Calculation of Steady State Capillary Rise from the Groundwater Table in Multi-Layered Soil Profiles

A computer program CRISP is presented to calculate steady state capillary rise from the groundwater table in single- and multi-layered soil profiles. The calculation is based on an integration of Darcy's equation and uses k-v functions, described with a modified formula of Brooks and Corey (1964). The three constants in this formula can be evaluated from texture and organic matter content, accounting for hysteresis (Bloemen, 1980). Application of the program CRISP shows that the heights of capillary rise from the groundwater table in single-layered soil profiles may be largely different. Of more interest for practical matters is capillary rise in multi-layered soil profiles, which may be strongly influenced by the depth of the groundwater table. The deepest admissable level of the groundwater table that guarantees a certain water supply to the root zone appears to vary widely for different soil profiles. This can be the starting point for a classification of soil profiles on the basis of their potentialities for capillary water supply. Characteristic textures and humus contents of upper soils and subsoils of different geo-genetical groups may thus become distinctive features. Possibly a subdivision of these groups is necessary. The depths of changes between distinguished layers are significant. The actual range of variations of the depth of the groundwater table should play part in a soil classification.     

Calculation of hydraulic conductivities and steady state capillary rise in peat soils from bulk density and solid matter volume

Values of the three constants in a convenient formula for the calculation of capillary conductivities of peat soils can be evaluated from bulk density and solid matter volume. Though some of the relative equations have a low significancy, they provide a means to calculate in a simple way a mean hydraulic conductivity function for peat soils with given values of the soil properties mentioned above. Examples of such functions are shown. The height of steady state capillary rise in homogeneous fen peats and high bog peats of increasing bulk density are comprised in some diagrams.     

Calculation of Hydraulic Conductivities of Soils from Texture and Organic matter Content

It is demonstrated how the values of the three constants in a convenient formula for the calculation capillary conductivities can be evaluated from texture and organic matter content of a soil. In this way it is possible to calculate the conductivity function of any soil provided that an adequate analysis of granular composition and humus content analysis is available. Particularly for practical purposes the quick availability of the results of this calculation is of great advantage over time consuming direct measurement or calculation with constants evaluated from moisture retention curves. To increase the practical significance of the method the effect of pore size hysteresis on the calculated conductivity functions is accounted for in a pragmatical way. Conductivity functions for a number of single-layered soil types in the Netherlands are given. In a subsequent paper it will be shown how the discussed method is put into practice in a computation of capillary rise from the groundwater table in multi-layered soil profiles.     

Comparison of the soil hydraulic conductivity predicted from its water retention expressed by the equation of Van Genuchten and different capillary models

Knowledge of water retention and conductivity is essential to study water transport in soil. Determination of the conductivity curve is difficult, and it is often predicted by application of a capillary model to the water retention relationship. Three expressions are predicted from the water retention described by the equation of Van Genuchten. Two expressions are obtained in combination with the capillary models of Mualem and Burdine . The third expression is obtained by a combination with the capillary model of Fatt & Dykstra . The three sets of soil properties were applied to clay in order to compute infiltration and infiltration rates according to the series solution of Philip . Comparison with this solution showed that the results of the first two combinations were severely under-estimated, while those of the third were satisfactory. Similar results were obtained for sand by comparison with experimental data. Because conductivity estimated from the capillary model of Fatt & Dykstra is complicated, it was expressed by a power equation, the exponent of which is obtained by applying the Fatt & Dykstra capillary model to the water retention curve expressed according to Brooks & Corey and having the same asymptotic behaviour as the Van Genuchten equation. Application of this procedure to fifty soils selected from a published database gave satisfactory results. It is concluded that the hydraulic conductivity of a soil can be predicted from its water retention as expressed by the equation of Van Genuchten subject to the condition of the capillary model of Fatt & Dykstra and as expressed by the equation of Brooks & Corey, for which the exponent is obtained according to the same capillary model.     

高矿化度对砂性土毛细水上升影响

为明晰极端干旱区高盐度潜水蒸发机理,指导盐荒地开发与水盐调控,开展了砂性土条件下,不同粒径及潜水矿化度（1、30、100、250 g/L）组合的毛细水上升试验。结果表明,试验后期毛细水上升规律稳定时段：粗砂土柱,潜水矿化度越大,毛细水上升高度越小;细砂土柱,1 g/L处理毛细水上升高度明显大于其他3组;粉土土柱,不同处理毛细水上升高度由大到小为：30、100、250、1 g/L。试验初始阶段,除粗砂外并不表现为矿化度越大,毛细水上升速度越小的趋势。相比高矿化度,粒径是控制毛细水上升的主要因素。研究表明,土体颗粒较细时（细砂、粉土）,高矿化度不仅改变毛细水重力,也使得土体孔隙结构发生不同程度变化,二者综合作用于毛细水上升过程。     

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。     

夹砂层土壤Green-Ampt入渗模型的改进与验证

对于土层夹砂结构,湿润锋穿过砂层上界面时,入渗率变为稳渗率。为确定各因素下夹砂层土壤的稳渗率,在Green-Ampt入渗模型基础上,引入导水度系数(小于1)来量化上层土壤的导水程度,建立了改进的夹砂层土壤Green-Ampt入渗模型。采用HYDRUS-1D软件,模拟了不同土壤质地、初始含水率、压力水头、砂层埋深和砂层厚度条件下的稳渗过程,根据模拟结果分析了夹砂层土壤的入渗规律及其影响因素,稳渗率主要受土壤质地、压力水头和砂层埋深的影响。在相同压力水头、初始含水率和砂层厚度下模拟获得不同砂层埋深的稳渗率,并采用改进的Green-Ampt入渗模型拟合,求得导水度系数和进水吸力值。分析发现导水度系数变化较小,为简化计算,取其平均值0.95。在此基础上,提出了由土壤物理特性参数进气值倒数估算进水吸力的计算公式。利用秦王川地区的夹砂层土壤积水入渗试验及已有文献资料验证所建模型的有效性,结果表明所建模型待定参数少,计算误差基本在5%以内,且试验设计较简单,可为农田水分管理及工程防渗技术提供理论依据。     

降雨条件下分层土壤入渗特性

通过室内模拟降雨入渗试验,研究了粘土、壤土和沙土3种土壤不同分层组合方式对土壤入渗特性的影响。结果表明:垂直入渗条件下,分层土壤入渗特征由土壤分层组合方式决定;分层土壤累计入渗量与湿润锋距离呈线性相关关系;分层土壤入渗过程中,当粗沙覆盖细沙且降雨强度大于下层细沙土稳定入渗率时,湿润锋以上剖面可以达到或接近饱和,当细沙覆盖粗沙且降雨强度大于细沙土稳定入渗率时,湿润锋剖面中,除细沙土部分剖面达到饱和,其余粗沙土剖面无法达到饱和,且其饱和程度和下层粗沙土土壤持水能力有关。     

Wetting of agricultural soil measured by a simplified capillary rise technique

We describe the use of a capillary rise method to measure the wettability of 10 samples of agricultural soil from Rothamsted long-term experimental sites. The samples have very similar clay contents, but organic carbon (C) contents range from 11.5 to 31.2 g kg⁻¹. Their wetting rates were interpreted by an improved method of data analysis, consistent with the Washburn equation, and showed an increase in the effective contact angle between the water meniscus and the soil with increasing C content. This corresponds to a decrease in wettability with increasing C content, and accords with other results reported in the literature. By contrast with water, we found that capillary rise for n -hexane into soil did not depend on the soil's bulk density or C content. A priori calculations of the expected wetting rates from fluid properties and an effective hydraulic radius estimated by other methods gave magnitudes and trends that agreed with our experimental data. The results show that estimates of effective hydraulic radius can provide a useful approximation for characterizing soil wetting, but that further modelling should be carried out.     

夹砂层土体构型毛管水上升的实验研究

针对西北地区土壤剖面多呈层状和春季强烈返盐季节土壤多处于裸露状态的特点，通过室内土柱实验，研究了有地下水位条件下夹砂层剖面的土壤蒸发问题。结果表明，砂层会影响毛管水上升的高度和速度，毛管水到达土表的时间随砂层厚度的增加、层位的升高以及级配的变差而延长，但砂层级配对于水分的阻滞作用最大。砂层影响各层土壤含水量的分布，在水分达到平衡状态时，砂层以下土壤含水量大于均质土，而砂层以上土壤含水量则小于或近似于均质土。砂层能明显降低地下水的蒸发量，但夹砂层土柱的蒸发量随砂层厚度的增加而降低，而表砂层土柱的蒸发量则随砂层厚度的增加而增强。在蒸发阶段，砂层层位的作用表现更为显著。     

应用HYDRUS-1D模拟砂质夹层土壤入渗特性

依据非饱和土壤水分运动理论,采用HYDRUS-1D软件,对砂质夹层土壤入渗特性进行数值模拟,分析各因素对砂质夹层土壤入渗规律的影响。结果表明：砂质夹层结构对土壤入渗特性有较大影响,具有暂时的阻水和减渗作用;湿润锋穿过砂层上界面后,入渗过程变为稳渗阶段,稳渗率主要受砂层质地、砂层埋深和压力水头影响,与土壤初始含水率和砂层厚度无关;砂质夹层土壤剖面水分分布不连续,上层土壤基本饱和,砂层土壤未饱和,土壤剖面含水率主要受砂层质地、砂层埋深和砂层厚度的影响。研究结果可为农业水资源利用及工程防渗技术提供理论依据。     

A transfer-function method for analysing breakthrough data in the time domain of the transport process

A transfer‐function method is proposed to determine transport parameters from solute breakthrough data. The method is based on the assumptions that a linear process governs the transport of solute through soil and that the soil is homogeneous. It needs breakthrough data at two different vertical locations from a pulse input of solute to the soil. The method predicts the response by convoluting the input with the transfer function in the time domain. Solute breakthrough data were measured in unsaturated soil columns by time‐domain reflectometry (TDR). An experimental soil column was placed over a supporting column filled with sandy soil. A constant hanging water table, maintained in the lower column, created suction in the upper column and maintained unsaturated conditions. A solution of calcium chloride (CaCl₂) was spread over the soil in the upper column during steady flow of water in the column. Resident concentrations of solute in terms of electrical conductivity were measured at two depths by TDR sensors. We analysed breakthrough curves of CaCl₂ in 81 experiments to determine the transport parameters in coarse sand, sandy loam soil and clay loam soil by the transfer‐function method. The transport parameters obtained were compared with those determined by the widely used deterministic equilibrium model of the CXTFIT program. The transfer‐function method provided a better fit between the measured and estimated breakthrough curves in almost all cases and resulted in stable values of the parameters. The method is robust against small errors in measurements. It is a mathematically sound and efficient method for analysing breakthrough data.     

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.     

应用HYDRUS-1D模拟砂质夹层土壤入渗特性

依据非饱和土壤水分运动理论,采用HYDRUS-1D软件,对砂质夹层土壤入渗特性进行数值模拟,分析各因素对砂质夹层土壤入渗规律的影响。结果表明:砂质夹层结构对土壤入渗特性有较大影响,具有暂时的阻水和减渗作用;湿润锋穿过砂层上界面后,入渗过程变为稳渗阶段,稳渗率主要受砂层质地、砂层埋深和压力水头影响,与土壤初始含水率和砂层厚度无关;砂质夹层土壤剖面水分分布不连续,上层土壤基本饱和,砂层土壤未饱和,土壤剖面含水率主要受砂层质地、砂层埋深和砂层厚度的影响。研究结果可为农业水资源利用及工程防渗技术提供理论依据。     

Zur Bestimmung der ungesättigten Wasserleitfähigkeit im Felde

Remarks on the determination of the unsaturated hydraulic conductivity based on field measurements In a previous paper hydraulic conductivity — moisture tension functions have been evaluated by a field method for various layers of a fallow loess soil (grey brown podzolic soil). The aim of the paper presented here, is to demonstrate the magnitude of error when the hydraulic functions are calculated in the presence of transpiring plants, as it was done elsewhere. The error arises, because the water uptake rates of roots are added to the capillary water flux through the soil matrix. From the present investigation it becomes clear that with advancing growing stages of winter wheat and the increasing water extraction ability the error becomes more and more pronounced. The conductivity values may then be overestimated by a factor of half the power of ten as compared to the results computed from the fallow soil.     

层状土中毛管水上升的实验研究

土壤盐渍化的重要机制是地下水中可溶盐在蒸发作用下不断向表土聚集;次生盐渍土的形成则可能是由于人为抬高地下水位,土壤毛管水重新分配土中盐分所致.不管怎样,防止土壤盐渍化的积极而彻底的方法是消除地下水位抬高的因素.但是,全面而根本的措施,往往限于客观条件不易做到,因此大多数都采取治标措施.例如,利用耕耘增加毛管水上升的困难,以及洪淤压碱等等.     

夹砂层状土壤潜水蒸发特性及计算模型

针对西北地区农田土壤常见的砂土夹层土壤结构，通过室内土柱实验，研究了浅层地下水埋深条件下夹砂层状土壤的潜水蒸发特性。研究结果表明：砂层对水分蒸发既有促进也有抑制作用，相同厚度时潜水蒸发强度随砂层层位的升高以及级配的变差而降低。以砂层的相对层位和相对有效粒径作为砂层的量化指标，建立了适用于不同层位和质地夹砂层土壤稳定蒸发强度的修正模型，并对其进行了验证。该实验为定量研究夹砂层状土壤潜水蒸发提供了新思路。     

Effect of reduced air pressure on soil thermal conductivity over a wide range of water content and temperature

To clarify the role of air molecules in coupled heat and mass transfer in soil, we measured the thermal conductivity of three kinds of soil (Ando soil, Red Yellow soil, and Toyoura sand) under reduced air pressure over a wide range of water content and temperature (10–75°C). The thermal conductivity increased sharply under reduced air pressure above a critical water content of the soil, becoming several times larger than that under normal pressure (101 kPa). The maximum thermal conductivity for each soil was obtained below 75°C and was similar to the thermal conductivity of some metals such as Mn, Hg and stainless steel. When the soil was drier than its critical water content, the thermal conductivity did not increase under reduced air pressure. The hydraulic diffusivity at the critical water content for each soil was of the order of 10^?8 m² s^?1. This suggests that the latent heat transfer is enhanced by the circulation of the condensed water. However, very little is known about the effect of circulating water on the latent heat transfer under reduced air pressure. To make this clear, the thermal conductivity would need to be measured in the steady state under reduced air pressure.     

科尔沁沙丘-草甸相间地区表土饱和导水率的土壤传递函数研究

以科尔沁沙地典型坨-甸相间地区为研究区,野外布设240个采样点,对流动沙丘、半固定沙丘、固定沙丘、沙丘区杨树林、沙丘区耕地、低覆盖度草甸、高覆盖度草甸、草甸区耕地、撂荒地9种地貌类型下的表层土壤进行了采样,测定了其含水率、干容重、有机质、饱和导水率等理化特性,分析了不同地貌类型下表层土壤理化参数差异。选取Campbell、Cosby、Wosten等、Saxton等4种土壤饱和导水率传递函数,对该地区表土饱和导水率进行了预测。结果显示这几种土壤传递函数预测值与实测值偏差较大,相关系数均小于0.3,精度难以满足本地区应用。在此基础上,选取土壤容重、有机质含量、饱和含水率、平均粒径、粒径标准偏差5种土壤特性参数作为输入变量,采用主成分分析与非线性回归分析相结合的方法,重新建立了预测本地区表土饱和导水率的土壤传递函数,结果显示预测值与实测值相关系数为0.661,该传递函数可用于科尔沁沙地表层土壤饱和导水率的预测。