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1.
A simple model was employed to interpret the results of a series of measurements of gas diffusion in soil cores. The model divides air-filled porosity into three functional categories: arterial, marginal and remote. Diffusion along the axis of the core occurs through arterial pores; marginal pores do not contribute to axial diffusion; remote pores are isolated from gas transport. Simulations based on the model closely resembled data acquired from real cores. Optimizing the fit between real and simulated data gave estimates of the three functional pore fractions which generally made sense (compaction or wetting of cores resulted in reduced arterial and increased marginal porosities, for example). Dividing the pores into the different classes specified by the model was functionally equivalent (i.e. observable results were identical) to the introduction of a tortuosity factor to represent pore convolution. In order to account for observed diffusion rates in terms of pore convolution alone it is sometimes necessary to invoke implausibly high tortuosities; the introduction of marginal porosity renders this unnecessary without in any way compromising the ability of the model to simulate real diffusion data. 相似文献
2.
To improve the predictive capability of transport models in soils we need experimental data that improve their understanding of properties at the scale of pores, including the effect of degree of fluid saturation. All transport occurs in the same soil pore space, so that one may intuitively expect a link between the different transport coefficients and key geometrical characteristics of the pores such as tortuosity and connectivity, and pore‐size distribution. To understand the combined effects of pore geometry and pore‐size distribution better, we measured the effect of degree of water saturation on hydraulic conductivity and bulk soil electrical conductivity, and of degree of air saturation on air conductivity and gaseous diffusion for a fine sand and a sandy loam soil. To all measured data were fitted a general transport model that includes both pore geometry and pore‐size distribution parameters. The results show that both pore geometry and pore‐size distribution determine the functional relations between degree of saturation, hydraulic conductivity and air conductivity. The control of pore size on convective transport is more for soils with a wider pore‐size distribution. However, the relative contribution of pore‐size distribution is much larger for the unsaturated hydraulic conductivity than for gaseous phase transport. For the other transport coefficients, their saturation dependency could be described solely by the pore‐geometry term. The contribution of the latter to transport was much larger for transport in the air phase than in the water phase, supporting the view that connectivity dominates gaseous transport. Although the relation between effective fluid saturation and all four relative transport coefficients for the sand could be described by a single functional relation, the presence of a universal relationship between fluid saturation and transport for all soils is doubtful. 相似文献
3.
We investigated the possibility of inferring effective hydraulic properties of soil from the structure of the pore space. The aim was to identify structural properties, which are essential for water flow, so that physical experiments may be replaced by direct morphological measurements. The pore structure was investigated in three dimensions by serial sections through impregnated samples. The complex geometry of pore space was quantified in terms of two characteristics: pore-size distribution and pore connectivity. Only pores larger than 0.04 mm were considered. The results were used as input parameters for a pore-scale network model. The main desorption branch of the soil-water characteristic and the corresponding hydraulic conductivity function of the network model were calculated by numerical simulation. The simulation results, which are exclusively based on morphological investigations, were compared with independently measured results from a multi-step outflow experiment. This approach was demonstrated for two centrasting soil materials: the A and B horizons of a silty agricultural soil. The simulations were close to the experimental data, except for the absolute values of the hydraulic conductivity. The pore-size distribution and pore connectivity govern the shape of hydraulic functions and the applied morphometric methods are suitable for predicting essential characteristics of hydraulic soil properties. 相似文献
4.
A multi-domain model for the transport of chemicals in soils is developed. The solute flux is related to the microscopic water flux, which is modelled using concepts to estimate the hydraulic conductivity of porous media. The pore space of the soil is divided into an arbitrarily large number of domains each representing an equivalent pore radius. The domains are arranged on a structural coordinate, perpendicular to the direction of mean water flow. Transport in the flow direction takes place in each domain by convection and diffusion with pore-size specific velocities. Solute mixing between the domains is simulated as convective-dispersive transport along the structural coordinate. The model is solved numerically for one-dimensional steady-state water flux under unit-gradient conditions. Required input parameters are the unsaturated conductivity function of a soil and a pore interaction coefficient which characterizes the solute exchange between the pore domains. Simulations show a gradual change from convection dominated transport (isolated tube model) to convective-dispersive transport. The length scale where this change takes place depends on the lateral mixing intensity, pore-size distribution of the medium, and saturation degree. 相似文献
5.
MODELLING OF SOIL PORES AS TUBES USING GAS PERMEABILITIES, GAS DIFFUSIVITIES AND WATER RELEASE 总被引：1，自引：0，他引：1
B. C. BALL 《European Journal of Soil Science》1981,32(4):465-481
Two models are presented describing the air-filled continuous pores in soil and how they change with soil water potential. In the first model (A), the pores are represented by tortuous tubes of uniform radius. The radius, length and number are calculated from air permeability, relative diffusivity and air-filled porosity measured at each soil water potential. In the second model (B), the pores are represented by tortuous tubes of three radii joined at random in series. The radii and total lengths of the tube sections are estimated by comparison of air permeability, diffusion coefficient and air-filled porosity at each water potential with values calculated for a large number of theoretical systems. The models were applied to the results from undisturbed cores of a silt loam taken from 30 to 80 mm depth. For both models, the sequences of continuous pores were estimated to be 2 to 7 times as long as the sample but shortened as the sample dried. From the second model the average pore radius in direct drilled soil, 0.3 mm, was half that in ploughed soil and the minimum radius, 0.1 mm, was one-quarter that in ploughed soil. 相似文献
6.
Three cross-sections of soil aggregates (2–5 mm diameter) were digitized at 5 μm resolution from montages (× 100) of scanning electron micrographs to produce binary images representing the soil pores and soil matrix. A three-dimensional random Boolean process was chosen as a model of the soil pores and matrix. The soil matrix was simulated by randomly positioned, overlapping spheres with radii drawn from an exponential distribution. Simulation of a 1 mm cube of one soil aggregate showed that all but 0.1 % of the pore space was connected to the exterior, although only 50% appeared to be connected to the exterior in a cross-sectional image. Pore spaces able to accommodate different sizes of microorganisms were also investigated. For example, a protozoan with a cross-sectional diameter of 20 μm could be accommodated in 17% of the pores in a 1 mm3 soil cube, although only 11 % of the pores would be accessible to those protozoa on the exterior of the cube. 相似文献
7.
Soil pore networks have a complex geometry, which is challenging to model in three dimensions. We use a Boolean model of pore space that has proved useful in modelling gas diffusion in dry structures to investigate the distribution of water in this pore space and to quantify the effects on pore connectivity to the soil surface. We first show how total porosity in dry soil influences connectivity via the percolation threshold. Then we show that our model simulation of the ‘ink-bottle effect’ can account for much of the hysteresis of the soil water. The differences in distribution of water between wetting and drying result in maintaining greater connectivity of the air-filled pore space during drying than during wetting. Hysteresis is large at small total porosities and slowly declines as porosity increases. During wetting much pore space is blocked when more than 40% of the pore space is filled with water, although during drying all non-isolated air-filled pores are connected to the surface. Even when soil is allowed to wet to near saturation, there are rapid increases in pore connectivity during drying, which may explain, for example, rapid increases in production and emission of nitrous oxide in soils near saturation. 相似文献
8.
基于全卷积网络的土壤断层扫描图像中孔隙分割 总被引：5，自引：5，他引：0
针对土壤断层扫描图像中存在部分容积效应及因孔隙成分复杂、结构不规则等引起的分割精度低的问题,该文提出一种全卷积网络(fully convolutional network,FCN)土壤孔隙分割方法,为土壤科学研究提供技术支持。该文以黑土土壤断层扫描图像为研究对象,通过卷积和池化运算输出不同尺度的孔隙特征图;将孔隙的深层特征和浅层特征相融合,采用上采样算子对融合特征进行插值操作,从而输出孔隙的二值图。与大津法、分水岭法、区域生长法和模糊C均值聚类法(Fuzzy C-means,FCM)4种常用孔隙分割方法的对比结果表明,FCN法在低,中,高3种孔隙密度的土壤图像中优于其他4种方法。FCN法的平均分割正确率为98.1%,比4种常用方法分别高25.6%,48.3%,55.7%和9.5%;FCN法的平均过分割率和欠分割率分别为2.2%和1.3%,仅为次优方法(FCM法)的33.8%和23.6%。通过融合土壤孔隙结构的多重特征,FCN法能够实现土壤孔隙整体和局部信息的精准判断,为土壤学的研究提供了一种更加智能化的技术手段。 相似文献
9.
土壤是地球表面由固、液、气三相组成的疏松多孔介质体,土壤物理、化学和生物学等过程主要发生在液相和气相填充的土壤孔隙中及其与固相的交界面。随着无损探测土壤孔隙结构、土壤生物化学原位分析和计算机模拟等技术的快速发展和计算能力的提升,从土壤孔隙的形态、结构和功能的角度,原位、直观、精确地研究土壤中动态发生的各种过程成为可能,推动了对真实土壤中各种微观过程与机制的研究。基于前期的研究进展,本文提出,研究透明土壤体的物理学—土壤孔隙学（Soilporelogy）的时代已经启航。土壤孔隙学主要针对土壤孔隙空间,研究其动态变化与土壤物理、土壤化学和土壤生物（生态）互作过程及其效应。以土壤孔隙学为主线,本文首先介绍了获取土壤孔隙方法的进步,进而论述了基于土壤孔隙的流体运动、生物化学过程、根系和生物活动以及土壤微观生态学等的试验和模拟研究。最后,本文对土壤孔隙学的研究方法和理论发展方向进行了展望,相信基于土壤孔隙的研究会推动土壤学研究新的发展。 相似文献
10.
The migration of colloids in soils can enhance the leaching of strongly sorbing contaminants. We present a model for the simulation of colloid leaching from unsaturated, aggregated soil media under stationary flow. Transport in the intra-aggregate pores is simulated by convection–dispersion, and transport in the interaggregate pores, and a stagnant layer of water surrounding the aggregates, is simulated by diffusion. The model describes the release of colloids from soil aggregates, sorption and desorption processes at the air–water interfaces, and flocculation and subsequent straining from the flowing water. All three processes were simulated as functions of ionic strength. Transport of ions in intra-aggregate pores was simulated by Fickian diffusion. The model was calibrated against experimental results of colloid leaching from columns packed with natural soil aggregates. The aggregates were of two soils differing in organic matter content. On each soil a single calibrated parameter set could describe the experiments with the three ionic strengths. The parameters for release of colloids from the aggregate surface and the sorption properties of the air–water interface were different for the two soils. The key parameters for leaching were the thickness of the stagnant layer of water surrounding the aggregates, the mechanical dispersion, the maximum concentration of colloids at the surface of the aggregates, the sorption capacity and rate coefficient of the colloids at the air–water interface, and the colloid diffusion coefficient. Simulations were also done with two additional irrigation intensities at one ionic strength. Simulated leaching was greater than measured leaching at both irrigation intensities, but the diffusion-controlled release of colloids from the aggregates was simulated correctly. 相似文献
11.
为解决膨胀土对工程结构以及农业生态环境的危害，进行煤矸石粉改良膨胀土的试验研究。对煤矸石粉掺量为0、3%、6%、9%的膨胀土土样进行压汞试验，测得微观孔隙特征值；选取Menger海绵模型建立孔隙分分形模型，计算土体孔隙分形维数，探究土体孔隙分形维数与孔隙特征参数以及煤矸石粉掺量变化的关系。结果表明：随着煤矸石粉掺量增加，土中大孔隙所占的含量较素膨胀减少61.5%，孔隙类型从团粒间孔隙转化为颗粒间孔隙；煤矸石粉的掺入改变了土体的孔隙结构特征，煤矸石粉与膨胀土发生胶结反应，孔隙连通性降低，使得总孔隙体积、孔隙率、孔隙平均孔径、孔隙临界孔径等孔隙特征参数呈减小趋势；基于分形理论分析孔隙分形维数，分形维数随煤矸石粉掺量的增加而增加，且与孔隙特征参数呈显著相关性。孔隙分形维数反应了孔隙特征参数以及孔隙发育程度，为土的孔隙表征提供方法借鉴。 相似文献
12.
13.
A casting method for the three-dimensional analysis of the intraprism structural pores in vertisols 总被引：2，自引：0，他引：2
The structural voids in vertisols contain easily available water for plants and their volume can be calculated from the shrinkage curve. Access by plants to that water depends also on the geometric arrangement of the pores so that the water can flow through them. We have devised a method for studying the structural porosity by casting the pores in resin. The intraprism pore space of wet soil clods is impregnated with a UV fluorescent polyester resin under vacuum. When this has set we use the swelling properties of the clay to separate the clay matrix from the resin. A cast so obtained is the real three-dimensional solid reproduction of the structural porosity. This representation of the pore system is easier to study than results from computerized reconstitution of the three-dimensional space from two-dimensional images of soil in thin sections. Channels, packing pores and planar voids can be observed directly in three dimensions as the method saves the integrity and continuity of pores as small as 10 μm in diameter. The geometry of the cast shapes agrees with the interpretation of shrinkage and moisture characteristic curves. The method offers direct qualitative observation of pore organization and volume measurements of the intraprism structural porosity in vertisols. 相似文献
14.
A model is described for solute diffusion in an aggregated medium in which a proportion of the water-filled pores are connected to a continuous pore network by narrow channels through which the diffusing ion can move only slowly. The experimental conditions for detecting departure from normal diffusion behaviour are discussed. A pulse technique for measuring the selfdiffision coefficient of 36Cl in both undisturbed and repacked soil samples at various moisture contents is described. No evidence of slow exchange between freely and slowly accessible pores was obtained. The self-diffusion coefficients in large undisturbed peds were somewhat greater than in repacked sieved samples at the same volumetric moisture content. 相似文献
15.
The quantification of the spatial heterogeneity of soil structure is one of the main difficulties to overcome for an adequate understanding of soil processes. There are different competing concepts for the type of heterogeneity, including macroscopic homogeneity, discrete hierarchy or fractal. With respect to these different concepts we investigate the structure of the pore space in one single sample (4 × 103 mm3) by analysing basic geometric quantities of the pores > 0.3 mm within gradually increasing subsamples. To demonstrate the relation between geometrical and functional properties we simulate gas diffusion within the three‐dimensional pore space of the different subsamples. An efficient tool to determine the geometric quantities is presented. As a result, no representative elementary volume (REV) is found in terms of pore‐volume density which increases with sample size. The same is true for the simulated gas diffusion coefficient. This effect is explained by two different types of pores, i.e. big root channels and smaller pores, having different levels of organization. We discuss the different concepts of structural organization which may be appropriate models for the structure investigated. We argue that the discrete hierarchical approach is the most profitable in practice. 相似文献
16.
利用CT数字图像和网络模型预测近饱和土壤水力学性质 总被引：7，自引：2，他引：5
在近饱和状态下,土壤的有效水力学性质主要取决于较大孔隙的结构特征,而这又决定了土壤中的优势流通道以及溶质的运移.基于孔隙形态学特征的网络模型可以很好的表现出较大孔隙的几何形态与拓扑特征对有效水力学性质的影响.本文通过对连续土壤切片CT图像的分析,定量获取了土壤孔隙的大小分布以及连通性参数.在此基础上建立了相关网络模型,在孔隙尺度上模拟了土壤中的水分运动过程并预测了近饱和土壤水力学性质.实验结果表明,虽然随机网络模型对室内填装土样本水力学性质的预测结果要优于相关网络模型,但是结合了实测孔隙形态特征的相关网络模型能够表现出田间原状土样本的双重孔隙度结构,其预测结果更符合实际的土壤结构特征. 相似文献
17.
通过测定两种土壤和一种玻璃珠的两相热导率随气压的变化,分析变压条件下气体分子碰撞平均自由程和多孔介质孔隙结构间的关系。研究计算了表征土壤平均孔隙结构的孔隙特征长度(d),同时依据静态几何学方法计算获取了颗粒平均间距(D)。结果表明,基于热传输方法获取的d值是从气体分子碰撞传热的动态观点获取的孔隙结构表征,标识着土壤颗粒间的热分离特征,是表征土壤孔隙结构的有效指标。由于土壤的d和D值相差3个数量级,但在玻璃珠上无量级差异,这说明d值可能只能表征土壤团聚体间的平均孔隙结构,不能反映团聚体内部及黏土颗粒内部的微细孔隙结构。 相似文献
18.
果蔬类多孔介质内部水中溶解有大量的营养物质(溶质),在干燥过程中溶质的迁移与湿分的传递同时进行,其内部微孔内的干燥传质机理尚不明确。为了揭示果蔬类多孔介质干燥过程中内部溶液的迁移机理,确定果蔬微孔结构特性对干燥传质过程的影响规律,该研究采用分子动力学方法模拟研究了果蔬类多孔介质微孔道中的干燥传质过程,构建了光滑壁面溶液扩散过程模型与粗糙壁面溶液扩散过程模型。模拟过程采用SPC/E水分子模型,选取OPLS-AA全原子力场和正则系综,溶液势函数选用静电库伦相互作用与Lennard-Jones相互作用,中心水分子的初始速度由高斯分布给出,采用Velocity-Verlet算法,用SHAKE算法固定水分子,x、y方向施加周期性边界条件,z方向上施加固定壁面边界条件。从分子水平模拟分析了果蔬类多孔介质内部溶液的扩散过程,并以马铃薯的热风干燥试验结果进行模型的验证。得出试验值与KCl溶液粗糙壁面模型的模拟值最为接近,其最大相对误差为17.39%;与纯水模型的模拟值相差最大,说明溶质的存在对水分扩散系数的影响不可忽略,且粗糙壁面模型更接近于真实孔道结构。从径向分布函数分布可以看出K+、Cl-对水分子... 相似文献
19.
Mathieu Lamandé Per Schjønning Nicola Dal Ferro Francesco Morari 《European Journal of Soil Science》2021,72(2):769-781
Combining digital imaging, physical models and laboratory measurements is a step further towards a better understanding of the complex relationships between the soil pore system and soil functions. Eight natural 100-cm3 soil cores were sampled in a cultivated Stagnic Luvisol from the topsoil and subsoil, which we assumed had contrasting pore systems. Artificial 100-cm3 cores were produced from plastic or from autoclaved aerated concrete (AAC). Eight vertical holes of each diameter (1.5 and 3 mm) were drilled for the plastic cylinder and for one of the two AAC cylinders. All natural and artificial cores were scanned in an X-ray CT scanner and printed in 3D. Effective air-filled porosity, true Darcian air permeability, apparent air permeability at a pressure gradient of 5 hPa and oxygen diffusion were measured on all cores. The active pore system characteristics differed between topsoil (sponge-like, network of macropores of similar size) and subsoil (dominated by large vertical macropores). Active soil pore characteristics measured on a simplified pore network, that is, from artificial and printed soil cores, supported the fundamental differences in air transport by convection and diffusion observed between top- and subsoil. The results confirm the suitability of using the conceptual model that partitions the pore system into arterial, marginal and remote pores to describe effects of soil structure on gas transport. This study showed the high potential of using 3D-printed soil cores to reconstruct the soil macropore network for a better understanding of soil pore functions. 相似文献
20.
Coefficients of gas diffusion (hydrogen through air) were measured on packings of Portland stone chips over a range of water contents. The chips were obtained by crushing blocks of Portland stone on which similar gas diffusion measurements had been previously made. The packings had a tri-modal pore-size distribution: pores between the stone chips; pores within the chips but between ooliths; pores within the chips and within ooliths. As the water content of the packings was progressively decreased, the diffusion coefficient for the packings increased in three steps corresponding to successive drainage of the three pore modes. The previous results for the blocks were used to give good theoretical prediction of the three steps obtained by measurement. These results support earlier speculation that a similar stepwise increase in diffusion coefficient in soils might have been caused by micro-pedal structure within soil crumbs. 相似文献