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• A novel method based on species diversity theory would assess preferential flow. • Effects of soil properties on the proportion of dye-stained areas (PDA) were studied. • Soil clay content and sand content primarily control PDA in upper soil layers. • Soil drainage capacity and bulk density primarily control PDA in lower soil layers. • Random forest algorithm could provide accurate and stable predictions for PDA. The preferential flow path development is potentially the result of spatial variations in soil properties with soil depth. However, visualizing the evolution of the preferential flow path with soil depth remains a challenge. This paper presents dye tracer and species diversity theory methods for characterizing preferential flow paths. Field dye tracer experiments were performed at three sites (tree, bush, and grass) in the Yellow River Delta wetland and dye distribution diversity indices (Simpson index (D s), Shannon-Wiener index (H), Margalef index (D m), and Pielou index (E)) were applied to verify their availability for preferential flow assessment. The results showed that the uniformity of the shallow-infiltrated dye at the tree site, non-uniformity of the shallow-infiltrated dye at the bush site, and deep dye infiltration at the grass site were the three typical infiltration types. The average proportion of dye-stained areas (PDA) gradually decreased with increasing soil depth. The quantitative effects of soil properties on PDA changes were profound, indicating that soil clay content at 0–10 cm depth, soil sand content at 10–20 cm depth, soil drainage capacity at 20–30 cm depth, and soil bulk density at 30–40 cm depth were the most predictive factors controlling PDA changes. Our results also showed that dye-stained patches with extremely high and high dye concentrations were the most distributed; D s , H , D m , and E were the highest at the tree site and E was the diversity index with the greatest importance for PDA change. The findings reveal the soil properties controlling the formation of preferential flow paths, which will improve our understanding of water resource management in the vadose zones of coastal wetlands. [ABSTRACT FROM AUTHOR]

• Fiber-optic sensing technique is innovatively applied in the field of hydrology and geotechnical engineering. • Fiber Bragg grating (FBG) is utilized to continuously monitor soil water potential. • FBG-suction method can measure a wide range of total suction accurately and efficiently. • The geophysical method holds significant potential for characterizing dynamic surface processes. Accurate measurement of soil water potential or suction is crucial for hydrology and geotechnical engineering. Inspired by recent advances in fiber-optic sensing techniques, we develop an innovative geophysical method for measuring the soil suction by using fiber Bragg grating (FBG), named FBG-suction method. In this study, working principles and theoretical background associated with the FBG-suction method are discussed. For verification purposes, a series of laboratory-scale tests including calibration, suction measurement, and evaporation monitoring are performed on two clayey soils of low to high plasticity under different degrees of saturation. The calibration and measurement results demonstrate the efficacy and accuracy of the developed method for measuring a wide range of soil suction. The curves of soil saturation with suction obtained by WP4C and FBG-suction method are comparable and reflect the difference in the water retention capacity of the tested soils. Additionally, the evaporation monitoring results show that the FBG-suction method can measure the low suction during the drying process and accurately obtain the high suction above the measuring range of the PST-55 psychrometer. The findings affirm the scientific viability of the fiber-optic sensing technique for acquiring the spatial distribution of water potential or suction in unsaturated soils and provide a novel perspective for future investigations into the spatiotemporal evolution characteristics of in-situ unsaturated soil under climate change. As interdisciplinary collaborations continue to advance, it becomes increasingly apparent that methods based on fiber-optic sensing have the potential to revolutionize traditional approaches to characterizing dynamic subsurface processes. [ABSTRACT FROM AUTHOR]

• A conceptual model is proposed to categorize soil pore water types. • Quantitative analysis on pore water redistribution during soil dry-wet processes. • The SWCC can be predicted using the NMR cumulative pore water distribution curve. The pore water distribution (PWD) of soil provides key information about soil shear strength, hydraulic conductivity, and water-retention ability. The PWD characteristics of the soil depend on the specimen preparation conditions (i.e., the variations in initial water content or compaction degree) and drying-wetting processes, which were investigated by using the nuclear magnetic resonance (NMR) technique. The NMR T 2 spectrum is mathematically transformed into the PWD curve to more accurately quantify the variations in pore water content stored with pores of different radii. The pore water in silty clay can be categorized into strongly bound water, intra-aggregate pore water, and inter-aggregate pore water, which are identified by the pore radius of <0.05 μm, 0.05-1.0 μm and >1.0 μm, respectively. The effects of compaction degree and initial water content on the PWD of the soil exhibit distinctions. For unsaturated compacted specimens, the PWDs remain unaffected by changes in compaction degree, while an increase in initial water content promotes the formation of clay aggregates, thereby increasing the maximum water-holding pore radius and the intra-aggregate pore water content. For saturated compacted specimens, the inter-aggregate and intra-aggregate pore water content increases with a decrease in the compaction degree, while the changes in the PWD shape from unimodal to bimodal with an increase in the initial water content. During the soil drying process, inter-aggregate pore water is rapidly drained, the intra-aggregate dominant pore water content increases first and then decreases, and the strongly bound water content remains constant. The relationship between the cumulative pore water distribution curve and the soil-water characteristic curve (SWCC) can be established through the Young-Laplace theory. The SWCC can be well predicted by using the envelope of the cumulative pore water distribution curves during soil drying. [ABSTRACT FROM AUTHOR]

• A modified permeability testing device is developed to test the permeability of subgrade soil under the overlying stress. • The hydraulic characteristics model, considering the influence of stress and wetting-drying cycles, is established. • A method, repaid obtaining the water migration law of subgrade under stress and wetting-drying cycles, is developed. The wetting–drying cycles and overlying stresses experienced by unsaturated subgrade alter soil pore structure, resulting in variations in its hydraulic characteristics, ultimately affecting subgrade water migration and moisture distribution. However, the combined effects of these two phenomena on key subgrade hydraulic characteristics including the soil–water characteristics and the permeability property remain unclear. This paper aimed to experimentally measure soil–water characteristic curve (SWCC) and saturated permeability of subgrade soil under various overlying stresses and wetting–drying cycles. A customized SWCC measurement system and a modified permeability testing device were developed to conduct the test program on compacted silty clay soil specimens. Subsequently, the influence of overlying stress and wetting–drying cycles on the hydraulic characteristics of the subgrade was analyzed, with a focus on the SWCC and saturation permeability coefficient, and the corresponding fitting model is established. Finally, the water transfer behavior of subgrade considering the effect of wetting–drying cycles and overlying stresses was analyzed using finite element method (FEM). The results indicated that an increased number of wetting–drying cycles and higher overlying stress led to a decrease in the saturated moisture content of subgrade soil, an increase in the air-entry value, and a flatter SWCC. The saturation permeability coefficient increased with the number of wetting–drying cycles, but decreased with increasing overlying stress. Furthermore, a high correlation was observed between the SWCC and permeability coefficient predictive model, considering both the number of wetting–drying cycles and overlying stress. Lastly, when considering the stress state and wetting–drying cycles, significant differences in the evolution of matric suction of the subgrade during rainfall were observed. The overlying stress amplifies the decrease in matric suction of the soil above the subgrade, while wetting–drying cycles reduce this decrease. [ABSTRACT FROM AUTHOR]

[Display omitted] • Stream power and soil clay content were used to model soil erosion rate. • Sediment sorting and transport were controlled by the soil properties. • A linear function was developed to quantify the sediment clay content. • The relative contributions of different transport mechanisms were quantified. Understanding the sediment sorting and transport processes is is crucial for enhancing the understanding of soil erosion mechanisms and improving soil erosion prediction. However, quantitative studies based on the coupling effects of both hydraulic parameters and soil properties are still lacking. In this study, five loessial soils with varying physicochemical properties were collected in the Loess Plateau of China, and simulated rainfall experiments were carried out in a metal box with a slope of 20 degrees under rainfall intensities of 1, 1.5, and 2 mm min−1. Runoff samples were collected to analyze the variation in erosion rates and effective particle size distribution of eroded sediments during the rainfall process. The results showed that the stream power was the optimal parameter to model the erosion rate, and a power function incorporating the clay content and stream power could well predict the erosion rate of different soils. The enrichment ratio indicated the enrichment of fine particles and depletion of coarse particles. And the sediment sorting degree increased with increasing soil clay content. The critical particle size for distinguishing different transport mechanisms varied among soils, with more than 55.46 % of the total sediment transported by suspension/saltation. A linear function and power function including the original soil particle size distribution and stream power were developed to quantify the sediment clay content and the relative contribution of different transport mechanisms. This study could be helpful to improve the soil erosion prediction model and provide basic guidelines for regional soil and water conservation. [ABSTRACT FROM AUTHOR]

• The seepage of water in clay deviated from that predicted by Darcy's law. • Macropore and mesopore are the dominant pore types in clay. • The weakly bound water (outer layer) is the main type of water in clay. • The movable water content was closely related to the permeability of the clay. Water flow in saturated clay soil at low hydraulic gradients usually occurs via pre-Darcy flow. The occurrence of the pre-Darcy flow phenomenon is attributed to the adsorption of water on pore walls via surface forces. However, a clear understanding of the amount of water that is movable in saturated clay soil and the distribution of movable/immovable in the microstructure of clay soil is lacking. Furthermore, it is difficult to accurately evaluate the effect of water mobility on the seepage characteristics of clay soil. To investigate the effects of water mobility and pore microstructure on pre-Darcy flow in clay soil, nine clay samples were used to conduct constant head permeability experiments. The results showed that the seepage of water in clay deviated from that predicted by Darcy's law, and with increasing hydraulic gradient, the apparent water permeability first increased and then tended to remain constant. The porosity ϕ , average pore radius r p , sorting coefficient S p , mean pore radius D m , dry density ρ and tortuosity τ had major influences on the stable apparent water permeability k s. In contrast, the influences of maximum pore radius and median pore radius on k s were relatively weak. Macropores (>1.0 μm) and mesopores (100–1000 nm) were the dominant pore types in the clay and accounted for almost 80 % of the overall porosity. The water in saturated clay can be divided into four types according to interfacial forces. The results showed that weakly bound water (in the outer layer) was the main type of water in the clay. Among the four types of water, k s had the strongest correlation with the proportion of free water. The mobility of water was affected by coupling between interfacial forces and hydraulic gradients. The movable water content was closely related to the permeability of the clay. [ABSTRACT FROM AUTHOR]

• SWDI is good at detecting severe/extremely droughts in humid region. • Sand are more prone to agricultural drought than clay. • Impact of groundwater on agricultural drought differ with soil textures. • 95th percentile is the best estimator of field capacity for clay and loam. • Min(maxgs) is the best estimator of field capacity for sand, loamy sand. Soil moisture is an important indicator for monitoring agricultural drought, and is closely related to soil texture. However, the role that soil textures play in agricultural drought evolution in humid climates is not well understood. In the study, we systematically assessed the feasibility of the Soil Water Deficit Index (SWDI) for agricultural drought monitoring in humid regions, examined the impact of soil texture and groundwater depth on agricultural drought evolution, and estimated the field holding capacity for different soil textures in humid areas based on a large-scale regional soil moisture monitoring network. The results show that: (1) The SWDI has reasonably good performance in detecting severe and extreme agricultural drought, but it tends to overestimate the drought intensity during non-severe/extreme drought periods in humid regions, particularly for clay soils. (2) Sand soils are more prone to agricultural drought than clay soils, which are more resistant to severe and extreme agricultural drought with high air temperatures and low precipitation. (3) During drought aggravation, clay soil showed a delay of about 0–18 weeks from the lowest SWDI to the maximum groundwater depth, while the delay was only 0–5 weeks for non-clay soil textures. (4) The 95th percentile is proposed as the best estimator of field holding capacity for clay and loam, while Min(maxgs) is the best estimator for sand, loamy sand, and sandy loam. Overall, these findings provide insights into the role of soil texture and groundwater depth in agricultural drought evolution in humid regions, and offer practical guidance for field holding capacity estimation for different soil textures. [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 influences of model parameters on the shape of the SFCC are analyzed. • The equation for double-porosity soil can characterize the bimodality of the SFCC. • An approach formulating the SFCC of the expansive soil is presented. • Theoretical SFCCs along hydration of an unsaturated expansive soil are given. Based on the similarity between the drying-wetting process and freezing-thawing process in the soil, a physically based equation predicting the soil freezing characteristic curve (SFCC) can be formulated combining the generalized Clapeyron equation. In this paper, by introducing the weighing factors in two scales of pore sizes, the physically based equation for single-porosity soil is extended for double-porosity soil, with the ability in well characterizing the bimodality of the SFCC. A systematic approach formulating the SFCC of the expansive soil is presented accounting for the pore size distribution and its evolution in two scenarios of the hydraulic conditions. Good agreements can be found between model predictions and experimental data for both scenarios. The effects of hydraulic condition on the pore structure of an expansive soil, as well as the consequences of the evolved pore structure on the SFCC, are emphasized and quantified. In addition, the framework presented herein demonstrates how an initially bimodal SFCC evolves into a unimodal shape along hydration and provides the theoretical SFCCs at different levels of saturation, which contributes to modelling the water-heat transfer in the freezing process of the unsaturated expansive soil. [ABSTRACT FROM AUTHOR]

• The influences of model parameters on the shape of the SFCC are analyzed. • The equation for double-porosity soil can characterize the bimodality of the SFCC. • An approach formulating the SFCC of the expansive soil is presented. • Theoretical SFCCs along hydration of an unsaturated expansive soil are given. Based on the similarity between the drying-wetting process and freezing-thawing process in the soil, a physically based equation predicting the soil freezing characteristic curve (SFCC) can be formulated combining the generalized Clapeyron equation. In this paper, by introducing the weighing factors in two scales of pore sizes, the physically based equation for single-porosity soil is extended for double-porosity soil, with the ability in well characterizing the bimodality of the SFCC. A systematic approach formulating the SFCC of the expansive soil is presented accounting for the pore size distribution and its evolution in two scenarios of the hydraulic conditions. Good agreements can be found between model predictions and experimental data for both scenarios. The effects of hydraulic condition on the pore structure of an expansive soil, as well as the consequences of the evolved pore structure on the SFCC, are emphasized and quantified. In addition, the framework presented herein demonstrates how an initially bimodal SFCC evolves into a unimodal shape along hydration and provides the theoretical SFCCs at different levels of saturation, which contributes to modelling the water-heat transfer in the freezing process of the unsaturated expansive soil. [ABSTRACT FROM AUTHOR]

[Display omitted] • Continuous simulations were performed to examine the effect of prestorm hydrological storage on the performance of kerbside leaky well systems. • The results reveal that storage volume is a critical variable which determines the performance efficiency of distributed storages. • The results reported the importance of emptying time for the performance of leaky well systems during storms of varying intensity. • The results provide information regarding loss in performance levels of distributed storages considering infill development as factor. The popularity of subsurface distributed storage-based kerbside stormwater infiltration systems is increasing because they can be integrated within the strict spatial constraints of existing infrastructure. Distributed systems operate based on the concept of spreading a limited amount of storage throughout the catchment area to achieve runoff management objectives. Storage can also limit the performance of urban stormwater systems. This study investigated the role of storage capacity, especially the volumetric capacity and the emptying time, in the performance of kerbside leaky well systems in a residential catchment area with a native clay soil environment. A detailed, process-based deterministic hydrological model was used to analyze hydrologic storage via a continuous simulation approach. The model was calibrated against catchment outflows measured with stormwater infiltration devices. Individual kerbside infiltration systems were also calibrated to monitor the behavior of on-site kerbside infiltration systems. By using approach of continuous simulations during designed scenarios, this study considered the prestorm storage availability in the hydrological analysis. The results demonstrated that hydrologic storage greatly influences the performance of kerbside leaky well systems during higher-intensity storms. In addition, the results show that distributing a stormwater storage volume equal to thirty-five 1-kL rainwater tanks across the catchment area as a kerbside infiltration system could notably reduce the runoff flow rates and volumes. The results further highlight that the addition of more homes within the connected area could diminish the stormwater reduction benefits achieved through the installation of a distributed kerbside infiltration system. [ABSTRACT FROM AUTHOR]

• Groundwater head increased up to 26.70 cm for the year 2060. • Land subsidence under the over-pumping scenario is specified. • The shoreline moves inland with a drawdown to 2, 9.50, and 14.80 m. • Future infrastructure planning should consider the land subsidence threat. Land subsidence (LS) due to the Sea Level Rise (SLR) and over-pumping was observed in many groundwater aquifers worldwide. The geotechnical properties and numerical simulation are considered a new integrative approach to investigate the LS hazard. This study investigates the environmental hazards related to LS in the quaternary sediments using laboratory and numerical modeling in the coastal aquifer of Nile delta, Egypt, due to SLR and over-pumping. Therefore, the geotechnical tests are conducted for the aquifer cap and classify the surficial soil as silty clay with moderately complex dispersive characteristics. In addition, hydraulic conductivity and one-dimensional consolidation of silty clay samples were analyzed to estimate the long-term settlement. Consequently, the groundwater head and drawdown were simulated for the current situation. Three proposed future scenarios are SLR and increased abstraction using the SEAWAT code. The results show that groundwater heads increased to 5.65, 14.50, and 26.70 cm for 2020, 2040, and 2060, respectively. Moreover, the estimated LS under the over-pumping scenario reached 7.60, 32.40, and 52.70 cm, with a maximum drawdown of 2.10, 9.70, and 15.10 m. SLR and over-pumping induce more inundation in the northern part due to SLR. The shoreline moves inland with a drawdown to 2, 9.50, and 14.80 m. Integration of measured and groundwater simulation settlements would provide appropriate information about the potential environmental hazards. Moreover, the future LS scenarios help stakeholders to make the right decision in developing effective measures in coastal aquifers to minimize the adverse impacts of LS on infrastructure waterways and human life. [ABSTRACT FROM AUTHOR]

• Depth specific EC estimates were used for mapping soil heterogeneities. • Including clay-rich zones improved the observed shallow groundwater levels. • The heterogeneous soil model better simulates the spatial hydraulic heads distribution. • The homogeneous and heterogeneous soil models simulated comparable tile drain flow. In agricultural fields, tile drains represent potential pathways for the migration of solutes, such as nitrates, in groundwater and surface water bodies. Tile drain flow is controlled by the temporal and spatial dynamics of the shallow groundwater table, which results from complex interactions between climate, topography and soil heterogeneity. Studies on the effect of topsoil heterogeneity on shallow water and drainage dynamics by fully 3D surface water and groundwater flow modeling are limited. The objective of our study is to demonstrate the use of depth specific electrical conductivity (EC) estimates to improve hydrological simulations in a tile-drained field. The model was applied to a field site in Denmark where times series of drainage discharge and water table elevations are available. Clay-rich soil zones were identified in a tile-drained field using depth specific electrical conductivity estimates generated by the inversion of apparent electrical conductivity data measured using an electromagnetic induction instrument. One model that included the low-permeability clayey zones in the soil layers down to a depth of 1.2 m was compared to a simpler model that assumed homogeneous soil layers. Both models simulate drainage discharge that compares well to the observations. However, including the clayey zones improves the simulation of hydraulic heads, and water table fluctuations, and generates flooded areas that are more representative of those observed during the wet seasons. Our results suggest that the simulation of water table fluctuations can be improved when the soil heterogeneity determined from depth specific EC estimates is included in integrated hydrological models. A better representation of the subsurface flow dynamics will also improve subsequent simulations of the transport and fate of agrochemical substances leaching from fields such as nitrate, which may deteriorate the quality of groundwater and surface water bodies. [ABSTRACT FROM AUTHOR]