A bimodal soil–water retention curve model for dual-porosity soils in the Three Gorges Reservoir hydro-fluctuation belt

As biocementation is beginning to be explored to improve the engineering properties of fine-grained soils such as clays, its performance under unsaturated conditions has become a key concern. In these geomaterials, the coupled interaction between the liquid and gas phases and their interaction with the solid skeleton plays a key role in the overall hydromechanical response. Accordingly, establishing a fundamental understanding of the unsaturated mechanical behavior of biocemented clays requires examining how calcium carbonate precipitation influences water retention and volumetric behavior. This study investigates these effects using two clayey soils biocemented at calcite contents of 5% and 10% using a novel mixing technique. Experimental results show that biocementation significantly reduces volumetric shrinkage under free-drying conditions, with volumetric strain reductions of up to 25%. In addition, treated soils exhibit a pronounced increase in air-entry value (AEV), reaching approximately a 70% increase at the 10% calcite level. Crucially, this increase in AEV is found to be independent of clay type, suggesting a generalizable trend for these geomaterials. By providing a detailed characterization of the water retention curves and volumetric response, this research offers a framework for understanding the unsaturated behavior of biocemented fine-grained soils. Ultimately, these results support the development of more reliable biocementation approaches for clays subjected to moisture variations, facilitating the broader adoption of this technique. • Developed a novel mixing protocol for biocementation, using ex-situ urea hydrolysis, that ensures uniform calcite distribution in clays and reduces environmental impact. • Biocementation reduced volumetric shrinkage by up to 25% in low-plasticity clay (LPC) and ~12% in high-plasticity clay (HPC) under free drying. • Air entry value (AEV) increased by ~40% at 5% calcite content and ~70% at 10% calcite content for both clay types. • Results suggest biocementation affects water retention and volumetric behavior through distinct microstructural changes to the clay fabric

Soil compaction is a major driver of soil degradation, expressed as a reduction in soil volume and an increase in bulk density (BD), primarily due to porosity loss. Given the agronomic and ecological importance of pore-size distribution, this study assessed the effects of different traffic intensities associated with grain hauling in a no-tillage wheat stubble field. Three treatments were established: no traffic (NT), one pass (1P) of a tractor-grain cart combination, and two passes (2P) of the same equipment combination. The evaluated parameters included soil moisture at the time of traffic (SM), bulk density (BD), degree of compactness (DG), soil water retention curve (SWRC), pore-size distribution, S-index (S), and stress propagation measured using pressure sensors. Under 1P, BD significantly increased down to 15 cm depth, whereas under 2P the effect extended to 20–35 cm depth. Total porosity declined to 25 cm under 1P and to 30 cm depth under 2P. The S-index dropped below critical threshold values between 10 and 25 cm under 2P, consistent with observed reductions in porosity. DG values aligned with S vs. DG curve inflections. Pressure sensors failed to decompress at 30 cm under 2P, indicating persistent compaction. These findings confirm the reliability of the S as a soil quality indicator and highlight the potential of pressure sensors for detecting soil deformation under traffic-induced stresses.

Increasing total soil porosity and optimizing pore distribution improve soil water-holding capacity, thereby alleviating drought impacts on crop yields in semi-arid regions. A three year split-plot field experiment was conducted, with organic fertilizer (sheep manure) rates as main plots and water-retaining agent (WRA) rates as subplots. Four organic fertilizer (0, 45, 60, and 75 Mg hm−2) and four WRA rates (0, 0.3, 0.6, and 0.9 Mg hm−2) were set, resulting in 16 combined treatments. Undisturbed soil samples were collected to analyze pore distribution and water availability using the soil water retention curve. The results showed significant variations in ameliorative effects with soil depth. Individual applications of either organic fertilizer or WRA significantly improved topsoil pore distribution and water availability but exerted negative effects on the subsoil. Combined application enhanced both soil layers, with a stronger synergistic effect in the subsoil. The combination of 45 Mg ha−2 organic fertilizer + 0.9 Mg ha−2 WRA achieved optimal soil improvement in the 0–20 cm layer, increasing aeration porosity by 21.89% compared to organic fertilizer alone; this improvement led to 14.99% and 15.65% increases in plant available water (PAW) and readily available water (RAW), respectively. For the 20–40 cm layer, the combination of 60 Mg ha−2 organic fertilizer + 0.9 Mg ha−2 WRA was optimal, increasing total, aeration, and capillary porosity by 24.18%, 183.50%, and 56.73%, respectively, compared to organic fertilizer alone. Consequently, subsoil water availability was enhanced, resulting in 57.53% and 61.18% higher PAW and RAW than the control without WRA. These findings highlight the necessity of layer-specific regulation and differentiated management. The optimal combinations (OF45+W0.9 for 0–20 cm and OF60+W0.9 for 20–40 cm) effectively optimize pore distribution and increase water availability through the complementary synergistic effects of organic fertilizer and WRA. Consequently, this strategy alleviates drought stress on crop yields in semi-arid regions.

Effect of Soil Textural Characteristics on Soil Water Retention Curve

A unified soil–water retention curve model for unsaturated soils considering freeze–thaw cycle effects

ABSTRACT In sandy soils, low organic carbon content and weak structural stability often constrain pore functionality, water availability, and gas exchange, highlighting the need for management strategies that improve soil physical quality. The aim of this study was to quantify the impact of cover crops on SOC, soil physical properties, and soil processes in a sandy loam dystrophic Ferralsol (Latossolo Vermelho-Amarelo Distrófico). The experiment followed a randomized complete block design composed of five treatments and five repetitions. The experimental treatments were: (1) Control (fallow plots subjected to weed desiccation), (2) G (single grass: Urochloa ruziziensis), (3) GG (two grasses intercropped: Pennisetum americanum + U. ruziziensis), (4) GL (one grass and one legume intercropped: P. americanum + Mucuna pruriens), and (5) MIX (two grasses and one legume intercropped: P. americanum + U. ruziziensis + M. pruriens). Undisturbed samples were collected from the layers of 0.00-0.10, 0.10-0.20, 0.20-0.40, and 0.40-0.60 m to determine physical indicators such as bulk density (Bd), total porosity (TP), field capacity (FC), permanent wilting point (PWP), plant-available water (PAW), and the soil water retention curve, pore size distribution, water storage capacity (FC/TP), saturated hydraulic conductivity (Ksat), air permeability (Kair), and pore continuity index (K1). Results demonstrated that, compared with the Control (absence of cover crops), GL (grass + legume), MIX (mixed species), and GG (grasses) improved air conductivity by 8, 3.5, and 2.9 times, and pore continuity by 5.8, 2.9, and 2.2 times, respectively. The MIX system led to a 39 % increase in SOC relative to the Control. Additionally, intercropping two grass species with one legume (MIX) and combining one grass with one legume (GL) resulted in a 19 % increase in plant-available water compared with the Control treatment. Importantly, these improvements in pore architecture occurred without significant changes in soil bulk density (Bd) or total porosity (TP), underscoring that cover crops can reorganize the pore network independently of mass–volume relationships. Combination of grasses and legumes (GL, MIX) has substantial potential to improve plant-available water and the overall soil physical quality of sandy soils.

The synergistic treatment of sandy soil with biopolymer and plant fiber as a soil improvement technique offers both stable strength and sustainability. However, due to the hydrophilic nature of both materials, soil structure is modified, thereby affecting its water retention capacity. To predict the effect of the combined treatment on the internal moisture of stabilized soil, it is of great significance to develop a non-saturated soil water retention model applicable to the synergistic improvement of biopolymer and plant fiber. Existing models do not account for the interactions between biopolymer and plant fiber with soil. In this study, various interaction mechanisms between biopolymer and soil are considered, including: (1) Biopolymer and plant fiber occupy part of the pore space, thereby altering pore volume of the soil; (2) Biopolymer and plant fiber themselves possess certain water-holding capacity; (3) The expansion of biopolymer and plant fiber may cause changes in soil volume, and their expansion is constrained by soil particles. To validate the new model, water retention curves were obtained through centrifuge measurements and computational verification of existing literature data. The experimental and calculated results demonstrated good consistency. The findings confirm that the new model effectively predicts the water retention characteristics of various soils after synergistic amendment with biopolymer and plant fiber.

Soil–water retention (SWR) is fundamental for understanding the hydro-mechanical behavior of unsaturated clay soils. The soil–water retention curve is typically obtained through extensive and costly laboratory testing. To offer a more efficient alternative, an extreme gradient boosting (XGBoost) model, optimized using a hybrid particle swarm optimization and genetic algorithm (PSO–GA), was developed. This hybrid model estimates the SWR across a broad suction range, accounting for both drying and wetting paths, along with key soil parameters. The performance of the model was evaluated through various statistical analyses and by comparing the predicted gravimetric water content with experimental data. A backward feature elimination method was employed to assess the impact of various input parameters on model accuracy and to offer a simplified model for scenarios with limited data availability. Additionally, Monte Carlo simulations were conducted to quantify the inherent uncertainties associated with the dataset, XGBoost hyperparameters, and model performance. The hybrid PSO–GA XGBoost model effectively estimates the water retention of clayey soils during both drying and wetting cycles, proving to be an alternative to traditional soil mechanics correlations. • PSO–GA XGBoost predicts soil–water retention with high accuracy across a wide suction range. • Model captures clay soil’s drying–wetting paths and quantifies uncertainties through Monte Carlo. • Reduced feature models maintain reliability, balancing simplicity and precision in applications.

Microplastics contamination in agricultural soils represents an emerging threat to soil health and ecosystem functioning. This study investigated the effects of polyester (PES) microplastic (MP) fibers on soil physical quality (SPQ) using indicators derived from soil water retention curve (SWRC) inflection point, based on Dexter's S-theory. Six soils with different textures were contaminated with PES fibers at concentrations of 0.25 %, 0.5 %, and 1 % (w/w) and incubated for about six months. Four SWRC models (van Genuchten with Mualem constraint, VGM; van Genuchten with Burdine constraint, VGB; van Genuchten unconstrained, VGN; and Kosugi, KSG) were fitted to experimental water retention data. The VGN and VGM models provided the best fitting accuracy across soil types. Overall, MP contamination altered key SPQ indicators determining: a decrement of the pressure head at inflection point (h*) and of effective porosity (Φ*), indicating larger modal pore diameters; an increment of the slope at inflection point (S*), suggesting enhanced soil aggregation. Effects were soil type and concentration dependent, with changes primarily occurring at 1 % MP concentration. Soils with moderate clay content (clay < 30 %) showed improved S* values, while clay rich soils showed minimal response. Capacitive indicators (air capacity and plant available water capacity) remained largely unaffected, suggesting preserved total porosity despite internal pore structure modifications. These findings demonstrate that PES MP fiber contamination can alter soil pore architecture and aggregation without substantially impacting bulk hydraulic properties, highlighting the complexity of MP-soil interactions and the value of inflection point indicators for detecting subtle changes in soil physical quality.

Abstract In soil physics, saturated hydraulic conductivity, , is a key soil hydraulic parameter governing the movement of water and solutes, with broad implications for transport processes under variably saturated conditions. Its accurate estimation, however, is challenging and requires precise characterization of pore space. In this study, we applied concepts of critical path analysis (CPA) to estimate from soil water retention curve. To evaluate the CPA, we used observations made on 313 undisturbed soil cores from the Kansas Mesonet soil physical property database in which the value of spans over five orders of magnitude in variation. We found that the CPA estimated reasonably well with root mean square log‐transformed error RMSLE = 0.87. For most samples, the predicted values were around the 1:1 line within a factor of 10 of the measurements. We also estimated using five other methods, but none was more accurate than the CPA. We found the RMSLE values were 1.42 for the Kozeny–Carman model, 1.28 for the Revil, Glover, Pezard, and Zamora model, 1.18 for the Johnson, Koplik, and Schwartz model, 2.68 for the Mishra and Parker model, and 1.40 for the Guarracino model. This study provides one of the first large‐sample, field‐based validations of the CPA framework for estimating in predominantly fine‐textured soils.

Lateritic soils, also known as red soils, are prone to actions of external environment such as wetting and drying processes and their cycling. However, in the practice of pavement design in red soil regions, there lacks experimental studies and analytical approaches to reveal and predict the evolution of the hydromechanical characteristics of red soils under complex environmental actions. This paper investigates the variation in the hydrostructural and dynamic characteristics of a compacted subgrade red clay collected from Nanning, Guangxi, China, before and after wetting-drying cycles. The pore structure and soil-water characteristics of the red clay before and after ten wetting-drying cycles were determined to reveal the influences of moisture fluctuation history. Besides, cyclic triaxial tests were performed to determine the resilient modulus (MR) and permanent strain (εp) of the red clay and reveal the influences of external stress, moisture content w, suction s, and WD cycles. It is found that (i) compacted red clay presents typical dual porosity with distinct inter-aggregate and intra-aggregate pores. Such pore structure results in bimodal soil water retention curves (SWRCs) of the red clay; (ii) upon wetting-drying cycles, the intra-aggregate pores shrink while the inter-aggregate pores swell. Besides, the global pore space (i.e., the overall void ratio) increases after WD cycles. This results in the elimination of the SWRC’s bimodal characteristics and the reduction in the clay’s water retention capacity in the low suction range and scale of shrinkage upon drying; (iii) the εp and MR vary non-linearly with σd, w, and s. Their relationships to the external stress and soil moisture change remarkably after WD cycles. A simple model was adopted to describe the variations of the εp and MR with w and s, which has achieved close agreements with the experimental measurements; (iv) the εp and MR of the tested red clay show a unique non-linear relationship regardless of the influences of σd, w, s, and WD cycles, which highlights possible intrinsic relationships between the elastic and plastic behaviors of compacted subgrade soils.

Using only bulk density, saturated water content and macroporosity measurements to infer time variability in soil water retention curves

Soil water retention and availability are influenced by intrinsic soil properties, management practices, and climate regimes. This study aimed to evaluate water retention and availability in an Ultisol under different integrated production systems in the Brazilian Cerrado. The systems analyzed included Crop–Livestock Integration (CLI), Livestock–Forest Integration (LFI), Crop–Forest Integration (CFI), no-tillage (NT) and native Cerrado vegetation (NV). Disturbed samples were collected for physical and chemical characterization, while undisturbed samples were used to determine water retention curves at depths of 0.00–0.10, 0.10–0.20, and 0.20–0.40 m. From these curves, water availability, pore-size distribution, differential log-pore-radius curves, most frequent pore radius, and relative hydraulic conductivity were estimated using the Mualem–van Genuchten model. Confidence intervals were used to evaluate differences between retention curves. The CLI system showed lower water content at saturation (14–30%) and field capacity (10–20%) compared to CFI, LFI, and NT. The NT system exhibited higher water availability across all layers (28, 48, and 46%, respectively) than CLI. Alterations in pore structure, likely due to the short integration period and monoculture history in CLI, resulted in lower water retention. Conversely, CFI, LFI, and NT showed higher retention and availability, attributed to higher organic matter content and more stable structural pores. Integrated production and no-tillage systems, especially when adopted long-term, enhance soil water retention and availability in the Brazilian Cerrado.

Intensive agricultural mechanization in Northeast China has exacerbated soil compaction and degraded water retention. Although biochar modifies soil hydraulics, its combined effect with matric suction on compressive behavior remains unclear. This study investigated the hydraulic and mechanical responses of repacked sandy clay brown soil to biochar (0, 0.5, 1 g kg−1) under varying matric suction (6–1000 kPa). We utilized water retention curves and uniaxial compression tests to assess mechanical properties, including pre-compression stress (σp), penetration resistance (PR), compression index (Cc) and swelling index (Cs). Additionally, an integrated model using the Entropy Weight Method (EWM), the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS), and the Adversarial Interpretive Structure Model (AISM) was developed to evaluate soil resistance and resilience. Results indicated that 1 g kg−1 biochar significantly enhanced field capacity (θFC) and readily extractable water (θMRE) (p &lt; 0.05). While individual factors influenced all mechanical properties, the biochar–suction interaction significantly affected pre-compression stress and the compression index (p &lt; 0.05). The model identified 1 g kg−1 biochar at 1000 kPa suction as the optimal combination for maximizing soil structural stability. These findings highlight the critical role of biochar–matric suction interactions in accurately assessing and managing soil mechanical behavior.

Abstract The development of an image‐based method for determining soil water retention curves (WRCs) is effective for predicting infiltration processes into soil. We investigated the effects of microscopy magnification on image resolution for estimating WRCs. Images of the soil surface were acquired at magnification of 50×, 100×, and 200×. Large‐sized pores (&gt;30 µm in radius) captured at lower magnifications of 50× and 100× were closer to values obtained from the conventional method, pressure plate method, because images at low magnification could capture wide areas and measure representative values. The image‐based method at high magnification (200×), showed spatial distributions and captured local tendencies. All parameters of hydraulic property of lognormal model ( θ e , ψ m , and σ ) had the same areas with large values caused by structural development. This means that WRCs had variabilities on the surface of the core samplers with 5‐cm inner diameter. Relationships between magnification and WRC parameters were investigated. The θ e and σ could be captured exactly at a magnification of 50×. In this study, ψ m was underestimated by the image‐based method at all magnifications. In conclusion, for estimating WRCs, image obtained at magnification of 50× showed the highest accuracy. However, images at 50× magnification could not capture the microscopic spatial distribution of soil pore structure.

The inflection point of the soil water retention curve (SWRC) is increasingly recognized as a key indicator of soil physical quality, as it reflects critical changes in pore structure and water availability. This study aims to develop and validate pedotransfer functions (PTFs) to estimate the water content (θi), matric suction head (hi), and slope (Si) at the SWRC inflexion point from basic soil physical properties. A dataset comprising 219 soil samples, including laboratory-measured and UNSODA database entries, was used. The inflection point parameters were computed analytically from van enuchten model fits. Linear, nonlinear, and polynomial regression techniques were applied to derive PTFs using soil organic matter, bulk density, geometric mean particle diameter, and geometric standard deviation as input variables. Model performance was evaluated using root mean square error (RMSE), normalized root mean square error (NRMSE), correlation coefficient (r), and Taylor diagrams. A dataset comprising 219 soil samples, including laboratory-measured and UNSODA database entries, was used. The inflection point parameters were computed analytically from van Genuchten model fits. Linear, nonlinear, and polynomial regression techniques were applied to derive PTFs using soil organic matter, bulk density, geometric mean particle diameter, and geometric standard deviation as input variables. Model performance was evaluated using root mean square error (RMSE), normalized root mean square error (NRMSE), correlation coefficient (r), and Taylor diagrams. Six PTFs were developed for θi (best model: r = 0.90; NRMSE = 8.6%), six for Si (best model: r = 0.71; NRMSE = 14.8%), and three for hi (best model: r = 0.46; NRMSE = 39.4%). θi and Si were estimated with good to excellent accuracy, while hi proved more difficult to predict due to its dependence on microstructural properties not captured by standard soil descriptors. The developed PTFs for θi and Si are reliable and practical tools for assessing soil hydraulic behavior and physical quality. In contrast, accurate estimation of hi remains challenging, suggesting the need for additional structural or imaging-based predictors in future models.

Bentonite serves as a critical engineered barrier in deep geological repositories, necessitating a reliable description of its temperature-dependent water retention behavior. This study determined soil water retention curves (SWRCs) for four bentonites (Sab65, GMZ, B75 powder, B75 pellet) along drying/wetting paths at 20–80 °C using vapor equilibrium technique, spanning a suction range of 5–300 MPa. All exhibit S-shaped SWRCs. Higher temperature systematically reduces water content at given suction, shifting SWRCs downward; this effect weakens at high suction where adsorption dominates. Material responses differ: Sab65 shows highest water content at low suction but strongest decrease at high suction and elevated temperature. Drying-wetting hysteresis is material-dependent. The van Genuchten (VG) model reproduced the drying-path SWRCs with high accuracy. The fitted parameter α increased with temperature (particularly for Sab65 and B75), whereas n showed only minor changes, indicating that heating primarily shifts the SWRC along the suction axis while the overall curve shape remains broadly similar. These findings elucidate the synergistic effects of temperature, mineralogy, and form on bentonite retention, thereby providing essential insights for the long-term performance assessment of engineered barriers under thermal conditions.

Identifying and characterizing bimodal soil hydraulic properties through water retention curve analysis

Drainage and imbibition along main and scanning curves: A pore-scale morphology approach