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.     

Ein autoregressives Verfahren zur Bestimmung der gesättigten und ungesättigten hydraulischen Leitfähigkeit

An autoregressive procedure to predict the hydraulic conductivity — Comparison of measured and predicted results An instantaneous profile method was used to measure the unsaturated hydraulic conductivity. Relatively new techniques involving undisturbed soil samples instrumented with minitensio-meters and Time-Domain-Reflectometry (TDR) mini-probes were used for the experiments. The laboratory method allows a high spatial and temporal resolution. Laboratory measurements were carried out for 40 soil horizons with a wide spectrum of texture and bulk density. In addition, retention curves were measured using the standard pressure plate apparatus. Using this homogeneous set of data, an autoregressive model was developed which allows a stepwise calculation of the hydraulic conductivity for a water potential range of —30 up to —600 hPa. This model was developed for loamy sands, sandy, silty and clayey soils in conjunction with data from the retention curves. The calculation procedure starts with the determination of an initial unsaturated conductivity (k) close to field capacity, i.e., for water potential from —60 hPa up to —100 hPa. This first value is then used to predict other conductivity values using appropriate changes in soil water content corresponding to a defined range of the soil water potential. Subsequently, the hydraulic conductivities for higher and lower potentials were estimated considering the k value of the previous step in combination with the data of the retention curve of the next water potential range. The advantage of this empirical model is the indirect consideration of soil structure, in contrast to the closed-form van Genuchten-Mualem (vGM) model. To demonstrate these effects on different fitting procedures, the vGM model was also used to describe soil hydraulic functions. The accuracy of both, the vGM model and the autoregressive one, were compared for various fitting procedures and soils.     

Hydraulic variability in space and time in a dark red latosol of the tropics

Detailed information of the variability of soil properties and processes in space and time is presented for a dark red latosol (Alfisol) of the county of Piracicaba, S.P., Brazil. Data were collected on 25 plots along a 125 m transect during the years 1989–1991, and consisted of the soil water content θ in the 0–150 cm soil layer and the water pressure heads h at 135 and 165 cm depths. These raw data were used to characterize variabilities in space and time using classical statistics and in a second step to analyse the difficulties in calculating soil water storage, soil hydraulic conductivities, hydraulic gradients, soil water fluxes and water balances. In general, there was a great variability of hydraulic properties and processes, which is fairly constant in time in the case of basic data like soil water content and potential, but not in the case of calculated data like hydraulic conductivities and gradients, and soil water flux densities. A discussion is presented of the difficulties of using Darcy's equation to estimate soil water flux densities due to the exponential K(θ) and K(h) relationships of the hydraulic conductivity K, and of the influence of variability in space and time on the establishment of water balance components.     

Soil physical quality: Part III: Unsaturated hydraulic conductivity and general conclusions about S-theory

The index of soil physical quality, S, which was proposed in Part I and which was applied to the problems of tillage in Part II is applied in Part III to the unsaturated hydraulic conductivity of soil. S is equal to the slope of the soil water retention curve at its inflection point. This curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg⁻¹). It is suggested that S is a measure of the micro-structural porosity of the soil. It is shown through an approximate theory, through simulations and through experimental results that the value of S at the inflection point is related to the unsaturated hydraulic conductivity of soil at the inflection point. It is proposed that the inflection point can be used as a “matching point” in studies of unsaturated hydraulic conductivity. Pedo-transfer functions are used to explore the predicted effects of soil texture class and bulk density on the values of the unsaturated hydraulic conductivity at the inflection point. The conclusions from this series of three papers are summarized and as a result, it is recommended that S be used as an index of soil physical quality that enables different soils and the effects of different management treatments and conditions to be compared directly. The use of S for prediction of a range of soil physical properties is summarized and is called S-theory.     

Bestimmung von hydraulischen Parametern für die Wasserhaushaltsuntersuchungen im natürlich gelagerten Boden. Ein Vergleich von Feld- und Laboratoriumsmethoden

Determination of hydraulic parameters to estimate water movement and water storage in undisturbed soil. A comparison of field and laboratory methods. The soil moisture characteristics and the unsaturated conductivity were measured under field and laboratory conditions. In an undisturbed soil monolith in continous connection with the underlying loess-soil-layer 60 tensiometer and a neutronprobe-accestube were used respectively to determine the matric potential and water content changes during a transient drainage experiment. Within the matric potential range of 0 to approximately ?50 to ?100 mbar the soil moisture characteristics determined in the laboratory and in the field are substantially different. When the matric potential is more negative than ?100 mbar the slope of these curves and hence the specific moisture capacities are relatively well comparable. The field-measured conductivity functions differ considerably from those values which were either measured under lab-conditions or computed from the soil moisture characteristics data. The conductivities are expressed as a function of the matric potential which is possibly the major reason for these remarkable differences. However, the changes in water content in this dense silty clay loam are too small to express the unsaturated conductivity as a function of the absolute water content. The most serious problems in extrapolating the results of the less time consuming laborcalculation methods to field conditions arise in that range of matric potentials where water movement is significant.     

Comparison of seven water retention functions used for modelling soil hydraulic conductivity due to film flow

The unsaturated soil hydraulic conductivity accounting for film flow is important for understanding soil hydrological and biological processes, especially in arid and semi‐arid regions. Recently, a theoretically based hydraulic conductivity model was developed to describe the hydraulic conductivity as a function of water content. We have used this model to compare seven soil water retention functions commonly used for predicting soil hydraulic conductivity due to film flow. A total of 30 soils, varying in basic properties, were selected from the Unsaturated Soil Hydraulic Database to evaluate the seven functions. The Webb method was applied to identify the critical soil matric potential (h _c) below which thin film flow controls water movement. Soil hydraulic conductivity measurements at matric potential below h _c were then used for curve fitting according to the seven functions. Slight differences were observed among the functions in predicting soil hydraulic conductivity due to film flow. Six of the seven functions in combination with the hydraulic conductivity model described the hydraulic conductivity due to film flow well, according to the terms of the coefficient of efficiency. The relatively poor performance of the one exception was due to the fact that the linear shape of the function made it less flexible at low matric potentials. In addition, the effect of textural class on its performance was substantial, showing a poorer fit for the sand soil compared with the loam and clay soils. These findings have important applications related to soil and water resources conservation especially in arid and semi‐arid regions.     

Influence of water content and soil properties on unsaturated hydraulic conductivity of some red and black soils

The unsaturated hydraulic conductivity was determined in the laboratory for some red and black soils, following water movement into a horizontal column of homogenous soil of uniform packing. A highly significant positive relationship was found between moisture content and hydraulic conductivity values in all the soils studied. Correlation coefficients calculated for the relationships between soil constituents/properties and the change in hydraulic conductivity per unit change in moisture content (regression coefficient between hydraulic conductivity and moisture content) have shown positive relationship to sand and negative relationships to silt, silt + clay, clay, carbonates, aggregates > 0.25 mm and saturated hydraulic conductivity. It is concluded that the unsaturated hydraulic conductivity decreases rapidly with decrease in moisture content and this decrease depends on the soil constituents/properties and differences between soil types are clear.     

灰色关联及非线性规划法构建传递函数估算黑土水力参数

土壤水分特征曲线和饱和导水率是重要的水力参数,为了简便准确获取这些参数,以松嫩平原黑土区南部为研究区域,采集136个采样点土样用于测定不同土层土壤水分特征曲线、饱和导水率以及土壤理化性质,并运用灰色关联分析确定影响土壤水力参数的主要土壤理化性质,采用非线性规划构建土壤分形维数、有机质、干容重、土壤颗粒组成与土壤水分特征曲线、饱和导水率之间的土壤传递函数,并通过与现有土壤传递函数对比分析进行精度验证。结果表明:1)土壤分形维数是估算土壤水分特征曲线模型参数和饱和导水率的主要参数之一,同时,干容重和有机质含量也在不同土层土壤传递函数中起到重要的作用;2)通过验证分析,不同土层各参数平均绝对误差接近于0,均方根误差值也都较小,其中在不同土层土壤传递函数估算的土壤含水率均方根误差分别为0.022、0.017cm~3/cm~3;3)对比分析其他已存的土壤水分特征曲线和饱和导水率的土壤传递函数,该文构建的土壤传递函数均方根误差值均较小,决定系数值都在0.66以上,表明估算精度较高,均好于其他方法估算精度,具有良好的区域适应性。综上,所构建的土壤水分特征曲线和饱和导水率土壤传递函数可以用于松嫩平原黑土区土壤水力参数估算。     

北京地区表层土壤分形特征研究

土壤是一种具有分形特征的复杂系统.分形维数可以反映很多土壤特性.本研究考虑了7种土壤类型、4种土地利用类型,在北京地区选取了30个典型样点,对复杂地理环境表层土壤的分形特征进行了分析,探讨了分形维数与土壤质地、土壤理化性质等方面的关系.研究结果表明,在复杂地理环境条件下,土壤颗粒分形维数总体变异性较弱;分形维数与黏粒含量呈显著正相关,与(砂粒含量+粉粒含量)和黏粒含量的比值呈显著负相关.分形维数可以反映土壤质地,但仅可指示部分重要土壤的理化性质.     

Using soil knowledge for the evaluation of mid‐infrared diffuse reflectance spectroscopy for predicting soil physical and mechanical properties

Mid‐infrared diffuse reflectance spectroscopy can provide rapid, cheap and relatively accurate predictions for a number of soil properties. Most studies have found that it is possible to estimate chemical properties that are related to surface and solid material composition. This paper focuses on prediction of physical and mechanical properties, with emphasis on the elucidation of possible mechanisms of prediction. Soil physical properties that are based on pore‐space relationships such as bulk density, water retention and hydraulic conductivity cannot be predicted well using MIR spectroscopy. Hydraulic conductivity was measured using a tension‐disc permeameter, excluding the macropore effect, but MIR spectroscopy did not give a good prediction. Properties based on the soil solid composition and surfaces such as clay content and shrink‐swell potential can be predicted reasonably well. Macro‐aggregate stability in water can be predicted reasonably as it has a strong correlation with carbon content in the soil. We found that most of the physical and mechanical properties can be related back to the fundamental soil properties such as clay content, carbon content, cation exchange capacity and bulk density. These connections have been explored previously in pedotransfer functions studies. The concept of a spectral soil inference system is reiterated: linking the spectra to basic soil properties and connecting basic soil properties to other functional soil properties via pedotransfer functions.     

WATER MOVEMENT IN DRY SOILS

hysteresis that occurs in the micro-hydrological characteristics of a soil (conductivity and diffusivity) as functions of water content and water potential between sorption and desorption is examined for three soils (sand, loam, and clay) and sepiolite. These materials, all aggregated, are sufficiently dry that both the vapour and liquid components of water movement are important. any water content or water potential the vapour conductivity is always bigger when wetting than when drying though differences may be small. There is hysteresis in water (vapour+liquid) conductivity as a function of water content when vapour flow but not liquid flow is dominant. Conversely, there is hysteresis in water conductivity as a function of water potential when liquid flow but not vapour flow is dominant. Both forms of this hysteresis are small. Hysteresis in diffusivity as a function of both water content and water potential exists, but is complicated. Variations caused by hysteresis, though real, are likely to be negligible in practice, swamped by effects caused by changes in the soil environment (e.g. temperature) or by changes in soil management (e.g. structure).     

砂壤质褐土饱和导水率与物理性质的多尺度关系——小波分析法

正确估算土壤水力特性是准确了解土壤水分运动和溶质运移过程的前提。土壤水力特性具有明显的空间变异特征,由于其空间异质性是各种物理、化学和生物过程(如生物活动、耕作、地形、土壤侵蚀)在不同尺度下综合作用的产物,导致其变异     

滩涂围垦农田土壤饱和导水率的影响因素及转换函数研究

确定苏北沿海滩涂围垦农田耕层土壤饱和导水率的影响因素,构建适合该区的土壤转换函数,是研究该区田间土壤水盐运动和盐渍化防控的重要前提。本文在该区典型地块实测土壤饱和导水率和相关土壤基本理化性质,探讨了该区土壤饱和导水率的剖面分布特点,对影响饱和导水率的土壤基本性质进行了主成分分析,并建立了用于该区饱和导水率间接估算的土壤转换函数。结果表明:滩涂围垦农田土壤饱和导水率随剖面深度增加呈表土层高、亚表层低、底土层又升高的趋势,20~40 cm土层饱和导水率最小,介于2.75~6.73 cm·d-1,属低透水强度;土壤容重随剖面深度增加表现出与饱和导水率相反的变化特点。除了容重、孔隙度、质地等物理因素外,土壤肥力、盐分等化学性质也是影响饱和导水率的重要因素;影响滩涂围垦农田土壤饱和导水率的因素可由持水特性、盐碱状况、养分特征和土壤质地4个主成分反映,其累计贡献率达78.17%。在Vereecken转换函数中引入土壤盐分后可提高预测精度,修正函数Vereecken_1是最适合滩涂围垦农区土壤、具有最佳预测精度的转换函数。本文构建的土壤转换函数,可通过较易获得的砂粒、黏粒、容重、盐分和有机质对耕层土壤饱和导水率进行较高精度的预测,其结果可为滩涂盐渍化农区田间尺度土壤饱和导水率间接估算以及水盐运动数值模拟提供支持。     

设计流量和土壤质地对微孔陶瓷灌水器入渗特性的影响

为探明微孔陶瓷灌水器土壤中入渗流量变化的原因,明确微孔陶瓷灌水器的出流原理,该研究基于土桶模拟试验,研究3种设计流量(0.72、1.87和4.40 L/h)的微孔陶瓷灌水器下2种土壤(黄绵土、塿土)的渗流特性。结果表明,使用不同灌水器灌溉后,短时间内入渗流量均迅速减小,而后缓慢减小趋于稳定。设计流量与土壤质地均影响灌水器的出流。灌水器周围土壤水势的变化是造成入渗流量变化的直接原因,土壤含水率的变化是入渗流量变化的根本原因。在没有淹没出流的情况下,土壤含水率越高,入渗流量越小。设计流量为1.87 L/h灌水器应用于塿土中,当土壤含水率由13%增大至40%时,入渗流量由1.4 L/h下降至0.3 L/h左右。灌水器周围土壤含水率对入渗流量具有反馈调节作用。采用微孔陶瓷灌水器作为灌溉系统的核心部件,在内部水头适宜(微压或零压)的情况下,通过灌水器入渗流量与土壤含水率的耦合作用,可实现土壤水分的自动调控,达到主动灌溉的目的。该文可为微孔陶瓷灌水器的推广应用提供参考。     

轻质土壤水分特征曲线估计的简便方法

以黄淮海平原封丘地区的潮土和风沙土为研究对象,根据大量的土壤基本物理性质和土壤持水数据,利用多元逐步回归分析方法,建立了轻质土壤在不同基质势下土壤含水量(θ)的传递函数模型,并进行了模型验证。结果表明,利用轻质土壤的基本物理性质估计其水分特征曲线是一种简便可行的方法,并且,在回归方程中,增加-30kPa含水量项可提高-30kPa以上土壤含水量的估计精度;增加-1500kPa含水量项可以明显提高-100kPa至-1500kPa间土壤含水量的估计精度。     

土壤含盐量与土壤电导率及水分含量关系的试验研究

应用不同盐分含量土壤，进行了土壤电导率与土壤含盐量关系的试验研究，获得了不同含盐量土壤的电导率随水分含量变化的规律，并且建立了土壤含盐量与土壤电导率及水分含量的关系．本文所用方法简便可行，试验结果具有较高精度．本研究为较精确测定土壤含盐量及快　速进行较大规模盐渍土壤调查提供了一种较佳方法．     

质地对土壤热性质的影响研究

土壤热性质是水热迁移研究中的重要参数之一。根据非稳态条件下热流方程的差分解和实测土壤温度资料计算了不同质地土壤的热扩散率，并得出了质地影响下的土壤导热率关系式。研究结果表明，对特定土壤而言，土壤导热率与含水率之间可建立幂函数关系；砂粒、粉粒和粘粒含量对土壤热性质有不同程度的影响；不同质地土壤的热性质与土壤水吸力之间存在良好的定量关系；此外，含盐土壤的导热率可表示为浓度的幂函数关系。     

A practical formula for quantifying the thermal conductivity of Tottori dune sand

Abstract

Sustainable agriculture needs appropriate management of water, chemicals and heat in soil. In this study, we focused on thermal conductivity, which is among the various soil physical properties that are crucial for the sustainable management of agricultural fields. To expand the Mochizuki model, which describes thermal conductivity as a function of water content and solution concentration, we considered the water content, solution concentration and temperature as independent variables. The thermal conductivity of Tottori dune sand was measured under conditions of various combinations of these three independent variables. We observed that the thermal conductivity increased linearly with increasing water content, 0.054–0.276 m³ m^?3, for fixed temperature and solution concentration, and varied linearly with solution concentration for fixed temperature and water content. These results are consistent with the Mochizuki model. Using the Mochizuki model, the experimental parameters, which are dependent variables of water content and solution concentration, are shown as functions of water content. From regression analyses of the relationships between the experimental parameters and temperature, we expanded the Mochizuki model into a new practical formula that quantifies the soil thermal conductivity as a function of water content, solution concentration and temperature.     

Comparison of experimentally Determined and Calculated Hydraulic Conductivities for two alluvial sandy loam Profiles

The dynamic water conductivity characteristics of two alluvial sandy loam profiles (Typic Ustochrepts) were determined following the ?instantaneous profile method”? by monitoring the temporal variation in soil moisture content and potential at different depths in the profile, as the downward movement of water in the nearly saturated profile continued with evaporation prevented. The experimental sites differed in bulk density, moisture retention functions as well as dynamic water conductivity characteristics K(O). The unsaturated hydraulic conductivities were also calculated from moisture retention functions following the methods suggested by Campbell(1974) and Ghosh (1977). The calculated conductivity values agreed fairly well with the field data for the light-textured soils studied and the calculated values can be used for all practical purposes.     

A new laboratory method to quickly determine the unsaturated hydraulic conductivity of undisturbed soil cores within a wide range of textures

A modified instantaneous profile method is described, which determines the hydraulic conductivity functions of soils with varying textures. Soil suction head as a function of time and depth, rate of outflow as a function of time and the final distribution of moisture content are monitored on undisturbed soil columns. Data is handled following a modification of the procedure of Weeks and Richards (1967). The apparatus consists of five measuring cells, a monitoring unit, five balances and equipment to section the soil columns for the gravimetric determination of soil moisture content. Each measuring cell contains a soil core of 250 cm³, into which five micro-tensiometers are inserted at 2 cm intervals. Exemplary hydraulic conductivity functions are shown for samples taken from an Arenosol, from silty and clayey-silty horizons of a Luvisol and from a clayey Vertisol. The functions showed good agreement with in situ data. Evaluations of variability show the method to be very reliable. On average, the functions of five samples can be determined in one week, making the method quick. At the same time, the method has great potential for the study of effects of soil structure on hydraulic conductivity.