gradients. The simulation results clearly demonstrate the progressively significant impact of porous walls on flow characteristics as porosity increases. Regression analyses indicate that the Forchheimer's law, which characterizes the nonlinear flow of rock fractures, remains valid in the presence of porous walls. Moreover, increased porosity enhances fluid exchange and leads to an increased ability for single fractures to transmit fluid. Two double-parameter equations were established to quantitatively characterize the effects of porous walls on hydraulic conductivity of the fracture and fluid exchange between porous walls and the fracture. Further analysis revealed that the angle between the fracture profile and main flow direction profoundly influences the inflow/outflow exchange between the fracture and its adjacent porous walls. Moreover, the pore-scale results indicate that it is unreasonable to equate the effect of porous walls with slip boundaries as suggested in previous studies. The results and findings of this study deepen our understanding of flow behavior at the interface between the rough-walled fracture and adjacent porous walls, and they serve as a basis for accurately simulating large-scale fracture networks that consider the hydraulic contribution of fracture walls. [ABSTRACT FROM AUTHOR]

• Correction methods were developed to remove wellbore effects from slug test data. • Successful characterization of synthetic aquifer heterogeneity using corrected data. • A field application was performed at a fractured porous media site. Slug tests provide an efficient way to obtain information about the spatial heterogeneity of an investigated site. Combining with the imaging method of hydraulic tomography, they can present broad application prospects in aquifer reconstruction. However, unlike pumping tests, the water level responses of slug tests may be significantly affected by wellbore effects, especially for underdamped cases. This study proposes a new characterization framework for characterization of aquifer heterogeneity based on water level responses recorded during cross well slug tests with a tomographical arrangement. The peak head of each water level response and its corresponding time, i.e. the hydraulic travel time, are utilized in the inversion process, rather than the whole water level time series. To eliminate the influence of wellbore effects, two correction methods with respect to the hydraulic travel time and peak head amplitude are developed. Based on corrected data, the distributions of hydraulic diffusivity (D) and specific storage (S s) are obtained through travel time-based and attenuation-based inversions, respectively. The new framework is verified by a series of simulated tomographic slug tests through a synthetic example based on an aquifer outcrop analog, where the hydraulic parameter distribution is known. The verification results indicate that the distributions of D and S s are well estimated, and their mean values are much improved compared to the old framework. With the same method, a field application is employed at a fractured rock experimental site located in Göttingen, Germany. Three potential transmissive fractures are revealed with hydraulic conductivities varying from 6.9 × 10–5 to 3.4 × 10–4 m/s. The uncertainty of the inversion results is discussed for future studies and related applications. [ABSTRACT FROM AUTHOR]

• Random forest model is trained on hydrogeological dataset for aquitards. • Dominant factors are clay fraction, depth, geology, sedimentology and location. • Detailed grain size fractions and grain size percentiles do not improve the model. Aquitards are common hydrogeological features and their hydraulic conductivity is an important property for various groundwater management issues. Predicting their hydraulic conductivity proves challenging, given its dependence on numerous variables. In this study, the dominant factors for predicting aquitard hydraulic conductivity are identified. To this end, a random forest model is trained on a dataset consisting of more than 1000 hydraulic conductivity measurements of core-scale sediment samples from a wide range of stratigraphic units and depths in the Netherlands. The dataset contains textural properties, such as the grain size distribution and porosity, as well as structural data, such as location, sampling depth, stratigraphical unit, lithofacies, organic carbon content, carbonate content and groundwater chloride concentration. Results show that clay fraction, stratigraphic unit, depth, lithofacies and x-coordinate are the most important features for predicting the hydraulic conductivity. Here, x-coordinate is presumably a proxy for distance from marine influence. Using a more detailed grain size distribution or using derived parameters such as the grain size percentiles does not improve the model any further. Our findings indicate that structural properties play a significant role in predicting aquitard conductivity, as they serve as indicators of processes such as compaction and soft-sediment deformation. The model is furthermore an effective method to estimate hydraulic conductivity for sediment samples without conducting costly and time-consuming hydraulic conductivity measurements. [ABSTRACT FROM AUTHOR]

• The effect of tunnel drainage in heterogeneous media is significantly different from the homogeneous model, and it aligns more closely with reality. • The water discharge distributes unevenly along the tunnel in heterogeneous realizations and is relatively higher compared to that in homogeneous media. • The drawdown caused by tunnel drainage is higher in homogeneous model, while large drawdown in specific small regions is observed in heterogeneous simulations. The mountainous tectonic zone exhibits high formation heterogeneity, which significantly affects groundwater seepage around the excavated tunnel. Traditional modeling methods that assume the surrounding rock to be a homogeneous field face challenges in accurately predicting and evaluating water discharge within the tunnel. To gain a better understanding of how heterogeneity influences tunnel water discharge, synthetic heterogeneous fields are conducted based on the conceptualization and parameters of the Daxuecheng Tunnel in Chongqing, southwest China. In the heterogeneous modeling, we consider four standard deviations of hydraulic conductivity distribution (0.21, 0.37, 0.55, 0.72) and four dip angles (0°, 30°, 60°, 90°). The results reveal that formation heterogeneity leads to localized instances of significant water inrush and irregular head drawdown distribution, which closely resemble the actual situation compared to the homogeneous model. Generally, the total water discharge into the tunnels increased with increasing heterogeneity and dip angles. Furthermore, the head drawdown in the tunnel region decreased with heterogeneity and increased with dip angles. These findings emphasize the importance of considering formation heterogeneity in tunnel zones and offer valuable guidance for evaluating engineering and environmental challenges in tunnel construction. [ABSTRACT FROM AUTHOR]

• Groundwater mounds resulting from nearshore recharge is identified in a sandy hillslope. • The thickness of vadose zone determines fluxes across water table along the hillslope. • Nearshore recharge temporarily changes streamlines and increases tortuosity of flowpaths. • Flowpaths near the wetland are mainly controlled by regional GW discharge and strong ET. Groundwater mounds and depressions induced by rainfall and evapotranspiration near wetlands/lakes have been widely observed, but their effects on groundwater flow field are poorly known. Here, a sandy hillslope near a wetland in the Ordos Plateau, China, which has a semi-arid climate, is selected for investigation. After identifying temporary groundwater mounds/depressions based on three water level observation wells, a HYDRUS-2D model is setup to obtain the highly transient daily flow field. The variable thickness of unsaturated zone near the wetland, which controls soil water content and unsaturated hydraulic conductivity, is found to be responsible for groundwater mounds resulting from focused recharge and groundwater depressions resulting from nonuniform evapotranspiration. On nonrainy days, evapotranspiration in the lowland is supported by flowpaths from regional groundwater discharge, and in the slope is supported by a flow cell from the upland to the upper part of the slope, which causes water table recession in the upland. On rainy days, focused recharge in the lowland leads to two temporary local flow cells, which break the regional groundwater discharge flowpaths into two temporary flow cells. The two temporary flow cells toward the upland contribute to water table rises in the uplandMoreover, we find focused recharge in rainy days does not change the overall direction of flowpaths controlled by regional groundwater discharge and strong evapotranspiration, but only increase the tortuosity of flowpaths. By coupling daily atmospheric conditions and regional groundwater discharge, this study has implications for groundwater recharge estimation and future groundwater modelling in arid regions. [ABSTRACT FROM AUTHOR]

• First-time examination of the effect of aquifer anisotropy on dam efficiency. • Higher hydraulic conductivity anisotropy enhances fresh groundwater discharge. • Optimizing dam design for environmental mitigation in anisotropic coastal aquifers. Hydraulic conductivity is a key parameter controlling groundwater flow and solute transport, and anisotropy in hydraulic conductivity is common in aquifers. Previous studies have investigated the effectiveness of subsurface dams in combating seawater intrusion (SWI) and associated environmental concerns. However, the quantitative impact of hydraulic conductivity anisotropy on the performance of subsurface dam has not been explored. This study goes beyond previous research by examining, for the first time, the effects of subsurface dams on SWI prevention in anisotropic settings. Furthermore, the study optimized subsurface dam designs in anisotropic coastal aquifers through numerical simulations that take into account the environmental impact. Our findings reveal the mechanism of hydraulic conductivity anisotropy's influence on the discharging zone and demonstrate that as the anisotropy ratio (r k) decreases, the coastal discharging zone expands landward, reducing SWI and ultimately lowering the minimum effective dam height (MEDH). This process effectively prevents SWI while maximizing fresh groundwater discharge. Increasing the r k values simultaneously expands both the MEDH and the peak fresh groundwater discharge (PFGD). Additionally, increasing the horizontal hydraulic conductivity of the aquifer significantly increases the range of PFGD, while the distance between the dam and shoreline emerges as the main factor influencing the extent of the MEDH. [ABSTRACT FROM AUTHOR]

• DNAPL accumulations indicate subtle vertical hydraulic conductivity contrasts. • shallow, fractured sedimentary rock aquitards are moderately to highly anisotropic. • bulk K h may not identify thin aquitards typical in shallow sedimentary rock systems. • fracture termination horizons can create poor vertical hydraulic connectivity. Aquitard assessment poses a challenge for practitioners and researchers managing dense non-aqueous phase liquid (DNAPL) contaminants in sedimentary rock aquifer systems. Within these systems, DNAPL migration and bulk hydraulic properties are controlled by fracture networks which are difficult to characterize at relevant scales with conventional methods. A previous study at a DNAPL contaminated site used contrasts in high-resolution vertical gradients to delineate four aquitards and two surfaces with poor vertical connectivity within a thick (>200 m) sedimentary rock aquifer system primarily composed of layered sandstones. In this study, we corroborate that method by using the existing DNAPL contamination as a tracer to identify and characterize the aquitards in the sequence and assess aquitard integrity. Closely spaced samples of rock from four continuous cores were collected and analyzed for volatile organic compounds (VOCs). The cores were collected along a transect perpendicular to groundwater flow and 75 m downgradient of the DNAPL source zone. Dissolved phase contaminant concentrations were collected using high-resolution multilevel systems placed along the plume centerline, downgradient from the source. The rock core and groundwater VOC concentration profiles suggest a primary aquitard limits vertical migration of both DNAPL and the associated dissolved phase contaminants, despite relatively large, downward gradients across its lower boundary. A calibrated, three-dimensional numerical groundwater flow model indicates an anisotropy factor of 1,000 for this aquitard. The K v values for all four aquitards are only slightly lower than the values for the aquifers. However, the aquitard K v values are 1–3 orders of magnitude less than their K h values, and the aquitard K h values are often similar to or even greater than the K h values for the aquifers. Therefore, large anisotropy, rather than exceptionally low K v values, distinguish the aquitards from the aquifers in this system. Fracture network characterization of nearby outcrops showed the anisotropy of the aquitards can be attributed to laterally extensive, bedding parallel partings coupled with short, frequently terminating, and poorly connected vertical fractures. In addition, the rock VOC profiles combined with fracture network data show that fracture termination surfaces can be powerful inhibitors of downward DNAPL migration providing additional evidence for the importance of these features in sedimentary rock flow systems. Ultimately, this study reveals the important influence of anisotropic aquitards on DNAPL migration, accumulation, and persistence, as well as associated plume migration. Furthermore, it demonstrates that subtle contrasts critical to aquitard integrity may be easily missed with long screened wells and other conventional, low resolution site characterization methods. [ABSTRACT FROM AUTHOR]

• Transience of a variable is treated as 1D spatial heterogeneity for optimization. • Transience of streambed K can be identified via geostatistical inverse modeling. • The new methodology is validated through synthetic and field data investigations. Quantitative investigation of the interaction between surface water and groundwater relies on the determination of transience of streambed hydraulic conductivity (K sb). The commonly used successive inversion approach (i.e., the inversion is repeated over successive time steps) of interpreting aquifer hydraulic response data belongs to different optimization windows consists of a sizable number of inversion sub-processes and tends to lose sharp changes in K sb. This study treats the transience of a parameter in the time domain as the heterogeneity in 1D space domain and uses geostatistical inverse modeling to characterize such transience. The methodology is first tested through a group of ten synthetic cases with independently generated random transient patterns of K sb. Results of these synthetic cases reveal the capability of geostatistical inverse modeling to effectively characterize the general varying pattern as well as sharp changes in K sb. Subsequently, the methodology is applied to interpret the aquifer hydraulic response data measured at the Rhône River site, Town of Fully, Switzerland. A significant improvement in model calibration is observed when the methodology is compared to the successive inversion approach. The field investigation reveals that the transience of in-situ K sb is closely related to stream stage variations. Particularly, multiple occurrences of sharp increases and decreases in K sb appear when the stream stage lowers below and rises above the aquifer head. The driving mechanism for these sharp variation events is that fine-grained sediments within the streambed can be partly washed away when the aquifer water flows toward the river, and vice versa. The geostatistical approach has the potential to help investigate other physical or chemical processes in groundwater hydrology associated with transient changes in K sb. [ABSTRACT FROM AUTHOR]

• Advective travel time along streamlines is analytically found for phreatic aquifers. • The Laplace/Poisson equations are solved in polygon's exterior/interior. • The water table, flownet, hydraulic gradients, and discharge streamlines are obtained. • Ecohydrology of parks and desert oases in arid zones is discussed. • Isoperimetric problems travel time – park's area are formulated. Strack's (1984, 2017) analysis of infiltration from a pond into a shallow aquifer, with a generated groundwater mound, is modified for steady phreatic groundwater flows in perched aquifers, typical for arid/hyperarid environments, with a zone of intensive transpiration and induced groundwater trough. The interior of an urban phreatophytic park is an n -polygon (in an aerial view), which degenerates into a circle at n → ∞. Transpiration acts as an areal groundwater sink. The water table drawdown is hydrologically symmetric with Strack's drawup. In the exterior of the park, the ground surface is phyto-sterile (e.g. a desert in Arabia or a paved megapolis) and evapotranspiration from the water table there is ignored. The travel (residence) time of advected marked particles along the flowpaths (streamlines) towards the park and under it are analytically evaluated. In terms of the Dupuit-Forchheimer model of a vertically averaged hydraulic head, the Strack potential obeys the Laplace and Poisson equations in polygon's exterior and interior (correspondingly), with a hydraulic commingling along the park boundary. The tracer particles start their topologically converging journey from a remote circular zone of contamination (e.g. sewage from urban septic tanks recharging the water table). Porosity, hydraulic conductivity of the aquifer, evapotranspiration rate and a piezometric head in a remote observational well are given. For a circular park flow is 1-D radial. In the park exterior, the Schwarz-Christoffel formula maps conformally the physical domain (an infinite trigon) onto a half-strip in the complex potential plane. Strack's potential is expressed via hypergeometric functions, whose parameters depend on n. The flow net is reconstructed with the help of the stream function. For a triangular park interior, there is no stream function and the Saint-Venant solution to the Poisson equation is engaged to find the water table, hydraulic gradients, and discharge streamlines. The total dewatered volume and groundwater storage under the water table and "gross travel time" are evaluated for circular parks. Implications for ecohydrology of parks and desert oases in the Gulf countries and intuitive isoperimetric problems are discussed. [ABSTRACT FROM AUTHOR]

• A novel deep learning model guided by analytical model was developed. • Analytical model helps the neural network to improve the accuracy of prediction. • The proposed model performs satisfactorily even with sparse training data. Deep learning models exhibit good interpolating ability, but their performance is often hindered by the scarcity of data in groundwater problems. Analytical models (solutions) provide a first-order physical principle for groundwater flow, but they are only applicable under specific conditions, such as when the aquifer is homogeneous. This study introduces a novel framework for deep transfer learning that integrates the strengths of both methods and overcomes their limitations. Specifically, we propose a deep learning model guided by a simple analytical model to predict groundwater flow in heterogeneous aquifers. It differs from previous deep learning model by incorporating the knowledge from the simple analytical model and utilizing transfer learning technique to improve the prediction in relatively complicated problems where the analytical model is not applicable. The model is tested against the traditional deep learning model Deep Back Propagation Neural Network (DBPNN) in the scenarios with unknown hydraulic conductivity fields. The results show that the proposed model significantly improve the accuracy of hydraulic head predictions by fusing analytical knowledge with neural networks. The hydraulic conductivity mainly affects the parameters of the shallow layers in the neural network, which enables the use of transfer learning techniques in more complex problems. In all test scenarios, the prediction errors of the proposed model are much smaller than those of the DBPNN. Additionally, the proposed model performs satisfactorily even with limited training data. [ABSTRACT FROM AUTHOR]

• Hydrochar derived from food waste was reused as a soil amendment. • Saturated hydraulic conductivity was increased by hydrochar. • Unsaturated hydraulic conductivity and sorptivity were decreased by hydrochar. • Infiltration was limited due to the soil water repellency induced by hydrochar. Hydrothermal carbonization of food waste (FW) and application of the solid product (hydrochar, HC) in agriculture, is expected to be an effective way to promote the circulation in food-energy-water nexus. However, little is known on the effects of applying FW-derived HC as a soil amendment on soil properties and functions. The current study investigated the effects of FW-derived HC on soil hydraulic properties and infiltration process. Two types of FW-derived HC were mixed into the top 0–20 cm soil layer at a rate of 1 % and 2 % (w/w) in a soil column experiment with grass growth. In-situ infiltration experiment was conducted within the soil column using a disc infiltrometer at different negative pressure (−20 cm, −6 cm, and −2 cm water head). Unsaturated hydraulic conductivity (K 0) and sorptivity (S 0) at each negative pressure head was estimated by fitting experimental cumulative infiltration data to a 4-terms infiltration model. Saturated hydraulic conductivity (K s) and soil water retention curve (SWRC) were measured using undisturbed soil cores taken from the soil column. Soil pore size distribution and soil water constants were estimated from the SWRCs. The results indicate that application of FW-derived HC significantly (p < 0.05) affected soil structure by increasing meso (0.03–0.0002 mm) and macro (> 0.03 mm) pores, which can be attributed to the promoted soil aggregation. Saturated hydraulic conductivity was significantly increased and water retention characteristics were changed with significantly (p < 0.05) increased soil water holding capacity and plant available water content. Besides, FW-derived HCs were found hydrophobic and can render wettable sandy soil water repellent. The K 0 and S 0 were significantly (p < 0.05) decreased and cumulative infiltration was reduced when soil was wetted from an initial dry condition. Negative effect was found on grass growth with significantly (p < 0.05) decreased aboveground biomass after HCs application. Our study indicates that agriculture application of FW-derived HC as a soil amendment could effectively improve soil fertility by recycling nutrient elements and improving soil structure. However, attention should be paid to the hydrophobicity and phototoxicity of FW-derived HC to avoid the related detrimental influences on plant growth and environment. [ABSTRACT FROM AUTHOR]

• Lognormal, unsaturated hydraulic conductivity model not requiring K s as input. • K s model derived from hydraulic parameters describing bimodal soil water retention function. • The novel hydraulic conductivity model does not require the use of integrals. • The model considers the clay-water interaction without using clay content data. • Lognormal distribution function parameters were constrained for physical consistency. We developed a novel, lognormal, pore-scale, unsaturated hydraulic conductivity model, K(ψ) M odel , which does not require saturated hydraulic conductivity, K s , as an input parameter. K(ψ) M odel is derived solely from hydraulic parameters describing a bimodal, lognormal, pore-scale, soil water retention curve θ(ψ). The K(ψ) M odel is based on the Hagen-Poiseuille equation, which represents the soil as a bundle of parallel, non-intersecting capillary tubes. To improve the modelling of fine-textured soils we introduced a novel model to consider the clay-water interaction. This model assumes that clay-water interaction occurs for soils having more than 30% of clay and an effective matrix porosity greater than 35%. Compared to previously developed models, the K(ψ) M odel does not require the use of integrals and can be computed from a spreadsheet and distinguishes between macropore (non-equilibrium) and matrix (equilibrium) flows. The K(ψ) M odel gives improved results when the hydraulic parameters are dynamically constrained and when θ(ψ) describes a bimodal, lognormal distribution. [ABSTRACT FROM AUTHOR]

• First hydrogeological model established for URL site of HLW disposal in China. • The effect of the hydraulic influence zones of faults is incorporated in the numerical model. • Different variations in the horizontal and vertical directions of hydraulic conductivity are considered. • A total of 120 stochastic parameterized models are presented. • Hydraulic conductivity < 10−9 m/s at high-level radioactive waste disposal depth. Deep geological disposal is internationally accepted as a feasible and safe approach for high-level radioactive waste (HLW) disposal. To evaluate hydrogeological conditions in support of safety assessments for HLW disposal at the Xinchang preselected site in China, stochastic inverse models of site-scale groundwater flow were developed in this study to generate highly parameterized hydraulic conductivity fields for the fractured medium by adopting pilot point and null-space Monte Carlo methods. The hydraulic influence zones of the faults were carefully considered to have homogenous parameter values across the zones. A total of 120 multiple random realizations were generated to characterize the 3D spatial variations in hydraulic conductivity by conditioning to 1218 fixed pilot points. The simulated hydraulic heads of the realizations were in good agreement with observational data. The hydraulic conductivity fields close to the boreholes could be characterized in detail and had relatively low uncertainties. Furthermore, the hydraulic conductivity (K) values around underground research laboratory (URL) at the HLW disposal zone depth were less than 10−9 m/s. The K values obtained for most of the influence zones of these faults at this depth were consistent with hydraulic test data and ranged from 10−10 to 10−9 m/s, thereby indicating that the surrounding rock has low permeability and is suitable for HLW disposal. These findings offer new insights into hydrogeological conditions by providing key information on the permeability characteristics that need further focus on HLW disposal. [ABSTRACT FROM AUTHOR]

• A modified KC equation is proposed to calculate hydraulic conductivity in nanopores. • Influence of surface activity and pore size on nano-flow is modelled by MD simulations. • A specific function is proposed to quantify the adsorbed proportion based on MD simulation results. • The modified KC equation is valid to be applied to both sandy and clayey soils. Hydraulic conductivity is a critical parameter for studying the behavior of clay-water systems. However, accurately estimating saturated hydraulic conductivity k in clayey soil by using the Kozeny-Carman (KC) equation is challenging due to the neglect of its active surface properties and nanoscale pores. Clay surfaces can have diverse characteristics resulting from various physicochemical processes such as isomorphous substitution and are typically characterized by the Cation Exchange Capacity (CEC). These properties can significantly impact fluid transport through the clay matrix. To address this issue, this study modifies the KC equation by incorporating an adsorbed water proportion φ that considers the occurrence state of pore water and its correlation with pore diameter, making it applicable to clayey soils. Molecular dynamics (MD) models are established to investigate the influence of surface interaction (CEC) and nanopore size (r) on saturated hydraulic conductivity k of seepage flow in clayey soils. Based on the MD results, a specific function is proposed to quantify the adsorbed proportion φ , which is linearly related to the root of CEC and reciprocal of pore size (r), and then incorporated into a proposed hydraulic correction ratio k * to modify the classical KC equation. The modified KC equation is validated using experimental data from literature, showing a high R 2 value of 0.96. This result demonstrates that the proposed correction ratio k * can extend the application scope of the classical KC equation to clayey soils. [ABSTRACT FROM AUTHOR]

• 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]

Ephemeral and intermittent flow in dryland streams plays a vital role in supporting delicate ecosystems and recharging groundwater in underlying aquifers. Despite extensive research, accurately predicting surface runoff, streambed water infiltration, and groundwater recharge during ephemeral streamflow events remains challenging due to the complex physical processes involved. Such processes are logistically difficult to observe in the field and should not be assessed through numerical studies alone. To tackle this complexity, we utilized an 8 m long, 0.25 m wide, and 0.6 m deep laboratory flume filled with fine sand as a physical model for event flows along a dry stream. This flume allowed us to measure both streamflow and streambed infiltration volumetrically as the stream wetting front advanced along its length during 8 steady inflow rates. The flume was divided into separate compartments to measure water across the sediment surface (streamflow) and water exiting through the flume's bottom (groundwater recharge). To validate our physical tests, we compared them with simulated flows in an integrated surface–subsurface hydrological numerical model. The model results exhibited good agreement with the observed stream and groundwater dynamics. Through a parametric numerical investigation, we identified that hydraulic conductivity significantly controlled the rate of surface infiltration and the advancement of the stream wetting front. Interestingly, we found a linear relationship between surface discharge and stream wetting front location, as well as groundwater recharge, under steady-state conditions, in line with the governing equations. However, our simulations of variable streambed heterogeneity and elevation unveiled the crucial influence of these factors in determining the timing of downstream flow progression. This insight is essential for establishing ecological connectivity along non-perennial rivers and has significant implications for delivering environmental flows in arid regions where ephemeral rivers are prevalent. Our study underscores the value of flume experiments in elucidating the transient physical processes during surface water and groundwater interactions in intermittent streams. By employing a combination of physical modelling and numerical simulations, we advance our understanding of these vital hydrological interactions, contributing to sustainable water management practices in semi-arid and arid environments. • Flume experiments demonstrated stream and groundwater dynamics during ephemeral flood wave advancement. • Location of the stream wetting front is linearly proportional to stream discharge. • Stream wetting front arrival is sensitive to depression storage and sediment heterogeneity. • Parametric analysis identified hydraulic conductivity as the primary control on stream wetting front. [ABSTRACT FROM AUTHOR]

• Geodesic kernel-based Gaussian process regression is proposed. • The method was used to estimate subsurface hydraulic conductivity. • We obtained excellent nonstationary predictions, considering often ignored factors. • A manifold with secondary information can estimate complex property distributions. • Indirect data reflection by a manifold amplifies the reproduced geological process. In this study, the combined application of the geodesic kernel and Gaussian process regression was explored to estimate nonstationary hydraulic conductivity fields in two-dimensional hydrogeological systems. Specifically, a semianalytical form of the geodesic distance based on the intrinsic geometry of the manifold was derived and used to define positive definite geodesic covariance matrices that were used for the Gaussian process regression. Furthermore, the proposed approach was applied to a series of synthetic hydraulic conductivity estimation problems. The results show that the incorporation of secondary information, such as interpretations of geophysical explorations or geological surveys, can considerably improve the accuracy of the estimation, especially in nonstationary fields. In addition, groundwater flow and solute transport simulations based on estimated hydraulic conductivity fields reveal that the accuracy of the simulation is strongly affected by the inclusion of secondary information. These results suggest that incorporating secondary information into manifold geometry can remarkably improve the accuracy of the estimation and provide new insights into the underlying structure of geological data. This proposed approach has critical implications for hydrogeological applications, such as groundwater resource management, safety assessments, and risk management strategies related to groundwater contamination. [ABSTRACT FROM AUTHOR]

• The mechanism of scour and siltation from high to low river stage was identified. • The response of leakage coefficient to scour and siltation varied by river section. • Conditions of thermal analytical models were compared and refined by seasons. Riverbed scour and siltation can change the hydraulic conductivity and thickness of sediments, leading to notable spatiotemporal variations of groundwater–surface water interaction. However, correlated quantitative research on the dynamic leakage coefficient (LC) remains lacking under riverbed scour and siltation. Based on dynamic monitoring and analysis of sediment medium properties, river-groundwater levels, and temperature profiles in the section of Liao River, this study compared the applicable conditions of different thermal analytical models and identified spatiotemporal variations in the LC values induced by riverbed scour and siltation. The results showed that: (1) The thermal tracer method combined with river-groundwater level monitoring can effectively quantify spatiotemporal variations in the LCs of riverbed sediment. Based on the sediment temperature presents diurnal fluctuations and gradient characteristics in different seasons, the LCs in summer, spring, and autumn compared to winter in seasonally frozen regions should be calculated using transient and steady-state analytical solution models, respectively. (2) The LCs ranged from 0.47 to 24.90 × 10-6 s−1, showed larger temporal variability than spatial, increased from the river bank to the center, and had larger values in autumn and winter. (3) Distinct from previous findings, the riverbed underwent a scouring process associated with reductions in the incoming sediment coefficient as the river stage shifted from high to low, eroding the surface 4–8 cm of sediment. During this process, the LC increased by 40 to 420%. Further, the influence of riverbed scour and siltation on the LC varied by river section. Near the riverbank, the sediment LC was primarily controlled by hydraulic conductivity related to the coarsening of sediment lithology; however, it was influenced by variations in both sediment thickness and hydraulic conductivity near the riverbed center. Collectively, these results further understand the mechanisms underlying changes in the sediment LC induced by riverbed scour and siltation, as well as provide a scientific basis for ensuring the sustainable development of riverside water sources under such changes. [ABSTRACT FROM AUTHOR]

• We present the SWRIM model for simulating infiltration curves in SWR substrates. • SWRIM represents the pore system as a set of disconnected pathways with dual K. • SWRIM estimates the K d distribution and the percentage of water-conducting pores. • SWRIM was validated on different substrates of 1.0 and 2.5 cm height. • Robust fits were obtained between experimental and modeled infiltration curves. The hydraulic characterization of hydrophobic substrates, which strongly depends on the water content, is an understudied topic in the field of soil physics. Based on laboratory experiments, this work presents a physically-based model, denoted as Soil Water Repellency Infiltration Model (SWRIM), to describe the double-slope cumulative infiltration curves commonly observed in hydrophobic substrates, whose behavior is conditioned by the dual hydraulic behavior (under dry and saturated conditions) of the water repellency materials. The experimental setup consists of 1D infiltration tests on hydrophobic substrates and tension of 0 cm, following by a free drainage. The SWRIM model assumes that water flow in the hydrophobic substrate is gravity driven, representing the pore system as a set of disconnected pathways or capillaries with dual behavior. Each capillary is governed by two hydraulic conductivity values corresponding to dry or partially saturated and saturated conditions, K d and K s respectively. During infiltration, water flow within each pathway is regulated by K d , but once the pathways are filled and the substrate is fully saturated, the water flow is governed by K s , whose value is directly measured from the slope of the drainage curve. In order to incorporate the variability of the pore system, unimodal and bimodal probability distributions of K d values were considered. The application of the SWRIM model allowed determining the distribution of K d and the percentage of water-conducting pores, δ , of the hydrophobic layer. Additional determinations of the soil hydraulic conductivity, K , and sorptivity under water repellent conditions were also obtained from the inverse analysis of the initial times of the infiltration test. The proposed model was validated on experimental soil columns of 5 cm internal diameter and 1.0 and 2.5 cm height filled with pine forest organic material, Pine, rosemary leaf litter, RS, and, blond peat, Peat. Overall, convex-to-linear curves with a monotonously increasing infiltration, typical of hydrophobic substrates, were obtained. SWRIM model allowed robust fits of the infiltration curves, showing in all cases a distribution of K d with a bimodal shape. Pine and Peat presented the largest values of δ. The estimated average K d was within the same order of magnitude as K values. Overall, the results showed that the proposed physically-based model allows to satisfactory describe and characterize the dynamic hydraulic behavior of hydrophobic soil substrates. [ABSTRACT FROM AUTHOR]

• Analytical solutions for steady vertical flux with generalized hydraulic conductivity. • The solutions generalize existing solutions based on Brooks-Corey and rational models. • The proposed model provides better estimate of drying fronts for evaporation problems. • The analytical solutions allow a better fitting of experimental data. In this paper, analytical solutions for one-dimensional nonlinear steady-state vertical flux through unsaturated soils are presented. The analytical solutions are applicable for homogeneous soils for which the hydraulic conductivity function is a generalized form of the well-known Brooks-Corey and rational models. The adopted generalized soil–water retention model involves two additional parameters which reflect the curvature of hydraulic functions near saturation. The main advantage of the used generalized hydraulic conductivity function is that it fits better experimental data. The soil domain is a finite-depth medium overlying a fixed groundwater level. The derived analytical solutions are obtained using the Maclaurin series expansion technique. Analytical solutions for both infiltration and evaporation are developed. For evaporation from a fixed groundwater level, the derived solutions can be used to predict the existence of a vaporization plane below the soil surface. For this case, analytical expression of the position of drying front separating liquid and gas regions is derived. The analytical results are compared to numerical ones for various infiltration and evaporation examples and excellent agreements are obtained. The generalized model is also used to fit experimental data obtained for two soils. The results show that the use of the standard Brooks-Corey model may lead to an overestimation of the position of the drying front. [ABSTRACT FROM AUTHOR]

• Seasonal groundwater level fluctuations accelerate residual saltwater removal. • Critical dam height of the fluctuating-head scenario is higher than that of the constant-head scenario. • Optimal dam height for fastest removal of residual saltwater is elucidated. Subsurface dams have been applied as an effective countermeasure against saltwater intrusion in coastal aquifers. However, part of the saltwater wedge ("residual saltwater") may be trapped upstream of the dam and take years to decades to remove, hindering groundwater exploitation. This study examines the mechanism of residual saltwater removal under seasonal groundwater level fluctuations induced by intermittent precipitation or pumping. The residual saltwater removal timescale for constant-head (CH) and fluctuating head (FH) scenarios are quantified and compared using field-scale numerical simulations. The results show that seasonal fluctuations in the groundwater table enhanced the dilution of residual saltwater, thereby accelerating the removal of residual seawater compared to the CH scenario. However, the critical dam height (i.e., the minimum height for complete removal of residual saltwater) is higher in the FH scenario than in the CH scenario. The sensitivity analysis indicates that the fastest residual saltwater removal in the FH scenario is generally obtained by a dam height slightly higher than the critical dam height. The impact of seasonal groundwater level fluctuations on residual saltwater removal timescale is more pronounced with a higher amplitude of inland hydraulic head variations, a lower average inland hydraulic head over the seasonal cycle, a larger dispersivity, a greater aquifer hydraulic conductivity, or a smaller porosity. These results provide valuable insights into the design of engineered subsurface dams and the management of existing subsurface barriers in coastal aquifers. [ABSTRACT FROM AUTHOR]

• Water vertical infiltration occurs along an extremum path with linear deviation. • The water content and wetting front evolution characteristics are easily shown. • A direct method for measuring hydraulic conductivity is presented. According to the principle of stationary action, water vertically infiltrates unsaturated soils along an infiltration-duration extremum path with linear deviation. The solution of the Euler–Lagrange equation with appropriate boundary conditions yields a linear relationship between the moisture diffusion function and the ratio of the spatial location to the wetting front distance. By integrating the law of mass conservation at the boundary, the temporal evolution of the wetting front is implicitly derived. A linear term is introduced to compensate for the deviation in calculating the position of the wetting front owing to the Taylor series approximation. Based on the representation of the diffusion function in relation to the boundary diffusivity, the spatial location and the position of the wetting front, a direct way to determine hydraulic conductivity is presented. Unlike existing methods, the proposed approach involves fewer hypotheses and is more theoretically robust. Moreover, the material parameters in the proposed approach have a clear physical meaning and can be explicitly determined. Using the Brooks–Corey hydraulic function, the evolution and distribution characteristics of the water content are explicitly determined, while the advancing wetting front is implicitly represented. The results of the proposed solution for different types of soil are in agreement with these numerical and analytical solutions. [ABSTRACT FROM AUTHOR]

• Water transport is most sensitive to the unsaturated region of K(h). • K(h) is commonly overestimated when only saturated K is measured. • Simulations without measured unsaturated K are the least accurate. The saturated hydraulic conductivity ( K s ) is considered to be one of the main soil hydraulic properties to simulate water transport in the vadose zone, yet it is notoriously difficult to assess due to its large field-scale variability and sensitivity to preferential flow paths. With the increasing availability of convenient methods to measure the unsaturated hydraulic conductivity ( K u ), the additional value of measuring K s could be questioned. This study investigates the relevance of measuring K s in two ways. Firstly, a sensitivity analysis was performed to evaluate which part of the hydraulic conductivity curve impacts the simulation of unsaturated water transport most. It was found that hydraulic conductivity curve between −10 cm and −1000 cm matric head was affecting the simulation more than at saturation. Secondly, a functional evaluation was performed by simulating water flow in six test fields according to three scenarios, in which the hydraulic properties were determined with the measured K s and/or K u . The simulation of saturation degree at three depths was the least accurate in the scenario where only K s was measured. Remarkably, the most accurate simulation was found in the scenario with only the measured K u . According to these results, measuring K u would be of greater importance than measuring K s for the simulation of water flow in the vadose zone. [ABSTRACT FROM AUTHOR]