Simplified method for quantifying the hydraulic properties of shrinking soils

In general, soils and their pore size systems are assumed to be rigid during the loss of water on drying. In reality, it is not the case for most soils, especially for soils with high quantities of clay or organic matter. As a result of shrinking, there are changes in the bulk density, the porosity, the pore size distribution, and the hydraulic properties of these soils. Currently, only a few methods enable the shrinkage behavior of soil samples to be determined while simultaneously quantifying the corresponding soil hydraulic properties. Either the methods need proprietary software for data processing, the equipment used is expensive or the calculation of the hydraulic properties is executed by inverse modelling. The aim of this study was to develop an alternative, simplified method for the simultaneous and automatic determination of the soil hydraulic properties, taking shrinkage into account. The HYPROP® evaporative device was combined with a circumference meter. A preliminary investigation found that the diameter of the cylindrical samples used for the HYPROP decreased linearly during evaporation from the bottom to the top. To sum up, recording the perimeter change in the middle position of the sample during drying‐out, together with the corresponding tension and water content, was sufficient to determine the hydraulic functions taking shrinkage into account. Measurements are presented for 6 samples which are different in texture and geological origin. The maximum shrinkage (19.5% by vol. between saturation and 5,000 hPa) was measured in the peat samples. The minimum shrinkage was quantified at 0.68% by vol. for the silty loam samples from Chile. The advantages of the method presented are: (1) the water retention curve and the hydraulic conductivity function can be determined simultaneously in the range between saturation and close to the wilting point, at a high resolution and taking into consideration shrinkage; (2) the method and device are simple and robust to use; (3) little time is required for measurement, between 3 and at most 10 d; (4) the functions are described over the whole tension range, using more than 100 user‐defined data points; (5) the evaluation of the volumetric soil water content measurement in shrinking soils is improved; and (6) common data models can be fitted to the hydraulic data as well as to the shrinkage data.     

The evaporation method: Extending the measurement range of soil hydraulic properties using the air‐entry pressure of the ceramic cup

Knowledge of hydraulic functions is required for various hydrological and plant‐physiological studies. The evaporation method is frequently used for the simultaneous determination of hydraulic functions of unsaturated soil samples, i.e., the water‐retention curve and hydraulic‐conductivity function. All methodic variants of the evaporation method suffer from the limitation that the hydraulic functions can only be determined to a mean tension of ≈ 60 kPa. This is caused by the limited measurement range of the tensiometers of typically 80 kPa on the dry end. We present a new, cost‐ and time‐saving approach which overcomes this restriction. Using the air‐entry pressure of the tensiometer's porous ceramic cup as additional defined tension value allows the quantification of hydraulic functions up to close to the wilting point. The procedure is described, uncertainties are discussed, and measured as well as simulated test results are presented for soil samples of various origins, different textures (sand, loam, silt, clay, and peat) and variable dry bulk density. The experimental setup followed the system HYPROP which is a commercial device with vertically aligned tensiometers that is optimized to perform evaporation measurements. During the experiment leaked water from the tensiometer interior wets the surrounding soil of the tensiometer cup and can lead to a tension retardation as shown by simulation results. This effect is negligible when the tensiometers are embedded vertically. For coarsely textured soils and horizontal tensiometer alignment, however, the retardation must be considered for data evaluation.     

Soil physical characteristics of peat soils

Drainage and intensive use of fens lead to alterations in the physical characteristics of peat soils. This was demonstrated using parameters of water balance (available water capacity) and the evaluated unsaturated hydraulic conductivity. Deriving the distribution of the pore size from the water retention curve was flawed because of shrinkage due to drainage, especially at high soil water potentials. These errors became greater as the peat was less influenced by soil‐genetic processes. The water retention curves (desorption) evaluated in the field and the laboratory satisfactorily corresponded. However, the wetting‐ and drainage‐curves obtained in the field differed up to 30 vol.‐% water content at same soil water potentials. These differences were largely due to a wetting inhibition.     

室内基于土壤水分再分布过程推求紫色土导水参数

选择三峡库区3种不同质地的紫色土,室内通过土壤水分再分布试验,探讨基于土壤水分再分布过程推求导水参数对于紫色土的适用性.结果显示,结合土壤水分垂直和水平再分布过程推求的紫色土水分扩散率与实测值具有很好的一致性,但推求的非饱和导水率偏差较大.然而,选用单一的土壤水分再分布过程结合实测水分特征曲线推求的紫色土非饱和导水率与实测值具有良好的一致性.湿润锋湿度与湿润剖面平均湿度不同函数关系对推求非饱和导水率和水分扩散率差异不明显.此外,基于土壤水分再分布过程推求导水参数方法比较适合低湿土壤的非饱和导水参数推求.     

Measuring near‐saturated hydraulic conductivity of soils by quasi unit‐gradient percolation—2. Application of the methodology

Sarkar et al. (this issue) proposed a laboratory measurement method for obtaining the hydraulic conductivity of soil at near‐saturated moisture conditions, bridging the gap between measurements that can be obtained with the evaporation method in the medium dry region, and measurements of the saturated conductivity by traditional methods. The method is based on a tension infiltration on a limited part of the surface of a soil sample and drainage of the sample at the same tension, leading to a divergent flow field. Despite equal tensions at top and bottom of the sample (“unit gradient”), the water flux in the sample is smaller than the corresponding value of the soil hydraulic conductivity at the applied tension. From numerical analysis of the flow problem, they concluded that unsaturated conductivity can be obtained with an accuracy of 10% for all texture classes of the USDA soil texture triangle. In this paper, we test the methodology for three different soil types using an appropriate apparatus. The results match well with independent saturated conductivity measurements on the wet side, and with unsaturated conductivity measurements in the medium moisture range that were obtained with the evaporation method.     

Measurement and evaluation of the hydraulic properties of horticultural substrates

Sustainable, environmentally friendly and resource-saving water and nutrient management in horticulture requires knowledge of the suitability of horticultural substrates for each specific application. One specific element is their hydraulic performance. To meet this requirement, methods are needed (1) to measure the hydraulic properties and (2) to evaluate the hydraulic quality of the horticultural substrates. The aim of this study was (1) to test the extended evaporation method and the HYPROP systems for quantifying the substrate’s hydraulic properties (the water retention curve, the unsaturated hydraulic conductivity function, the shrinkage dynamics, the bulk density), and (2) to develop a rating framework for assessing the hydraulic suitability of the substrates. The hydraulic evaluation was split into cultivation under free drainage in the ground and cultivation in containers. The hydraulic criteria related to high-quality horticultural substrates were defined as the amount of easily plant-available water, the air capacity and the height of capillary rise. Limiting factors could be water repellency effects and shrinkage. The rating framework consists of five classes between non-satisfactory and very good. It was tested on 23 commercial horticultural substrates. Both the measurement methods for quantifying soil hydraulic properties and the evaluation procedure proved applicable.     

Simplifying the evaporation method for quantifying soil hydraulic properties

Since about 25 years, we have measured the unsaturated hydraulic conductivity function and water retention curve with the evaporation method of more than 1500 mineral and organic soils samples. From this data base, 104 representative samples of varying texture and dry bulk density were selected and the temporal dynamics of the basic measured values (mass or water loss, respectively, and tension change over time) was analyzed. With the exception of sand, water loss per time interval was constant in all other mineral and organic soils during the measuring time in the tension range between 0 and about 60 kPa. In sands, the nonlinear water loss over time by evaporation can be described by a quadratic function with high accuracy (r² > 0.99). For all other soils, a linear function is sufficient (r² > 0.99). The use of evaporation functions enables extending weighing intervals. This reduces costs for the measuring equipment and increases the effectiveness of the method while maintaining the same quality of unsaturated hydraulic conductivity and water retention functions. It was confirmed that measuring with two tensiometers is sufficient for accurate hydraulic conductivity and water retention function. Reducing evaporation by screening the sample surface helps to decrease hydraulic gradients and keeps tension distributions approximately linear with depth. This is recommended in particular for clayey soils.     

A relatively simple scaling method for describing the unsaturated hydraulic functions of stony soils

Few if any methods exist to estimate the effects of stone content (stoniness) on the unsaturated soil hydraulic properties. A relatively simple scaling method is presented to estimate the hydraulic conductivity of unsaturated stony soils having different stone contents. A key assumption of the method is that van Genuchten's water retention parameters α and n of the fine soil fraction are the same as those of the stony soil. The method further assumes a linearly decreasing relationship between the saturated hydraulic conductivity and the stone content, based on previous numerical simulations. Using the proposed method, it is possible to calculate the hydraulic conductivity of unsaturated stony soils, knowing the saturated hydraulic conductivity of the fine soil fraction, the retention curve of the fine soil fraction, and the particular stoniness of the soil.     

基于遗传算法的土壤水分运动参数识别

土壤水分运动参数的识别是研究土壤水分运动的基础。该文以反映土壤含水率实测值和计算值吻合程度的均方差最小为优化目标,以土壤导水率和扩散率经验参数上下限为约束条件,建立了土壤水分运动参数识别的优化计算模型。采用遗传算法和田间均质土壤一维非饱和运动数值计算相结合的方法,获得土壤导水率和扩散率经验参数最优值。经验证计算,土壤含水率实测值和计算值吻合程度较高,表明这一方法是可行的。     

Prüfung von 3 Verfahren zur Vorhersage der hydraulischen Leitfähigkeit ungesättigter Böden aus Wasserretentionsdaten oder aus der Bodenart

Testing of three methods to predict unsaturated soil hydraulic conductivity from water retention data or from texture class Using 60 soils taken from UNSODA (Leij et al., 1996) the method proposed by Renger et al. (1999) and the prediction according to Mualem (1976)/van Genuchten (1980) were tested. The parameters of the Mualem/vanGenuchten model were estimated either from water retention data or from a table of reference values. Using the reference values requires only the knowledge of texture class (German classification system). An advantage of the method proposed by Renger et al. (1999) is its capability to predict saturated conductivity too. The model of Mualem/vanGenuchten using reference values of parameters yields the best results. The standard deviation between observed and predicted values of unsaturated hydraulic conductivity was 0.93 lg (K) for the Mualem/vanGenuchten model and 1.3 lg (K) for the Renger et al. (1999) predictions.     

Soil hydraulic properties of two loess soils in China measured by various field-scale and laboratory methods

Knowledge of hydraulic properties is essential for understanding water movement in soil. However, very few data on these properties are available from the Loess Plateau of China. We determined the hydraulic properties of two silty loam soils on agricultural land at sites in Mizhi and Heyang in the region. Undisturbed soil cores were collected from seven layers to one meter depth to determine saturated hydraulic conductivity, soil water retention curves and unsaturated hydraulic conductivity (by the hot-air method). Additional field methods (internal drainage and Guelph permeameter) were applied at the Heyang site to compare differences between methods. Soil water retention curves were flatter at Mizhi than at Heyang. Water contents at saturation and wilting point (1500 kPa) were higher at Heyang than at Mizhi. However, unsaturated hydraulic conductivity was lower at Heyang than at Mizhi, with maximum differences of more than six orders of magnitude. Nevertheless, the two soils had similar saturated hydraulic conductivities of about 60 cm day^− 1. Comparison between the methods showed that soil water retention curves obtained in the laboratory generally agreed well with the field data. Field-saturated conductivities had similar values to those obtained using the soil core method. Unsaturated hydraulic conductivities predicted by the Brooks–Corey model were closer to field data than corresponding values predicted by the van Genuchten model.     

Effects of Bentonite, Hydrogel and Biochar Amendments on Soil Hydraulic Properties from Saturation to Oven Dryness

Effects of hydrogel, bentonite, and biochar as soil amendments on soil hydraulic properties and improving water availability from saturation to oven dryness were investigated. Soils were mixed with hydrogel (0.10%, 0.25%, and 0.50%), bentonite (0.5%, 1.0%, and 2.5%), and biochar (1.0%, 2.5%, and 5.0%) as soil amendments (weight:weight). Three methods (extended multistep outflow (XMSO), evaporation (EVA), and WP4 dewpoint potentiometer) were used to measure soil hydraulic properties from saturation to oven dryness. The cumulative XMSO results were more uniform across all the applied pressure steps for the amended soils. The EVA exhibited a shorter linear decrease during the first evaporation stage and a lower evaporation rate during the second evaporation stage. The WP4 results also exhibited that soil amendments increased the soil water content of the amended soils at low matric potentials. The results of soil water retention curves revealed that the unamended soil retained less water at any matric potential compared to the amended soils. Soil hydraulic conductivity decreased with increasing amount of soil amendments. The saturated hydraulic conductivity was higher for the unamended soil than the soils amended with 2.5% bentonite, 0.50% hydrogel, and 5.0% biochar by 11, 3, and 18 times, respectively. These results suggested that soil amendments improved soil water retentivity, which confirmed the appropriateness of these soil amendments for potential use in sandy soil improvements. However, field experiments and economical perception studies should be considered for further investigation.     

The Effects of Long‐term Fertiliser Applications on Soil Organic Carbon and Hydraulic Properties of a Loess Soil in China

Based on a 28‐year in situ experiment, this paper investigated the impacts of organic and inorganic fertiliser applications on soil organic carbon (SOC) content and soil hydraulic properties of the silt loam (Eumorthic Anthrosols) soils derived from loess soil in the Guanzhong Plain of China. There were two crop (winter wheat and summer maize) rotations with conventional tillage. The treatments included control without fertiliser application, organic manure application (M), chemical fertiliser application (NP), and the application of organic manure with chemical fertiliser (MNP). The results showed that the 28‐year organic manure applications (M and MNP) significantly (p < 0·05) increased SOC content at surface layer (0–10 cm), but the effect of chemical fertilisers alone on SOC was not significant. Organic manure treatments (M and MNP) apparently improved soil hydraulic properties. Compared with control, field capacity and total porosity significantly (p < 0·05) increased while soil bulk density significantly (p < 0·05) decreased for organic manure applications. The M and MNP treatments increased soil water retentions by 3·2–10·8%, which was dependent of suction tensions. However, the NP treatment had no significantly impact on soil water retention compared with control. Neither organic nor inorganic fertiliser applications significantly changed saturated hydraulic conductivity. However, a clear difference was observed for unsaturated hydraulic conductivity between the M and the control at 0–5 cm. Overall, long‐term applications of organic manuring increased SOC content and amended soil hydraulic properties. However, the effects of chemical fertilisers on these soil properties were limited. Copyright © 2015 John Wiley & Sons, Ltd.     

两种非饱和导水参数推求方法在紫色土上的应用

土壤非饱和导水参数的难于获取,限制了非饱和水流数值模拟技术的实际应用。本研究选择颇具代表性的土壤水分特征曲线推求法和简单入渗法两种方法,分别推求不同质地紫色土导水参数,并进行了分析比较。结果显示,土壤水分特征曲线推求法和简单入渗法,推求的非饱和导水率与计算值均具有较好的一致性。鉴于土壤水分特征曲线的易测优势和简单入渗法实验简便省时特点,采用这两种方法进行紫色土非饱和导水参数的推求或预报是可行的。     

两种非饱和导水参数推求方法在紫色土上的应用

土壤非饱和导水参数的难于获取,限制了非饱和水流数值模拟技术的实际应用。本研究选择颇具代表性的土壤水分特征曲线推求法和简单入渗法两种方法,分别推求不同质地紫色土导水参数,并进行了分析比较。结果显示,土壤水分特征曲线推求法和简单入渗法,推求的非饱和导水率与计算值均具有较好的一致性。鉴于土壤水分特征曲线的易测优势和简单入渗法实验简便省时特点,采用这两种方法进行紫色土非饱和导水参数的推求或预报是可行的。     

Water retention characteristics of soils over the whole moisture range: a comparison of laboratory methods

The exact characterization of soil hydraulic properties is essential for various purposes, such as agricultural and groundwater management. Various laboratory methods for the determination of soil water retention characteristics are frequently applied. Standard methods comprise: (i) the hanging‐water column and (ii) the simplified evaporation method (HYPROP^©) for wet to moderate moisture ranges; and (iii) the pressure plate and (iv) the dew point method (WP4C PotentiaMeter®) for moderate to dry moisture ranges. These four methods were evaluated and compared, with the aim of demonstrating their compatibility. The hanging‐water column method and the evaporation method provided comparable retention data in the wet to moderate moisture range. The dew point method provided corresponding results over the dry range, while retention data obtained from the pressure plate method sometimes had a tendency to over‐estimate water contents. Hysteresis was observed for retention data in the dry range when applying the dew point method to previously wetted or dried samples.     

Free‐Form estimation of soil hydraulic properties using Wind’s method

Transient evaporation experiments offer the potential to determine simultaneously the soil hydraulic properties necessary to simulate water flow in unsaturated soils. We present a new algorithm for determining the retention and conductivity curve from evaporation experiments which uses Wind’s method with a free‐form soil water retention function. Our algorithm estimates nodal values of volumetric water content and derives a smooth and monotone retention curve by cubic Hermite interpolation. A multilevel routine increases the number of nodes and their adequate number is identified by a performance criterion which balances goodness of fit, the cross correlation between the estimated water contents and the number of degrees of freedom. We calculate point values of unsaturated hydraulic conductivity by the instantaneous profile method and discard unreliable conductivity estimates by a statistical filter criterion. Results for three synthetic data sets including an uncertainty analysis of the estimated retention curves show that the algorithm is suitable to identify, both correctly and precisely, the soil hydraulic properties. An application to a real data set confirms these results. In order to enable the free‐form functions to be used in numerical flow simulations, we extrapolate the retention function to the dry range and compute a coupled conductivity function based on the Mualem model. Major advantages of the proposed method are the enormous flexibility provided by the free‐form functions, the low level of parameter cross‐correlation in comparison with classic parametric functions, and the possibility of assessing the uncertainty of the retention curve individually in different ranges of pressure head.     

Suitability of a hydraulic‐conductivity model for predicting salt effects on swelling soils

Many empirical approaches have been developed to analyze changes in hydraulic conductivity due to concentration and composition of equilibrium solution. However, in swelling soils these approaches fail to perform satisfactorily, mainly due to the complex nature of clay minerals and soil–water interactions. The present study describes the changes in hydraulic conductivity of clay (Typic Haplustert) and clay‐loam (Vertic Haplustept) soils with change in electrolyte concentration (TEC) and sodium‐adsorption ratio (SAR) of equilibrium solution and assesses the suitability of a model developed by Russo and Bresler (1977) to describe the effects of mixed Na‐Ca‐Mg solutions on hydraulic conductivity. Four solutions encompassing two TEC levels viz., 5 and 50 mmol_c L^–1 and two SAR levels viz., 2.5 and 30 mmol^1/2 L^–1/2 were synthesized to equilibrate the soil samples using pure chloride salts of Ca, Mg, and Na at Ca : Mg = 2:1. Diluting 50 mmol_c L^–1 solution to 5 mmol_c L^–1 reduced saturated hydraulic conductivity of both soils by 66%, and increasing SAR from 2.5 to 30 mmol^1/2 L^–1/2 decreased saturated hydraulic conductivity by 82% and 79% in clay and clay‐loam soils, respectively. Near saturation, the magnitude of the change in unsaturated hydraulic conductivity due to the change in TEC and SAR was of 10³‐ and 10²‐fold, and at volumetric water content of 0.20 cm³ cm^–3, it was of 10¹⁴‐ and 10⁶‐fold in clay and clay‐loam soils, respectively. Differences between experimental and predicted values of saturated hydraulic conductivity ranged between 0.6% and 11% in clay and between 0.06% and 2.1% in clay‐loam soils. Difference between experimental and predicted values of unsaturated hydraulic conductivity widened with drying in both soils. Predicted values were in good agreement with the experimental values of hydraulic conductivity in clay and clay‐loam soils with R² values of 0.98 and 0.94, respectively. The model can be satisfactorily used to describe salt effects on hydraulic conductivity of swelling soils in arid and semiarid areas, where groundwater quality is poor.     

Long‐term‐fertilization effects on soil organic carbon,physical properties,and wheat yield of a loess soil

The large dryland area of the Loess Plateau (China) is subject of developing strategies for a sustainable crop production, e.g., by modifications of nutrient management affecting soil quality and crop productivity. A 19 y long‐term experiment was employed to evaluate the effects of fertilization regimes on soil organic C (SOC) dynamics, soil physical properties, and wheat yield. The SOC content in the top 20 cm soil layer remained unchanged over time under the unfertilized plot (CK), whereas it significantly increased under both inorganic N, P, and K fertilizers (NPK) and combined manure (M) with NPK (MNPK) treatments. After 18 y, the SOC in the MNPK and NPK treatments remained significantly higher than in the control in the top 20 cm and top 10 cm soil layers, respectively. The MNPK‐treated soil retained significant more water than CK at tension ranges from 0 to 0.25 kPa and from 8 to 33 kPa for the 0–5 cm layer. The MNPK‐treated soil also retained markedly more water than the NPK‐treated and CK soils at tensions from 0 to 0.75 kPa and more water than CK from 100 to 300 kPa for the 10–15 cm layer. There were no significant differences of saturated hydraulic conductivity between three treatments both at 0–5 and 10–15 cm depths. In contrast, the unsaturated hydraulic conductivity in the MNPK plot was lower than in the CK plot at depths of 0–5 cm and 10–15 cm. On average, wheat yields were similar under MNPK and NPK treatments and significantly higher than under the CK treatment. Thus, considering soil‐quality conservation and sustainable crop productivity, reasonably combined application of NPK and organic manure is a better nutrient‐management option in this rainfed wheat–fallow cropping system.