Search Results (192)

The soil–water retention curve (SWRC) is a fundamental property that governs the hydraulic and mechanical behavior of unsaturated soils. Laboratory SWRC determination remains time-consuming and costly, promoting indirect estimation methods. However, existing methods often oversimplify the pore structure and particle arrangement of soils and neglect the effect of capillary menisci, resulting in discrepancies from natural soil behavior. This study proposes a novel method to estimate the SWRC of coarse-grained soils based on grain size distribution (GSD) and relative density. In the proposed method, soil particles are idealized as spheres in a two-dimensional (2D) plane, and the packing structure is modeled using representative quadrilaterals composed of four poly-disperse particles. The GSD is employed to calculate the probability of different particle sizes occupying the corners of the quadrilateral elements, while the relative density defines their geometric configuration. The water retention behavior is then evaluated using the geometric relationships between the air–water interface and particle radii. The predicted SWRCs are in good agreement with experimental data, indicating that the method can effectively capture the water retention characteristics of coarse-grained soils governed by capillary effects. The method’s applicability is limited to coarse-grained soils and excludes clayey soils where adsorbed water dominates retention mechanisms. Full article

The soil–water characteristic curve (SWCC) is essential for understanding hydraulic behavior in geotechnical applications involving coarse granular materials. However, existing models often overlook the coupled effects of key factors. This study systematically investigates the influence of grain size distribution, density, and contact angle on the SWCC using a numerical approach that combines the discrete element method (DEM) with an enhanced pore morphology method incorporating locally variable contact angles (Lvca-PMM). The results show that smaller uniformity coefficients ($C_u$), larger median grain sizes ($D_{50}$), higher porosity ($\phi$), and larger contact angles ($\theta$) shift the SWCC to the left, reducing both the air entry value (${\Psi }_a$) and residual suction (${\Psi }_r$). Specifically, linear relationships were identified between ${\Psi }_a$, ${\Psi }_r$, $C_u$, $\phi$, and cos($\theta$), while a power-law relationship was observed with $D_{50}$. Furthermore, the interaction of these factors plays a critical role, where a change in one property can amplify or diminish the effects of others. Based on these findings, empirical equations for predicting ${\Psi }_a$ and ${\Psi }_r$ were developed, offering practical tools for engineers to efficiently estimate the SWCC. This research provides deeper insight into the water retention properties of coarse soils and supports the optimized design of granular fills and drainage systems. Full article

Preferential flow, which primarily drains via vertical and interconnected macropores under gravity, allows water and solutes to transport non-uniformly through the soil matrix. Such a feature exacerbates the leaching risk of pollutants to groundwater. However, there is still a lack of knowledge of how the soil macropores affect the migration of manure-sourced veterinary antibiotics (VAs) in agricultural soils. This study used a series of techniques, including field dye tracing experiments, measurements of soil water retention curves (SWRCs), and micro-CT scanning, to explore macropore characteristics for a typical Entisol. The leaching behavior of sulfadiazine (SDZ) and sulfamethazine (SMZ) was then investigated using undisturbed columns (15 cm ID × 20 cm) under simulated rainfall. The results revealed the great lateral diffusion ability of the soil (up to 65 cm) as compared to vertical penetration (50 cm depth) in the field. The soil was abundant in macropores with equivalent diameter > 200 µm, and the macroporosity was higher in the lower layer (40–60 cm) than the upper layers, where cultivation may lead to the fragmentation of the soil structure and the formation of more isolated pores. Breakthrough curves (BTCs) and hydrological modeling indicated a faster penetration time and greater leaching of sulfonamides with increased macropores in the soil. Such an effect was, however, strengthened under rainstorm conditions (25 mm h⁻¹). Antibiotics leaching was strongly correlated with the mean macropore diameter (MD), compactness (CP), and connectivity (Γ) parameters and significantly affected by MD and CP (p < 0.05), particularly at a moderate rainfall intensity (11 mm h⁻¹). This study has linked antibiotics migration with the soil structure and highlighted macropores’ contribution to their accelerated leaching, thus providing evidence for environmental risk assessments and promoting sustainable soil and water management in real scenarios of soil macropore flow. Full article

Accurately constructing a soil–water characteristic curve (SWCC) model that accounts for the combined effects of multiple factors is of great significance for in-depth understanding of the physical and mechanical behaviors of soils in complex environments. Based on the van Genuchten (vG) model, this study systematically analyzed the effect of the coupling mechanism of void ratio, temperature, and salinity on SWCC. An SWCC model capable of characterizing multi-factor coupling effects was established by incorporating multi-factor influence terms. Fitting verification with experimental data demonstrates that the proposed model can effectively depict soil water retention characteristics under the combined action of multiple factors. Furthermore, parameter sensitivity analysis clarifies the influence laws of each model parameter on the air entry value and the slope of the transition segment of SWCC. To address the challenge of cumbersome determination of model parameters, a parameter prediction method based on the Bayesian regularized neural network (BRNN) was proposed. By training a large volume of SWCC experimental data under multi-factor conditions, effective prediction of model parameters was achieved, with the input being the basic physical properties of soil and environmental variables and the output being the target model parameters. Considering that the influence of salinity introduces additional parameters, the training set was divided into two scenarios (saline and non-saline conditions) for separate modeling to enhance the pertinence and accuracy of parameter prediction. Prediction results indicate that the proposed method exhibits reliable parameter prediction capability, and its prediction accuracy is mainly influenced by the quantity and quality of training data. Full article

The accurate determination of the soil-water characteristic curve (SWCC) and unsaturated hydraulic conductivity is vital across multiple disciplines, including hydrogeology, soil science and geotechnical engineering. Nevertheless, conventional techniques for measuring these unsaturated soil parameters are often laborious and time-consuming, posing significant practical challenges. This research presents a new technique for estimating SWCC and unsaturated hydraulic conductivity by employing fractal theory and utilizing a three-dimensional fractal dimension (D_s). The results revealed that all three soils exhibited fractal characteristics in their particle surfaces, with D_s values of 2.611 for Malan loess, 2.688 for paleosol, and 2.771 for remolded loess. The complexity of the pore structure was in the order of remolded loess > paleosol > Malan loess. The test results of the soil-water characteristic curve indicate that the water storage capacity of the three soils was in the order of paleosol > remolded loess > Malan loess. Compared with the Brooks-Correy fitting curve, the fractal model is feasible in predicting the soil-water characteristic curve. Two models were used to predict the unsaturated hydraulic conductivities of three types of soil, and the results were compared with the measured values. By comparing the R² and RMSE values of the fractal model and the Brooks-Corey model, it was found that the fractal model proposed in this paper can effectively predict the unsaturated hydraulic properties of these three types of soil. This study provides a simple and effective alternative for predicting the SWCC and unsaturated hydraulic conductivity of unsaturated soils, with potential applications in various earth science fields. Full article

Water scarcity in semi-arid regions necessitates accurate soil water modeling to optimize irrigation management. This study compares three HYDRUS-1D parameterization approaches—based on the drying-branch soil water retention curve (SWRC), wetting-branch SWRC (using Shani’s drip method), and inverse modeling—to simulating soil water content at 15 cm and 45 cm depths under center-pivot irrigation in a semi-arid region. Field experiments in three maize fields provided daily soil water, soil hydraulic, and meteorological data. Inverse modeling achieved the highest accuracy (NRMSE: 2.29–7.40%; RMSE: 0.006–0.023 cm³ cm⁻³), particularly at 15 cm depth, by calibrating van Genuchten parameters against observed water content. The wetting-branch approach outperformed the drying branch at the same depth, capturing irrigation-induced wetting processes more effectively. Statistical validation confirmed the robustness of inverse modeling in reproducing temporal patterns, while wetting-branch data improved deep-layer accuracy. The results demonstrate that inverse modeling is a reliable approach for soil water simulation and irrigation management, whereas the wetting-branch parameterization offers a practical, field-adaptable alternative. This study provides one of the first side-by-side evaluations of these three modeling approaches under real-world semi-arid conditions. Full article

The water retention and permeability characteristics of loess are core factors governing geological disaster prevention and engineering stability in the loess regions of northwest China. This study focuses on Yangling loess, systematically conducting soil water characteristic curve (SWCC) measurements and saturated permeability tests under different initial compaction degrees and water contents using a pressure plate apparatus and a TST-55 permeameter. By combining fitting analyses of the Gardner, Fredlund–Xing, and Van Genuchten SWCC models, the study reveals the influence mechanism of initial conditions on the water retention properties of Yangling loess. Furthermore, the unsaturated hydraulic conductivity of loess was predicted using the Van Genuchten–Mualem model. Finally, a quantitative relationship model between hydraulic conductivity and multiple factors (initial compaction degree, water content, and matric suction) was constructed using the response surface methodology. The results indicate the following: (1) A higher initial compaction degree and water content lead to a higher air entry value of loess, resulting in stronger water retention capacity. Among the three models, the Van Genuchten model exhibits the optimal fitting effect for the SWCC of Yangling loess. Its parameter a (related to the air entry value) decreases significantly with increasing compaction degree, while parameter n (pore size distribution index) increases linearly. The SWCC model, considering compaction degree, established based on these findings, can accurately predict the water retention characteristics in the high suction range (0~1200 kPa). This model’s precision in the high-suction segment is particularly valuable, as it addresses a critical range for engineering applications where soil behavior transitions from near-saturated to highly unsaturated states. (2) When loess transitions from a saturated to an unsaturated state, the hydraulic conductivity decreases up to 10⁴ times. Both increased initial compaction degree and water content lead to a significant reduction in hydraulic conductivity. This drastic reduction highlights the sensitivity of loess permeability to saturation changes, which is attributed to the rapid reduction in interconnected pore channels as soil suction increases and pore spaces are filled or compressed under higher compaction. (3) The response surface prediction model quantitatively reveals the influence weights of various factors on hydraulic conductivity in the order of matric suction > initial compaction degree > initial water content. The model exhibits a high coefficient of determination (R² = 0.9861), enabling rapid and accurate prediction of the hydraulic conductivity of Yangling loess. This high precision confirms that the model effectively captures the complex interactions between the factors, providing a reliable tool for practical engineering calculations. This study provides a new model and experimental basis for the accurate prediction of unsaturated loess hydraulic properties. The proposed SWCC model, considering compaction degree and the response surface model for hydraulic conductivity, offers practical tools for engineers and researchers, facilitating more precise design and risk assessment in collapsible loess areas. Full article

Biopolymers have recently been introduced as eco-friendly alternatives to other chemical cementitious additives for sandy soil stabilization, especially in pavement construction. The resilient modulus (M_R) is a key metric considered in the mechanistic design of pavement layers that ensures a safe and economic design based on guaranteed accurate values. This study investigated the effects of dehydration on the M_R of biopolymer-treated sand. Prepared specimens were subjected to two different curing conditions. The first set underwent closed-system curing (CSC) for periods of 7, 14, and 28 days. The second set of specimens was cured at different levels of suction by controlling relative humidity (RH) using different salt solutions (0.27, 1.0, 9.7, 21.0, 54.6, 113.7, and 294 MPa), referred to as dehydration curing (DC). The soil water retention curve (SWRC) was measured over the entire suction range to evaluate the dehydration curing and to link the results of suction levels and dehydration regime. M_R tests were conducted on both sets of specimens using a dynamic triaxial system to simulate different confining, traffic, and dynamic stresses. The results showed a significant increase in M_R (i.e., up to eight times) for specimens cured under DC conditions that was proportional to the suction level across different zones of the SWRC. Scanning electron microscopy revealed a phase change from hydrogel to film, which enhanced cementation and bonding between particles. in addition, CSC treatment resulted in a 10–30% reduction in M_R. A new regression model is proposed to predict the M_R of biopolymer-treated sand as a function of confining stresses, dynamic stresses, and suction. These findings will assist pavement engineers and designers in achieving safe, sustainable, and economic designs. Full article

Annually, about 2.4 million tons of superabsorbent polymers (SAPs) used in disposable diapers are thrown away, polluting our planet. This study aims to explore the potential for reusing SAPs removed from diapers to enhance soil water retention. To this end, the swelling and water retention properties of SAP gels from three different types of diapers were compared to those of an agricultural gel, Aquasorb. Sand was used as a model for soil. When mixed with sand, diaper gels have a swelling degree of ca. 100 g per gram of dried polymer, and a swelling pressure of 12–26 kPa, which are similar to those of Aquasorb gel. Using a synthesized poly(acrylamide-co-sodium acrylate) gel as an example, the correlation between the swelling pressure and the compression modulus of the swollen gel was demonstrated. Soil-hydrological constants were estimated from water retention curves obtained by equilibrium centrifugation of gel/sand mixtures. It was observed that adding 0.3 vol% of diaper gels to sand leads to a 3–4-fold increase in water range available to plants, which is close to that provided by agricultural gel Aquasorb. The water-holding properties were shown to be maintained during several swelling/deswelling cycles in the sand medium. The addition of diaper gels to soil had a significant positive impact on mustard (Brassica juncea L.) seed germination and seedling growth, similar to the agricultural gel Aquasorb. This suggests high potential for the reuse of SAPs from diaper waste to improve soil water retention and water accessibility to plants. This would provide both economic and environmental benefits, conserving energy and raw materials to produce new agricultural gels and limiting the amount of waste. Full article

Soil contamination with heavy metals (HM) poses a risk to human health and can impact different soil functions. This study aimed to determine the influence of heavy metal pollution on the physical and physicochemical characteristics of the two profiles of alluvial–deluvial soil under grassland located at different distances from the Aurubis-Pirdop Copper smelter in Bulgaria. Data for soil particle-size distribution, soil bulk and particle densities, mineralogical composition, soil organic carbon contents, cation exchange properties, surface charge, soil water retention curves, pore size distribution—obtained by mercury intrusion porosimetry (MIP)—and thermal properties were obtained. The contents of Pb, Cu, As, Zn, and Cd were above the maximum permissible level in the humic horizon and decreased with depth and distance from the Copper smelter. Depending on HM speciation, the correlations are established with SOC and most physicochemical parameters. It can be concluded that the HMs impact the clay content, specific surface area, distribution of pores, and the water stability of soil aggregate fraction 1–3 mm to varying degrees. Full article

Tillage practices can significantly impact soil structure and pore size distribution and connectivity, consequently affecting the shape of the soil water retention curve (SWRC) and its related estimated hydraulic parameters in the top layer of soil. This study investigated the effect of no-tillage (NT) and conventional tillage (CT) practices on SWRCs and their soil hydraulic parameters, estimated by the Brooks–Corey (BC) function at 0–15 and 15–30 cm depths within sugarbeet and corn planting rows in clay loam and sandy loam soils, respectively. Soil water retention curves were measured using the evaporative method (HYPROP). Measured SWRC results were modeled for both untilled and tilled soils using the BC function for each depth in both soils. In clay loam, results indicated that all soil parameters of the BC function, water contents at 330 (θ₃₃₀) and 15,000 (θ_15,000) hPa, and plant available soil water content (AW) were not significantly affected by the type of tillage at either soil depth. The lack of difference in results between NT and CT may be due to considerable soil disturbance, primarily by the harvest process of sugarbeet roots. However, in sandy loam, results indicated that differences occurred in SWRC’s estimated parameters between the NT and CT practices. Averaged across 4 years and two soil depths, the pore size distribution index (λ) and saturated water content (θ_s) were significantly larger under CT than under NT due to greater soil loosening and disturbance caused by multiple passes of the CT process, thereby developing more soil macroporosity. However, the θ₃₃₀ and AW were significantly larger in NT than in CT due to reduced soil disturbance and improved soil structure under NT compared to CT practices. Regardless of tillage, measurements of SWRC are important for determining better irrigation management practices, enabling producers to optimize crop productivity, while saving water and sustaining water quality. Full article

Water stress represents one of the most critical limiting factors affecting plant distribution, growth rate, biomass accumulation, and crop yield across diverse growth stages. Variations in species’ drought tolerance fundamentally shape global biodiversity patterns by influencing survival rates, distribution ranges, and community composition under changing environmental conditions. This study investigated the physiological responses of six plant species (Haloxylon ammodendron (H.A.), Nitraria tangutorum Bobr. (N.T.B.), Sympegma regelii Bge. (S.R.B.), Tamarix chinensis (T.C.), Potentilla fruticosa (P.F.R.), and Sabina chinensis (Linn.) Ant. (S.C.A.)) to varying water stress levels through controlled water gradient experiments. Four treatment levels were established: W1 (full water supply, >70% field water holding capacity); W2 (mild stress, 50–55%); W3 (moderate stress, 35–40%); and W4 (severe stress, 20–25%). Height growth and leaf mass per area decreased significantly with increasing water stress across all species. S.C.A. consistently exhibited the highest leaf mass per area among the six species, while H.A. showed the lowest values across all treatments. Leaf water content declined progressively with intensifying water stress, with T.C. and P.F.R. showing the most pronounced reductions (T.C.: 16.53%, 18.07%, and 33.37% under W2, W3, and W4, respectively; P.F.R.: 19.45%, 28.52%, and 36.08%), whereas N.T.B. and H.A. demonstrated superior water retention capacity (N.T.B.: 2.44%, 6.64%, and 9.76%; H.A.: 1.44%, 4.39%, and 5.52%). Water saturation deficit increased correspondingly with declining soil moisture. Diurnal leaf water potential patterns exhibited a characteristic V-shaped curve under well-watered (W1) and mildly stressed (W2) conditions, transitioning to a double-valley pattern with unstable fluctuations under moderate (W3) and severe (W4) stress. Leaf water potential showed linear relationships with air temperature and relative humidity, and a quadratic relationship with atmospheric water potential. For all six species, the relationship between pre-dawn leaf water potential and soil water content followed the curve equation y = a + b/x. Under water-deficient conditions, S.C.A. exhibited the greatest water physiological changes, followed by P.F.R. Both logarithmic and power function relationships between leaf and soil water potentials were highly significant (all F > 55.275, all p < 0.01). T.C. leaf water potential was the most sensitive to soil water potential changes, followed by S.C.A., while H.A. demonstrated the least sensitivity. These findings provide essential theoretical foundations for selecting drought-resistant plant species in arid regions of the Qaidam Basin. This study elucidates the response mechanisms of six distinct drought-tolerant plant species under water stress. It provides critical theoretical support for selecting drought-tolerant species, designing community configurations, and implementing water management strategies in vegetation restoration projects within the arid Qaidam Basin. Furthermore, it contributes empirical data at the plant physiological level to understanding the mechanisms sustaining species diversity in arid ecosystems. Full article

Soil erosion in granite-derived weathering mantles poses serious threats to slope stability and ecological sustainability in subtropical regions. While polyacrylamide (PAM) is widely used to improve soil structure, its concentration-dependent effects on multiple soil functions remain unclear. This study developed a multifunctional Soil Function Index (SFI) framework integrating erosion resistance (SFI₁), water regulation (SFI₂), and ecological function (SFI₃) to evaluate the effects of PAM application (0‰, 1‰, 3‰, 5‰, 7‰) on gully-prone sandy material. Herein, SFI₁ was quantified through shear strength (τ) and soil erodibility (K_r); SFI₂ was assessed using soil hydraulic parameters (saturated hydraulic conductivity and water retention curves) and SFI₃ was derived from the grass root system analysis. The results showed that SFI₁ and SFI₂ increased nonlinearly with PAM concentration, reaching maximum values of 0.983 and 0.980 at 7‰, with K_r reduced by 77.3% and non-capillary porosity (NAP) increased by 8.1%. In contrast, SFI₃ peaked at 0.858 under 3‰ and declined sharply to 0.000 at 7‰, due to micropore over-compaction, reduced aeration, and limited plant-available water. The total SFI exhibited a unimodal trend, with a maximum of 0.755 at 3‰, beyond which ecological suppression offset physical improvements. These findings demonstrate that PAM modifies soil multifunctionality through pore-scale restructuring, inducing function-specific thresholds and trade-offs. A PAM concentration of 3‰ is identified as optimal, achieving a balance between erosion control, hydrological performance, and ecological viability in the management of subtropical granite-derived sandy slopes. Full article

The dynamic non-equilibrium effect (DNE) describes the non-unique character of saturation–capillary pressure relationships observed under static, steady-state, or monotonic hydrodynamic conditions. Macroscopically, the DNE manifests as variations in soil hydraulic characteristic curves arising from varying hydrodynamic testing conditions and is fundamentally governed by soil matrix particle size distribution. Changes in the DNE across porous media with discrete particle size fractions are investigated via stepwise drying experiments. Through quantification of saturation–capillary pressure hysteresis and DNE metrics, three critical signatures are identified: (1) the temporal lag between peak capillary pressure and minimum water saturation; (2) the pressure gap between transient and equilibrium states; and (3) residual water saturation. In the four experimental sets, with the finest material (Test 1), the peak capillary pressure consistently precedes the minimum water saturation by up to 60 s. Conversely, with the coarsest material (Test 4), peak capillary pressure does not consistently precede minimum saturation, with a maximum lag of only 30 s. The pressure gap between transient and equilibrium states reached 14.04 cm H₂O in the finest sand, compared to only 2.65 cm H₂O in the coarsest sand. Simultaneously, residual water saturation was significantly higher in the finest sand (0.364) than in the coarsest sand (0.086). The results further reveal that the intensity of the DNE scales inversely with particle size and linearly with wetting phase saturation (Sw), exhibiting systematic decay as Sw decreases. Coarse media exhibit negligible hysteresis due to suppressed capillary retention; this is in stark contrast with fine sands, in which the DNE is observed to persist in advanced drying stages. These results establish pore geometry and capillary dominance as fundamental factors controlling non-equilibrium fluid dynamics, providing a mechanistic framework for the refinement of multi-phase flow models in heterogeneous porous systems. Full article

This review provides a comprehensive overview of the closed-form expressions developed for estimating the soil water retention curve (SWRC) from 1964 to the present. Since the concept of the SWRC was introduced in 1907, numerous closed-form functions have been proposed to describe the relationship between soil matric suction and volumetric water content, each with distinct strengths and limitations. Given the variability in SWRC shapes influenced by soil texture, structure, and organic matter, models in the form of sigmoidal, multi-exponential, lognormal, hyperbolic, and hybrid functions have been designed to fit experimental SWRC data. Based on the number of adjustable parameters, these models are categorized into three main groups: three-, four-, and five-parameter models. They can also be classified as one-, two-, or three-segment functions depending on their structural complexity. A review of the developed models indicates that most are effective in representing the SWRC between the residual and saturated water content range. To capture the full range of the SWRC, hybrid functions have been proposed by combining traditional models. This review presents and discusses these models in chronological order of publication. Full article