Estimating the wetting branch of the soil water retention curve from grain-size fractions

The development of a theoretical method for estimating the wetting branch of the soil water retention curve (SWRC) is required for knowledge of the soil infiltration process. The aim of this study was to derive the theoretical functions to represent the wetting branch of the SWRC based on the Jensen method, and then compare the Jensen method and Kool & Parker (KP87) model for estimating the wetting branch of the SWRC. Fifteen soil samples with varying basic properties (e.g., grain-size distribution and bulk density (BD)) were selected from the Unsaturated Soil Hydraulic Database (UNSODA) to test these two methods. Results showed that the Jensen method (root mean squared error (RMSE) = 0.057 cm³ cm⁻³) produced a substantially better performance in predicting the wetting branch of the SWRC than the KP87 (RMSE = 0.089 cm³ cm⁻³) for the 15 samples. The range of the scaled mean bias error (SMBE) between the Jensen method-predicted and measured soil water contents at all pressure heads was −0.529 to 0.402. A positive linear relationship between the SMBE and silt content was observed for the Jensen method. The findings of this study have practical significance for simulating the soil infiltration in the unsaturated zone.     

使用积分法估算原状土van Genuchten模型参数

The van Genuchten model is the most widely used soil water retention curve (SWRC) model. Two undisturbed soils (clay and loam) were used to evaluate the accuracy of the integral method to estimate van Genuchten model parameters and to determine SWRCs of undisturbed soils. SWRCs calculated by the integral method were compared with those measured by a high speed centrifuge technique. The accuracy of the calculated results was evaluated graphically, as well as by root mean square error (RMSE), normalized root mean square error (NRMSE) and Willmott’s index of agreement (l). The results obtained from the integral method were quite similar to those by the centrifuge technique. The RMSEs (4.61 × 10-5 for Eum-Orthic Anthrosol and 2.74 × 10-4 for Los-Orthic Entisol) and NRMSEs (1.56 × 10-4 for Eum-Orthic Anthrosol and 1.45 × 10-3 for Los-Orthic Entisol) were relatively small. The l values were 0.973 and 0.943 for Eum-Orthic Anthrosol and Los-Orthic Entisol, respectively, indicating a good agreement between the integral method values and the centrifuge values. Therefore, the integral method could be used to estimate SWRCs of undisturbed clay and loam soils.     

基于土壤质地数据估算土壤保水性能Van Genuchten参数

The van Genuchten (vG) function is often used to describe the soil water retention curve (SWRC) of unsaturated soils and fractured rock. The objective of this study was to develop a method to determine the vG model parameter m from the fractal dimension. We compared two approaches previously proposed by van Genuchten and Lenhard et al. for estimating m from the pore size distribution index of the Brooks and Corey (BC) model. In both approaches we used a relationship between the pore size distribution index of the BC model and the fractal dimension of the SWRC. A dataset containing 75 samples from the UNSODA unsaturated soil hydraulic database was used to evaluate the two approaches. The statistical parameters showed that the approach by Lenhard et al. provided better estimates of the parameter m. Another dataset containing 72 samples from the literature was used to validate Lenhard’s approach in which the SWRC fractal dimension was estimated from the clay content. The estimated SWRC of the second dataset was compared with those obtained with the Rosetta model using sand, silt, and clay contents. Root mean square error values of the proposed fractal approach and Rosetta were 0.081 and 0.136, respectively, indicating that the proposed fractal approach performed better than the Rosetta model.     

Deforestation effects on soil quality and water retention curve parameters in eastern Ardabil,Iran

The land use change from natural to managed ecosystems causes serious soil degradation. The main objective of this research was to assess deforestation effects on soil physical quality attributes and soil water retention curve (SWRC) parameters in the Fandoghlou region of Ardabil province, Iran. Totally 36 surface and subsurface soil samples were taken and soil water contents measured at 13 suctions. Alfa (α) and n parameters in van Genuchten (1980) model were estimated by fitting SWRC data by using RETC software. The slope of SWRC at inflection point (S_P) was calculated by Dexter (2004) equation. The results indicated that with changing land use from forest (F) to range land (R) and cultivated land (C), and also with increasing soil depth from 0–25 to 75–100 cm in each land use, organic carbon, micropores, saturated and available water contents decreased and macropores and bulk density increased significantly (P < 0.05). The position of SWRC shape in F was higher than R and C lands at all soil depths. Changing F to R and C lands and also increasing soil depth in each land use significantly (P < 0.05) increased α and decreased n and S_P. The average values of S_P were obtained 0.093, 0.051 and 0.031 for F, R and C, respectively. As a result, deforestation reduced soil physical quality by affecting SWRC parameters.     

Water repellency of organic growing media related to hysteretic water retention properties

With respect to soils, most growing media can exhibit hysteresis during drying/wetting cycles, which greatly affects their hydraulic properties. In the case of organic substrates, hydrophobicity during desiccation could be considered as one of the main factors leading to hysteretic behaviour. The purpose of this study was to estimate the influence of changes in wettability on the water retention properties, θ(ψ), of peat and pine bark during a drying/wetting cycle. Major differences in the hydraulic behaviour of the two organic materials studied were observed. For peat, hysteresis was found in the water retention curve (21%) and also in the contact angle/water potential relationship, (α(ψ), 20%), whereas in pine bark, this phenomenon was less pronounced in the water retention curve (10%) and even more limited in the α(ψ) curve (> 5%). Water retention hysteresis was successfully modelled using a modified van Genuchten‐Durner approach (VG_α model), which took into account the local hydrophobicity of each poral domain of the porous media, regardless of the extent of hysteresis. Incorporating the parameters of the VG_α water retention model into a α(ψ) equation to characterize overall or average changes in the hydrophobicity of the material during desiccation resulted in values very similar to those of the contact angles calculated with the capillary rise method. These results indicate that water retention properties of these organic substrates are strongly influenced by hydrophobicity.     

Hysterese der Wasserspannungskurve in organogenen Böden

Soil water hysteresis in organic soils In 20 undisturbed samples of peat, muck, peaty soil and humic sand, the hysteresis of the moisture characteristic by soil suction from 0 to –9,8 kPa and from 0 to –29,4 kPa was measured. The greatest hysteresis was found in peat. Suction values from –1 to –3 kPa caused differences of 0,076 cm³/cm³ in water content during drying and wetting. In muck with low ash content and in humic sand the hysteresis was smaller. In peaty soil and muck with high ash content (> 40 %) it diminished to twice or three times lower values in comparison with those in peat. In organic soils the hysteresis decreases clearly by soil suction higher than –6 kPa. Repetition of drying and wetting gave a diminution (ca. 30 %) of the hysteresis loop in peat and displaces it towards the field of lower water content. The hysteresis of muck did not change considerably. The change of the porosity structure (pore ϕ > 30 μm) during the repetition of drying and wetting indicate that some soil shrinkage has occured.     

General scaling rules of the hysteretic water retention function based on Mualem's domain theory

Based on the domain theory of hysteresis, the present study rigorously derives a unified scaling transformation for predicting the wetting scanning retention function, θ_w ( ψ ), following any sequence of wetting and drying processes, from the measured main wetting curve. It is proved theoretically that a shape-similarity exists among the different wetting curves of a given soil. Each wetting scanning curve and the main wetting curve are described by the same normalized equation, over the common interval of ψ values. This shape-similarity is a direct result of the Mualem domain theory. On this basis, a general unique equation is formulated for prediction of all wetting scanning curves, of all orders, which is compatible with the Mualem dependent, as well as independent, domain theory. On the same theoretical basis, it is proved that no similarity exists among the drying scanning curves. Thus, the use of the scaling transformation for prediction of the drying scans is subjected to inconsistency with the physical principles underlying the dependent domain theory. As a result, scaling in this case would inevitably lead to inaccuracies in the calculated drying curves. A significant step forward has been made in the present study, from Mualem's dependent domain model (1984) with its implicit predictive formulae of the drying scanning curves. A unique explicit equation was theoretically derived, applicable for describing the drying scanning curves of all orders. This unique equation of the drying scanning curves, together with the general single equation of the wetting scanning curves, provided closure of all theoretical wetting and drying scans within the main hysteresis loop. Consequently, the retention function following any sequence of successive wetting and drying processes can be predicted by these two equations determined by the measured boundary curves.     

Differences between field-monitored and laboratory-measured soil moisture characteristics

A soil water retention curve (SWRC) is usually measured in a laboratory (lab SWRC), and is used to analyze in-situ soil moisture conditions. However, it is rarely verified whether and how a lab SWRC is in agreement with its equivalent relation between matric potential (h) and volumetric water content (θ) in a natural field (in-situ SWRC). In addition, most SWRCs show moisture hysteresis through which the drying process gives a larger θ at a given h than the wetting process, while an in-situ SWRC must be produced through the cycles of drying and wetting in the field. Thus, it can be hypothesized that an in-situ SWRC shows a lower value of θ than a lab SWRC for any h that the soil layer ordinarily experiences. To give experimental proofs for this hypothesis, this study aimed at quantifying seasonal behaviors of in-situ SWRCs and at comparing them with their corresponding lab SWRCs. To obtain a series of in-situ SWRCs, the h and θ were coincidently monitored at four points with three depths each in a meadow for 2.5 years using tensiometers and a capacitance-type soil moisture sensing system. As the equivalent to the in-situ SWRCs, the lab SWRCs were also measured. The in-situ SWRCs tended to have roughly 10% smaller θ than the lab SWRCs for the series of h observed in the study site, suggesting that an in-situ SWRC can hardly be reproduced by a lab SWRC only. In addition, when the driest condition in the recent 3 years was exerted on the study site, some in-situ SWRCs shifted along the θ axis on the θ(h) charts, suggesting that the most dried condition had changed the soil moisture regime of these soil layers, resulting in the reduction of monthly or annual means of soil water content in the field. Since the shifts of the in-situ SWRCs were accompanied by the increases in both the gradients ‘dθ/dh’ and the variation of measured h, it was implied that an extraordinary drying of a soil layer promotes the development of soil pore structure or an increase in the fraction of plant available water.     

Pedo-transfer functions for estimating the hydraulic properties of paddy soils in subtropical central China

Pedo-transfer functions (PTFs) have been widely used to estimate soil hydraulic properties in the simulation of catchment eco-hydrological processes. However, the accuracy of existing PTFs is usually inadequate for use. To develop PTFs for local use, soil columns were collected from a double rice-cropped agricultural catchment in subtropical central China. The PTFs for saturated soil hydraulic conductivity (K_s) and parameters (θ_s, α, and n) of the van Genuchten model for the soil water retention curve (SWRC) were obtained based on soil’s basic properties, and compared with models developed by Li et al. in 2007 and Wösten et al. in 1999, respectively. Our results indicated that K_s in the range of 0.04–1087 cm d^?1 and θ_s in the range of 0.34–0.51 cm³ cm^?3 were both well estimated with the R²_adj of 0.72 and 0.87, respectively, but α (0.04–0.65 cm^?1) and n (1.05–1.21) were relatively poorly predicted with the respective R²_adj of 0.38 and 0.55, despite the use of more input parameters. Our local derived PTFs outperformed the other two existing models. However, if the local PTFs for paddy soils are not available, the Wösten et al. 1999 model can be proposed as a useful alternative. Therefore, this study can improve our understanding of the development and application of PTFs for predicting paddy soil hydraulic properties in China.     

A New Approach in Comparison and Evaluation of the Overall Accuracy of Six Soil-Water Retention Models Using Statistical Benchmarks and Fuzzy Method

Eurasian Soil Science - In this research, six samples of valid and widely soil-water retention curve (SWRC) estimation models, including van Genuchten, Brooks and Corey, Fredlund and Xing, Durner,...     

Inverse Estimation of the Hydrodispersive Properties of Unsaturated Soil Using Complex-Variable-Differentiation Method under Field Experiments Conditions

Because soil hydraulic properties are indispensable for determining soil water retention and soil solute movement, their input for simulation models is essential. Many of these parameters cannot be estimated directly at the scale of interest, but can only be derived through inverse modeling. During this process, the parameters are generally adjusted using least-squares approach with Levenberg–Marquardt (LM) algorithms in which numerically simulated models are fitted to measured data. In this study we used a new inverse method to estimate the unsaturated soil hydro-dispersive properties from in-situ experiments. The method employs complex-variable-differentiation method (CVDM) to accurately predict of the hydraulic properties of the van Genuchten–Mualem models (θ_r, θ_s, α, k_s, n). To the knowledge of the authors, it is first study use CVDM in soil physics. The optimization procedure was performed by using a continuous data set of daily in situ soil water content and bromide concentration measurements. Estimated parameters during the inversion showed high correlation (R² = 0.88, RMSE = 0.013 and the model efficiency CE = 0.77) by using the CVDM-methods with the actual field measurements, compared with the traditional LM-algorithm (R² = 0.81, RMSE = 0.021 and CE = 0.626). The results show that the new inverse analysis in the present work has the high accuracy, validity, uniqueness, and higher inversion efficiency. Meanwhile, the convergence and stability of the modified LM-algorithm are improved. Overall, it was concluded that the CVDM is promising method to estimate hydro-dispersive parameters in soil physics.     

Optimizing Pedotransfer Functions for Predicting Soil Moisture of Wetting Curve Based on the Effective Degree of Saturation

Eurasian Soil Science - Hysteresis of soil water retention curve (SWRC hysteresis) is widely used for modeling water flux in an unsaturated soil during infiltration and irrigation processes. The...     

Estimating the water retention curve from soil properties: comparison of linear, nonlinear and concomitant variable methods

The unsaturated soil hydraulic functions involving the soil–water retention curve (SWRC) and the hydraulic conductivity provide useful integrated indices of soil quality. Existing and newly devised methods were used to formulate pedotransfer functions (PTFs) that predict the SWRC from readily available soil data. The PTFs were calibrated using a large soils database from Hungary. The database contains measured soil–water retention data, the dry bulk density, sand, silt and clay percentages, and the organic matter content of 305 soil layers from some 80 soil profiles. A three-parameter van Genuchten type function was fitted to the measured retention data to obtain SWRC parameters for each soil sample in the database. Using a quasi-random procedure, the database was divided into “evaluation” (EVAL) and “test” (TEST) parts containing 225 and 80 soil samples, respectively. Linear PTFs for the SWRC parameters were calculated for the EVAL database. The PTFs used for this purpose particle-size percentages, dry bulk density, organic matter content, and the sand/silt ratio, as well as simple transforms (such as logarithms and products) of these independent variables. Of the various independent variables, the eight most significant were used to calculate the different PTFs. A nonlinear (NL) predictive method was obtained by substituting the linear PTFs directly into the SWRC equation, and subsequently adjusting the PTF parameters to all retention data of the EVAL database. The estimation error (SSQ) and efficiency (EE) were used to compare the effectiveness of the linear and nonlinearly adjusted PTFs. We found that EE of the EVAL and the TEST databases increased by 4 and 7%, respectively, using the second nonlinear optimization approach. To further increase EE, one measured retention data point was used as an additional (concomitant) variable in the PTFs. Using the 20 kPa water retention data point in the linear PTFs improved the EE by about 25% for the TEST data set. Nonlinear adjustment of the concomitant variable PTF using the 20 kPa retention data point as concomitant variable produced the best PTF. This PTF produced EE values of 93 and 88% for the EVAL and TEST soil data sets, respectively.     

Re-evaluation of the structural properties of some British swelling soils

The structural condition of swelling soils can be assessed from their shrinkage curves. We re‐evaluated data on six British swelling soils using modern methods to model the void ratio, e, as a function of the moisture ratio, ?. The points on the e–? curve were fitted with a constitutive shrinkage equation using an unbiased least‐squares, curve‐fitting program. The shrinkage curves were then differentiated to obtain their slopes, σ(?), which were used to calculate the overburden potentials, Ω. The slope functions were subsequently differentiated to obtain the curvatures, κ(?), from which the maximum curvature at the wet end was used to separate the structural shrinkage, S_c, from the proportional (unsaturated) shrinkage. At the point of maximum curvature, S_c and the volumetric air content, θ_ac, were calculated and found to correspond closely to those reported previously. Water retention curves were constructed and fitted using the van Genuchten equation, from which the α coefficient appears an important structural parameter. The structural condition of a swelling soil appears to be well described by its air content at the point of maximum curvature, its van Genuchten α coefficient, and a parameter describing the effect of the overburden potential.     

Analysis of water retention characteristics of oil-polluted earthy materials with different textures based on van Genuchten model

Purpose

The water retention characteristics of polluted soil systems are affected not only by the properties of the soil (e.g., texture) but also by those of the pollutants. This study aimed to evaluate the effect of oil pollution on the water-holding capacity of earthy materials with different textures.

Materials and methods

Three earthy materials (Lou, Loessial, and Aeolian sandy earthy materials) with different textures were treated with crude oil at five pollution levels (0, 0.5, 1, 2, and 4%). The soil water retention curve (SWRC) obtained for each treated sample was analyzed using the van Genuchten model.

Results and discussion

Oil pollution resulted in lower soil water retention in each case, with a leftward shift of the corresponding SWRC, and led to a decrease in the saturated water content of the earthy material, characterized by a marked increase in incomplete saturation, and a decrease in the residual water content (i.e., irreducible saturation). Oil pollution also determined a marked increase in the slope of the SWRC. The response of the SWRC to oil pollution was significantly influenced by the texture of earthy material, and the saturated water content of all earthy materials was strongly affected by the level of oil pollution. The residual water content of the heavier-textured Lou earthy material was also strongly affected by the oil pollution level, but no clear influence was found for the lighter-textured Loessial or Aeolian sandy earthy materials. The SWRC slope of the Aeolian sandy earthy material was sensitive to oil pollution, unlike those of the Lou and Loessial earthy materials.

Conclusions

Oil pollution reduced the water-holding capacity of different earthy materials to an extent depending on the soil texture. Under low-suction conditions, a significant effect of oil pollution on earthy materials of different texture was generally observed, while under high-suction conditions, the effect of oil pollution was greater for heavier-textured earthy materials.

     

低水势下土壤水分磁滞现象研究

Knowledge of the soil water characteristic curve is fundamental for understanding unsaturated soils. The objective of this work was to find scanning hysteresis loops of two fine textured soils at water potentials below wilting point. This was done by equilibration over NaCl solutions with water potentials of -6.6 to -18.8 MPa at 25 ℃. When cycled repeatedly through a series of potentials in the range noted previously both soils exhibited a hysteresis effect. The experimental differences in water content between the drying and wetting soils at the same water potential were much too large to be accounted for by failure to allow sufficient time to attain equilibrium as predicted by the exponential decay model. The wetting versus drying differences were relatively small, however, at only 4 mg g^-1 or less in absolute terms and about 3% of the mean of wetting and drying, in relative terms. Hysteresis should be a consideration when modeling biological and physical soil processes at water contents below the wilting point, where small differences in water content result in large potential energy changes.     

Shrinkage Characteristics of Lime Concretion Black Soil as Affected by Biochar Amendment

Studies have reported that biochar is a sustainable amendment that improves the chemical and physical properties of soil.In this study,an incubation experiment was conducted to investigate the effects of different application rates of biochar on the cracking pattern and shrinkage characteristics of lime concretion black soil after three wetting and drying cycles.Biochar derived from the corn straw and peanut shell mixture was applied to the soil at rates of 0,50,100,and 150 g kg~(-1)dry weight,representing the treatments T_(0),T_(50),T_(100),and T_(150),respectively.During the wetting and drying cycles,the cracking pattern and shrinkage characteristics of the unamended and amended soil samples were recorded.Application of biochar significantly increased soil organic carbon content in the samples.During soil desiccation,biochar significantly reduced the rate of water loss.Cracks propagated slowly and stopped due to the relatively higher water content in the soil applied with biochar.The cracking area density(ρ_c),equivalent width,fractal dimension,and cracking connectivity index decreased during the drying process with increasing application rate of biochar.Theρ_(c )value of the T_(50),T_(100),and T_(150) treatments decreased by 33.6%,52.1%,and 56.9%,respectively,after three wetting and drying cycles,whereas the T_(0) treatment exhibited a marginal change.The coefficient of linear extensibility,an index used to describe onedimentional shrinkage,of the unamended soil sample(T_(0))was approximately 0.23.Application of 100 and 150 g kg~(-1)biochar to the soil significantly reduced the shrinkage capacity by 41.45%and 45.54%,respectively.The slope of the shrinkage characteristics curve,which indicates the ralationship between soil void ratio and moisture ratio,decreased with increase in the application rate of biochar.Furthermore,compared with the T_(0) treatment,the proportional shrinkage zone of the shrinkage characteristic curve of the T_(50),T_(100),and T_(150) treatments decreased by 5.8%,13.1%,and 12.1%,respectively.Differences were not observed in the moisture ratio at the maximum curvature of the shrinkage characteristic curve among the treatments.The results indicate that biochar can alter the cracking pattern and shrinkage characteristics of lime concretion black soil.However,the effects of biochar on the shrinkage of lime concretion black soil are dependent on the number of wetting and drying cycles.     

求van Genuchten模型参数的AM-MCMC方法

采用基于自适应采样算法的马尔科夫链蒙特卡罗方法(简称AM-MCMC)来估算描述土壤水分特征曲线的van Genuchten模型的参数,并推求出模型参数的后验分布,从而为模型参数的不确定性分析提供依据。结果表明,对于van Genuchten模型而言,采用AM-MCMC算法能得到模型参数后验均值和方差的分布,并且能推求出模型参数的置信区间,所以用这种算法来求解van Genuchten方程的参数是行之有效的,为求解van Genuchten模型参数提供了一种新的思路。     

几种常用绿地改良材料对土壤水分特征的影响

分析了几种常用绿地土壤改良材料及其不同配比对土壤水分特征曲线和水分常数的影响,结果表明:利用RETC软件对各配比土壤水分特征曲线van Genuchten方程的参数拟合效果较好,R2均大于0.99;随着砂粒含量的增加,土壤田间持水量降低,土壤中水分有效性比例增加,但砂粒粒径对土壤水分常数影响不显著;绿化植物废弃物能提高土壤田间持水量和有效水含量,降低土壤凋萎含水量;绿化植物废弃物还能提高有效水占田间持水量的比例,以20%绿化植物废弃物的用量为最大,为49.59%;聚丙烯酰胺(PAM)虽然能提高土壤田间持水量,但阻碍土壤水分的释放,降低土壤水分的有效性;脱硫石膏可以增加土壤田间持水量和水分的有效性。综合而言,以70%土、10%砂、20%绿化植物废弃物和0.5 kg/m~3脱硫石膏的配比相对最佳。