Aerodynamic method for obtaining the soil water retention curve

A new method for the rapid plotting of the soil water retention curve (SWRC) has been proposed that considers the soil water as an environment limited by the soil solid phase on one side and by the soil air on the other side. Both contact surfaces have surface energies, which play the main role in water retention. The use of an idealized soil model with consideration for the nonequilibrium thermodynamic laws and the aerodynamic similarity principles allows us to estimate the volumetric specific surface areas of soils and, using the proposed pedotransfer function (PTF), to plot the SWRC. The volumetric specific surface area of the solid phase, the porosity, and the specific free surface energy at the water-air interface are used as the SWRC parameters. Devices for measuring the parameters are briefly described. The differences between the proposed PTF and the experimental data have been analyzed using the statistical processing of the data.     

Pedotransfer functions for estimating soil water retention curve of Sicilian soils

Pedotransfer functions (PTFs) make use of routinely surveyed soil data to estimate soil properties but their application to soils different from those used for their development can yield inaccurate estimates. This investigation aimed at evaluating the water retention prediction accuracy of eight existing PTFs using a database of 217 Sicilian soils exploring 11 USDA textural classes. PTFs performance was assessed by root mean square differences (RMSD) and average differences (AD) between estimated and measured data. Extended Nonlinear Regression (ENR) technique was adopted to recalibrate or develop four new PTFs and Wind’s evaporation method was applied to validate the effectiveness of the relationships proposed. PTFs evaluation resulted in RMSD and AD values in the range 0.0630–0.0972 and 0.0021–0.0618 cm³ cm^–3, respectively. Best and worst performances were obtained respectively by PTF-MI and PTF-ZW. ENR allowed to recalibrate PTF-MI and PTF-ZW with improvements of RMSD (0.0594 and 0.0508 cm³ cm^–3) and to develop two relationships that improved RMSD by 75–78% as compared to PTF-MI. The results confirmed the potential of ENR technique in calibrating existing PTFs or developing new ones. Validation conducted with an independent dataset suggested that recalibrated/developed PTFs represent a viable alternative for water retention estimation of Sicilian soils.     

土壤水分特征曲线的分形模拟

Many empirical models have been developed to describe the soil water retention curve （SWRC）. In this study, a fractal model for SWRC was derived with a specially constructed Menger sponge to describe the fractal scaling behavior of soil; relationships were established among the fractal dimension of SWRC, the fractal dimension of soil mass, and soil texture; and the model was used to estimate SWRC with the estimated results being compared to experimental data for verification. The derived fractal model was in a power-law form, similar to the Brooks-Corey and Campbell empirical functions. Experimental data of particle size distribution （PSD）, texture, and soil water retention for 10 soils collected at different places in China were used to estimate the fractal dimension of SWRC and the mass fractal dimension. The fractal dimension of SWRC and the mass fractal dimension were linearly related. Also, both of the fractal dimensions were dependent on soil texture, i.e., clay and sand contents. Expressions were proposed to quantify the relationships. Based on the relationships, four methods were used to determine the fractal dimension of SWRC and the model was applied to estimate soil water content at a wide range of tension values. The estimated results compared well with the measured data having relative errors less than 10% for over 60% of the measurements. Thus, this model, estimating the fractal dimension using soil textural data, offered an alternative for predicting SWRC.     

The effect of earthworm coprolites on the soil water retention curve

The effect of earthworm coprolites on the water retention curves in soils of different geneses and textures was investigated by the method of equilibrium centrifuging. Coprolites sampled in the field were compared with the surrounding soil. The effect of earthworms on a soddy-podzolic light loamy soil (from Moscow oblast) was comprehensively analyzed in the course of a special model experiment in a laboratory. This experiment was necessary because it was difficult to separate the coprolites from the soil, in which additional coprolites could appear under natural conditions. In all the variants of the experiment, the differences between the water retention curves of the coprolites and the surrounding soil (or control substrates unaffected by earthworms) were statistically significant. The development of coprolites favored a considerable increase (up to 20 wt.% and more) of the soil water retention capacity upon equivalent water potentials within the range from 0 to ?1000 kPa. In most cases, the soil water retention capacity increased within the entire range of the soil moisture contents. This could be explained by the fact that strongly swelling hygroscopic plant remains (detritus) were included into the coprolites and by the formation of a specific highly porous aggregate structure.     

A new model for the soil-water retention curve that solves the problem of residual water contents

We present a new model for the soil‐water retention curve, θ(h_m), which, in contrast to earlier models, anchors the curve at zero water content and does away with the unspecified residual water content. The proposed equation covers the complete retention curve, with the pressure head, h_m, stretching over approximately seven orders of magnitude. We review the concept of pF from its origin in the papers of Schofield and discuss what Schofield meant by the ‘free energy, F ’. We deal with (historical) criticisms regarding the use of the log scale of the pressure head, which, unfortunately, led to the apparent demise of the pF. We espouse the advantages of using the log scale in a model for which the pF is the independent variable, and we present a method to deal with the problem of the saturated water content on the semi‐log graph being located at a pF of minus infinity. Where a smaller range of the water retention is being considered, the model also gives an excellent fit on a linear scale using the pressure head, h_m, itself as the independent variable. We applied the model to pF curves found in the literature for a great variety of soil textures ranging from dune‐sand to river‐basin clay. We found the equation for the model to be capable of fitting the pF curves with remarkable success over the complete range from saturation to oven dryness. However, because interest generally lies in the plant‐available water range (i.e. saturation, θ_s, to wilting point, θ_wp), the following relation, which can be plotted on a linear scale, is sufficient for most purposes: , where k₀, k₁ and n are adjustable fitting parameters.     

估计太湖地区水稻土水分特征曲线的物理-经验方法研究

为寻求适于太湖地区水稻土水分特征曲线估算的简便方法,本文以实测土壤基本性质和土壤水分特征曲线为基础,对13种物理-经验方法估计土壤水分特征曲线的效果进行综合评价。结果表明,基于形状相似性的物理-经验方法预测效果要优于多数分形方法,但均差于基于Kravchenko-Zhang分维方法和B rooks-Corey水分特征曲线方程的孔隙表面分形模型(简称KZBC模型)。物理-经验方法在不同压力水头段对含水量的预测精度有差异,在实际工作中不仅要考虑模型的适用性,还要考虑其适用的压力水头范围。KZBC模型估计土壤水分特征曲线精度最高,实用性也较强,是适用于本研究区水分特征曲线估计的最佳方法。     

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.     

Methods for predicting the optimum and the range of soil water contents for tillage based on the water retention curve

Information is needed on the range of soil water contents for tillage. The objective of the work was to develop methods for the prediction of the soil water contents at which tillage may be done satisfactorily. Three water contents are considered: the lower (dry) limit, the optimum water content, and the upper (wet) limit. This paper makes a synthesis of published results from tillage and soil physics experiments and also includes some new experimental results. The effects of tillage are considered in relation to some “fixed points” including the lower plastic limit, field capacity and a new fixed point “the inflection point”. These considerations lead to methods for prediction of the lower (dry) tillage limit, the optimum water content, and the upper (wet) tillage limit in terms of the parameters of the van Genuchten equation for soil water retention. Predictions can be made in terms of soil composition through the use of pedotransfer functions for the parameters of the van Genuchten equation. The new methods will enable the effects of soil degradation and climate change on tillage work days to be estimated. The results are potentially mappable using geographic information systems.     

Soil porous system changes quantified by analyzing soil water retention curve modifications

Soil water retention curves (SWRCs) relate soil water pressure head (h) to soil water content (θ) and can also be used to find information regarding soil pore distribution. To analyze SWRCs in relation to pore size distribution (PSD), changes due to wetting and drying (W–D) cycles were studied in three different tropical soils (Geric Ferralsol, GF; Eutric Nitosol, EN; Rhodic Ferralsol, RF), using three different treatments: T₀, the control with samples not submitted to W–D cycles; T₃, samples submitted to three consecutive W–D cycles; T₉, samples submitted to nine consecutive W–D cycles. Log-normal PSD equations for each treatment were obtained using the S-theory. For the GF soil, the pressure heads separating structural and matrix domains (h_s) were 17.7, 12.2 and 14.7 kPa for T₀, T₃, and T₉, respectively. These values are equivalent to pore radia of 8.4 μm (T₀), 12 μm (T₃), and 10 μm (T₉). For the RF soil, h_s values were 8.5 kPa (T₀), 20.5 kPa (T₃), and 15.1 kPa (T₉), equivalent to radia of 18 μm (T₀), 7.3 μm (T₃), and 9.9 μm (T₉); and finally, for the EN soil, h_s were 18.1 kPa (T₀), 9.1 kPa (T₃), and 13.5 kPa (T₉), equivalent to radia of 8.2 μm (T₀), 16 μm (T₃), and 11 μm (T₉). It was found that the soil structure presented important changes in PSD due to W–D cycles for all the investigated soils. It was also observed that the W–D cycles increased the S_inf (slope of SWRC) value for the GF soil for all treatments; S_inf did not substantially change in all treatments for the EN soil; S_inf decreased between T₀ and T₃, and T₀ and T₉ for the RF soil. According to the S-theory, it is possible to infer that W–D cycles improved the soil structure of GF, made the RF soil structure worse and did not substantially change the EN soil structure.     

根据土壤粒径分形估计紫色土水分特征曲线

选择不同质地紫色土,分别根据土壤颗粒的数量和重量分布计算土壤粒径分形维数,并与Tyler和Wheatcraft土壤水分特征曲线模型的拟合分形维数进行比较分析。结果表明,三种途径获得的分形维数与土壤质地密切相关,均表现出土壤黏粒含量越高,质地越细,分形维数越高。土壤粒径数量分布分形维数(2.98～3.26)和土壤粒径重量分布分形维数(2.73～2.81)存在较大的差异,但二者间却存在显著的线性关系。土壤粒径分形维数与土壤水分特征曲线模型拟合分形维数(2.72～2.84)均存在显著的线性关系,尤其是土壤粒径重量分布分形维数与土壤水分特征曲线模型拟合分形维数数值十分接近。通过建立的土壤水分特征曲线模型分形维数与土壤粒径分布分形维数关系式,结合Tyler和Wheatcraft模型进行土壤水分特征曲线预测,预测值与实测值具有良好的一致性,因而根据土壤粒径分形预测紫色土水分特征曲线是可行的。     

用于估算伊朗盐土和盐碱土土壤水分特征曲线的土壤传递函数(PTFs)研究

Soil water retention data are essential for irrigation scheduling and determination of irrigation frequency.However,direct measurement of this characteristic is time consuming and expensive and furthermore its spatial and temporal variabilities in field scales increase the number of measurements.Different pedotransfer functions,such as Saxton et al.,Campbell,Vereecken et al.,Rawls and Brakensiek,Wo¨sten et al.,Rajkai et al.,Ghorbani Dashtaki and Homaee,Zacharias and Wessolek,and Rosetta,were evaluated to estimate soil water retention of saline and saline-alkali soils collected from south of Tehran,Iran.The saturation-extract conductivity of all the 68 samples and exchangeable sodium percentage of more than half of them were measured to be greater than 4 dS m-1 and 15%,respectively.The calculated Akaike’s information criterion values showed that Saxton et al.and Campbell models were the best in estimation of soil water retention curve and total available water,respectively.     

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.     

砒砂岩风化物对土壤水分特征曲线及蒸发的影响

运用高速离心机法和室内模拟蒸发的方法,对比研究了红色(RS)、灰色(GS)和白色(WS)三种颜色砒砂岩风化物在不同添加比例条件下对沙黄土和风沙土水分特征曲线、供水能力和干旱过程中失水过程的影响。研究表明:砒砂岩风化物的添加对沙黄土和风沙土水分特征曲线有较明显的影响。砒砂岩风化物的持水能力均高于沙黄土和风沙土,砒砂岩风化物添加提高了风沙土和沙黄土的持水能力,降低了沙黄土和风沙土失水速率;较低比例的砒砂岩风化物提高了沙黄土和风沙土的比水容量,但对风沙土和沙黄土低吸力段供水能力的提高作用并不明显。整个蒸发期间,砒砂岩风化物降低了沙黄土的标准蒸发量,而对于风沙土,其作用是蒸发前期降低,中后期增加。红色、灰色、白色三种砒砂岩风化物分别使风沙土的蒸发失水比提高了11.59%、10.14%和0.01%,添加三种砒砂岩风化物后的处理的最终平均含水量分别是风沙土的4倍、2.33倍、1.33倍,使沙黄土的蒸发失水比降低了13.33%、13.33%、29.52%,最终含水量降低了45.83%、54.17%、66.67%。因此,从提高土壤持水能力方面考虑,砒砂岩可以用于当地矿区和排土场区土壤改良,以促进生态修复。     

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.     

A TDR-pressure cell design for measuring the soil-water retention curve

This paper presents a new type of pressure cell associated with a zigzag-shaped time domain reflectometry (TDR) probe for determining the soil-water retention (θ(ψ)) curve of disturbed thin soil samples. The pressure cell, designed for pressures ranging between 0 and −500 kPa, consisted of a zigzag copper rod (150-mm long, 2 mm in diameter) vertically installed in a clear plastic cylinder (60-mm high, 50 mm in internal diameter) with six vertical copper rods (60-mm long, 2 mm in diameter) arranged around the inner wall of the plastic cylinder. The cylinder was closed at the base with a nylon cloth and placed on a porous ceramic disc. The inner rod and the six-rod grille of the cell were connected respectively to the inner and outer conductors of a coaxial cable. The results showed that the correlation between the apparent dielectric constant measured with a standard three-rod TDR probe and the zigzag-shaped TDR probe, both immersed in five different non-conductive fluids, was excellent (R² = 0.99). On the other hand, the volumetric water content measured with the TDR probe of the pressure cell filled up with sand, 2-mm sieved loam and clay-loam soils was highly correlated to the corresponding values calculated from the gravimetric water content and the soil bulk density (R² = 0.97; RMSE = 2.32 × 10⁻²). The parameters of the θ(ψ) curves measured for these three different soils with the TDR-pressure cell were within the range of values found in the literature. The cell was also used to study the θ(ψ) changes of a 2–4-mm sample of loam soil aggregates after a slow and a fast-wetting process. While negligible changes in both the soil structure and θ(ψ) were observed following slow wetting, fast wetting resulted in disintegration of aggregates and drastic changes in the shape of the θ(ψ) curve.     

鲁中南山地典型植被土壤颗粒与土壤水分特征曲线的分形学特征

土壤粒径分布与水分特征曲线是土壤的重要物理性质,对土壤侵蚀状况和土壤肥力有显著影响。以鲁中南山地典型植被下土壤为研究对象,运用分形学理论研究5种典型植被土壤颗粒与水分特征曲线的分形学特征。结果表明:1)不同植被类型的土壤颗粒单重分形维数、多重分形参数和土壤水分特征曲线分形维数具有显著差异,均表现为麻栎+刺槐混交林>黑松+黄连木混交林>黑松林>核桃林>荒草地;2)土壤分形维数有林地大于荒草地,混交林大于纯林;3)土壤分形维数与黏粒体积分数、粉粒体积分数呈显著正相关,与砂粒体积分数呈显著负相关;4)土壤颗粒单重分形维数与土壤水分特征曲线分形维数呈显著的正相关关系。说明土壤颗粒分布与土壤水分特征曲线的分形参数可以作为反映土壤结构性状变化的定量指标,可利用土壤颗粒分形与土壤水分特征曲线分形维数的相关关系来描述对应的土壤水分特征曲线。研究成果可为鲁中南山地退耕还林与生态造林工程建设及其生态效益评价提供理论参考。