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Advances in modeling of water in the unsaturated zone   总被引:8,自引:0,他引:8  
This paper reviews recent advances in analytical and numerical solution of problems of water flow through rigid soils in the unsaturated zone. The Richards model remains the most widely accepted and fertile framework for water flow analyses. More general formulations are reserved for the analysis of problems involving macroporosity, thermal effects, and air pressure effects. Many exact and approximate solutions have been derived for particular boundary value problems of homogeneous soils using methods such as quasi-linear analysis, Green-Ampt analysis, perturbation, and the kinematic wave approximation. Numerical simulators have become bigger and more accurate due to improvements in the areas of nonlinear solution procedures, mass conservation, computational efficiency, and computer hardware. Problems of natural heterogeneity have been addressed primarily through application of various stochastic methods to the Richards model. The stochastic formulations generally refute the concept of simple equivalent homogeneous properties, but do themselves offer a certain limited potential for a predictive capability.  相似文献
2.
Knowledge of scale dependent variation of soil properties is important where upscaling and generalization from plot scale studies to field and larger scale is desired. We used conventional statistics, geostatistics, and fractal analysis to characterize and compare the apparent soil electrical conductivity (ECa) of six contiguous agricultural fields each ranging between 9.5 and 14.0 ha in size. Factor analysis revealed that ECa was strongly related to ammonium extractable K, organic matter (OM), pH and Bray-2 Phosphorus, but not to ammonium extractable Ca and sum of bases. All six fields were spatially structured and well described by exponential semivariograms. Fractal dimensions estimated from the linear portion of the semivariogram using a linear plateau model were statistically different (p = 0.05) among some of the fields, and the differences may have been caused by management differences. Fractal analysis identified at least two scales of variation for the fields. The first scale of variation, common to all six fields, was for distances less than 9 m. The second scale of variation was for distances ranging between 9 and 46 m (field NC), 9 and 79 m (fields SC and SW); and 9 and 126 m (field SE). Two of the fields (fields NW and NE) did not have a plateau on the log–log plot of the semivariograms, indicating a scaling behavior at larger distances. The study showed that although the semivariogram forms are similar among the six fields, the rate of change of the semivariograms (as indicated by the fractal dimension) differs for some of the fields at distances greater than 9 m.  相似文献
3.
This paper presents a new theoretical model to describe the spatial variability in tillage forces for the purpose of fatigue analysis of tillage machines. The proposed model took into account both the variability in tillage system parameters (soil engineering properties, tool design parameters and operational conditions) and the cyclic effects of mechanical behavior of the soil during failure ahead of tillage tools on the spatial variability in tillage forces. The stress-based fatigue life approach was used to determine the life time of tillage machines, based on the fact that the applied stress on tillage machines is primarily within the elastic range of the material. Stress cycles with their mean values and amplitudes were determined by the rainflow algorithm. The damage friction caused by each cycle of stress was computed according to the Soderberg criterion and the total damage was calculated by the Miner’s law. The proposed model was applied to determine the spatial variability in tillage forces on the shank of a chisel plough. The equivalent stress history resulted from these forces were calculated by means of a finite element model and the Von misses criterion. The histograms of mean stress and stress amplitude obtained by the rainflow algorithm showed significant dispersions. Although the equivalent stress is smaller than the yield stress of the material, the failure by fatigue will occur after a certain travel distance. The expected distance to failure was found to be df = 0.825 × 106 km. It is concluded that the spatial variability in tillage forces has significant effect on the life time of tillage machines and should be considered in the design analysis of tillage machines to predict the life time. Further investigations are required to correlate the results achieved by the proposed model with field tests and to validate the proposed assumptions to model the spatial variability in tillage forces.  相似文献
4.
In this paper, the macroscopic dispersion resulting from one and twodimensional flows through a semiconfined aquifer with spatially variable hydraulic conductivity K which is represented by a stationary (statistically homogeneous) random process is analyzed using the spectral representation technique. Stochastic fluctuation equations of the steady flow and solute transport are solved to construct the macroscopic dispersive flux and evaluate the resulting macrodispersivity tensor in terms of the leakage factor and input covariances describing the hydraulic conductivity in a semiconfined aquifer bounded by a leaky layer above and an impervious stratum below. The macrodispersivity tensor is studied using some convenient forms of the log hydraulic conductivity process. The sensitivity of the resulting macrodispersivity to the input covariances is discussed along with the influence of the leakage factor for both one and twodimensional flows. It is found that the longitudinal macrodispersivities are increased due to the presence of leakage, while the transverse macrodispersivities are reduced due to leakage.  相似文献
5.
现有研究大多采用简单的摩尔库伦模型针对土的空间变异性对边坡或基础的安全系数或失效概率做计算分析.事实上临界状态本构模型,如SIMSAND,能更准确地反映土的应力-应变关系.为此,本文采用SIM-SAND模型,针对砂土初始孔隙比的空间变异性对其力学特性及破坏模式的影响做详细分析,算例采用简单的室内平面应变双轴试验,分为松砂排水、密砂排水、松砂不排水和密砂不排水四种情况.每一种情况均采用蒙特卡罗方法进行初始孔隙比的随机分布生成,并做大量计算,以此来分析初始孔隙比的不均匀性对剪切带生成和破坏模式和竖向承载力发展及其概率密度分布的影响.  相似文献
6.
A saturated flow problem with spatially varying conductivity is studied in a rectangular domain. An expansion of the flow equation with respect to small perturbations of the conductivity is given. Discrete spectra are used to calculate the expected flux across the outflow boundary and its variability. The results obtained are compared with results based on Monte Carlo studies. Another way to deal with heterogeneous soils is to replace the actual conductivity by a smooth, so-called, effective conductivity. A comparison is made between results based on that approach and our results.  相似文献
7.
A computer based numerical method is presented for the analysis of water and solute movement in unsaturated heterogeneous porous materials. Such a method is necessary since, for those field studies where solute movement is of concern, the soil profiles under consideration are invariably heterogeneous. The numerical analysis is based on a general one-dimensional finite difference soil water flow model which includes a numerical technique combining the concepts of scale heterogeneity with an interpolative soil water hysteresis model. An explicit finite difference solute movement subroutine is incorporated into the unsaturated flow model to describe the transport of nonreactive solutes. A velocity dependent longitudinal dispersion coefficient is used in the solution of the hydrodynamic dispersion equation. The resulting hysteretic scale heterogeneous solute movement model permits the study of solute dynamics during infiltrating and redistribution in realistically complex spatially varying soil profiles. Results are presented for the leaching of both coarse grading to fine and fine grading to coarse sand profiles. Both vertical and horizontal profiles are studied using either a constant flux or a constant concentration input boundary condition. The four cases studied demonstrate the versatility of the numerical method and emphasise the substantial differences in transport behavior that can arise between heterogeneous and homogeneous profiles.Now with BHP Petroleum Pty. Ltd., GPO Box 1911R, Melbourne, Vic. 3001, Australia.  相似文献
8.
空间变异性是结构参数的固有属性,对于工程结构的随机响应和可靠度分析具有重要影响。结合随机场离散的局部平均理论和随机响应分析的摄动随机有限元法,提出一种考虑参数空间变异性的平面框架结构可靠度分析方法,并定量分析了参数空间变异性对结构可靠度的影响规律。首先,考虑随机因素的空间变异性,采用二维线性随机场离散的局部平均理论将平面框架结构的连续随机场离散为一组随机变量,并通过理论推导建立了随机场局部平均间协方差矩阵的二重积分表达式;然后,采用摄动随机有限元法分析结构随机响应及其对基本随机变量的梯度向量,并利用可靠度分析的梯度优化法计算结构可靠指标,从而提出了一种考虑参数空间变异性的平面框架结构可靠度分析方法。分析表明,该方法具有较高的计算精度和计算效率;随机场离散的局部平均理论对相关结构类型不敏感;随着随机场相关偏度和变异性的增大,框架结构的可靠指标逐渐减小,说明结构参数的空间变异性对结构可靠度的影响不容忽视。  相似文献
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