THE MEASUREMENT OF SOIL-WATER HYSTERETIC RELATIONSHIPS ON A SOIL MONOLITH

Soil water properties of an undisturbed uniform soil monolith were studied under laboratory conditions. Water content, hydraulic conductivity and soil water suction were measured simultaneously on a full cycle starting from saturation, drying to a small water content and then wetting until saturation was reached again. The results showed clearly the hysteretic nature of the relations between water content and suction and between hydraulic conductivity and suction, but not between hydraulic conductivity and water content.     

SOIL SUCTION BY THE RAPID METHOD: AN APPARATUS WITH EXTENDED RANGE

In the rapid method of measuring soil suction the sample is placed on a porous plate attached to a capillary flow tube containing water. A suction is applied to the flow tube and rapidly adjusted to equal the suction of the soil, without change of soil-moisture content. A form of the apparatus is described which has provision for very convenient and rapid adjustment of the applied suction by the use of either a vacuum pump or an air piston. The range of the apparatus is extended from the usual limit of pF 3 up to pF 3.35 by applying air pressure to a chamber enclosing the sample, using a hand pump. Suction curves for a sand, a silty clay, and a heavy clay illustrate the application of the apparatus, which can readily be constructed in a form suitable for field use.     

Water-facilitated dispersal of inoculant Bradyrhizobium japonicum in soils

Summary Studies were performed to assess the influence of percolating water and an advancing wetting front on the transport of Bradyrhizobium japonicum in sand and silt loam soils, and to assess the influence of clay content on water-facilitated dispersal of these bacteria in a sand amended with various amounts of kaolin. The data obtained showed that movement of B. japonicum in soil was dependent upon water movement and that both percolating water and an advancing wetting front readily transported bacteria in coarse-textured soils. Percolation with the equivalent of 10 cm of rainfall dispersed B. japonicum throughout 40-cm columns containing sand and silt loam soils. Percolation with 5 cm of water was sufficient to disperse B. japonicum throughout 20-cm columns of these soils but did not transport these bacteria below the surface 4 cm of a sand amended with 12% kaolin. Our finding that cells of B. japonicum are readily transported by an advancing wetting front indicates that non-saturated flow of soil water contributes to dispersal of inoculum in soils.     

土壤初始含水率对优先流的影响简

土壤中优先流的存在会造成肥料利用率降低以及土壤深层甚至地下水污染.为确定土壤初始含水率对优先流的影响,通过室内含大孔隙土柱定水头入渗实验,以长武黑垆土及渭河砂土为研究对象,分析不同土壤初始含水率对优先流的影响.结果表明：1）相同质地,当土壤初始含水率大于其斥水性的峰值含水率时,湿润锋运移深度随着土壤初始含水率的增大而增大,反之湿润锋运移深度随着初始含水率的增加而减少,不同质地土壤湿润锋运移速度随土壤粉粒质量分数升高而减慢;2）累积入渗量受到土壤储水性及斥水性的双重影响,导致与湿润锋的运移趋势并不一致,黑垆土对照CK组及优先流O-C-30组（除7.04％含水率外）累积入渗量随着土壤初始含水率的增大而减小,渭河砂土CK组及O-C-30组均呈现2阶段特征,累积入渗量先随着土壤初始含水率的增加而减少,之后随土壤初始含水率的增加而增加;3）Kostiakov入渗模型能很好地拟合累积入渗量随时间的变化过程,适用于存在优先流的土壤入渗模拟.该研究结果对土壤优先流水分模拟具有一定的指导意义.     

Hysterese der Wasserspannungskurve in organogenen Böden

Soil water hysteresis in organic soils In 20 undisturbed samples of peat, muck, peaty soil and humic sand, the hysteresis of the moisture characteristic by soil suction from 0 to –9,8 kPa and from 0 to –29,4 kPa was measured. The greatest hysteresis was found in peat. Suction values from –1 to –3 kPa caused differences of 0,076 cm³/cm³ in water content during drying and wetting. In muck with low ash content and in humic sand the hysteresis was smaller. In peaty soil and muck with high ash content (> 40 %) it diminished to twice or three times lower values in comparison with those in peat. In organic soils the hysteresis decreases clearly by soil suction higher than –6 kPa. Repetition of drying and wetting gave a diminution (ca. 30 %) of the hysteresis loop in peat and displaces it towards the field of lower water content. The hysteresis of muck did not change considerably. The change of the porosity structure (pore ϕ > 30 μm) during the repetition of drying and wetting indicate that some soil shrinkage has occured.     

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.     

红壤积水入渗及土壤水分再分布规律室内模拟试验研究

利用室内模拟土柱试验研究了红壤积水入渗及土壤水分再分布规律。结果表明:入渗过程中湿润锋距离、累积入渗量与时间的平方根呈线性关系,湿润锋运移速率与时间呈乘幂关系,而入渗率与时间平方根的倒数呈线性关系;再分布过程中,在隔绝蒸发条件下,土壤含水量随土层深度的增加而下降,同一深度土层含水量则随时间推移而下降。无论在隔绝蒸发还是在自然蒸发条件下,短期内土壤水吸力有随土层深度的增加呈先下降后升高的趋势,而同一深度土层水吸力则随时间的推移而升高;土壤初始含水量对水分的入渗及再分布有较大影响,初始含水量较低时,水分入渗较快,而水分再分布则较慢。     

Soybean seedling root growth as influenced by soil texture,matric suction and bulk density

Abstract

Laboratory experiments were conducted under controlled conditions to determine the effect of five matric suctions (0.05, 0.10, 0.30, 1.00 and 3.00 bars) and three bulk densities (1.10, 1.30 and 1.50 g.cm^?3) on the moisture content, penetrometer resistance and soybean (Glycine max L.) root growth in six different soil textural groups (sand, silt, clay and their combinations).

The different textural groups were compacted in PVC pipes 4.4 cm ID and 10 cm long and placed in pressure cells to obtain the desired matric suction. After equilibrium five pregerminated soybean seedlings were fixed on the soil surface. At the end of 48 hours root elongation was measured.

There was an increase in root growth in all the textural groups at all the bulk densities when the matric suction was increased from 0.05 to 0.30 bar. There was however a gradual decrease in root growth as the matric suction increased from 0.30 to 3.0 bars. The reduction in root growth at low and high matric suctions was related to moisture content, change in soil resistance and redox status of the soil system.

The measured penetrometer resistance values were directly related to the level of compaction, soil matric suction and also were dependent upon the texture. Close relationships were recorded between redox potentials and soil matric suction.     

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.     

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。     

Einfluß physikalischer Eigenschaften und anthropogener Tätigkeit auf die Hysterese der Wasserspannungskurve organogener Böden

Influence of the soil properties and human activity on the hysteresis of the moisture retention function in organic soils The hysteresis of the moisture retention function is characterized by the greatest difference of water content during drying and wetting (Θ_AΘ_B) at the same suction. It diminishes by an increased degree of peat decomposition, ash content, bulk density and pH and increases with the volume of macropores (Ø > 50 μm). In the soil suction range Ψ = –1 to –6 kPa the hysteresis can be calculated by multiple regression equations (Tab. 3). Under arable land compared with grassland the hysteresis increases depending on the soil looseness and diminution of soil aggregates. A sand cover on peatland leads in the upper (20–30 cm) layer to a diminution of hysteresis proportional to the sand thickness.     

压砂条件下灰钙土地区农田土壤水分垂直入渗特征研究

为探讨压砂条件下灰钙土水分垂直入渗特征,采用土柱模拟法,分别研究了压砂厚度、砂石粒径、土壤初始含水率及灌水量,对压砂条件下水分累计入渗量和土壤湿润锋的影响.结果 表明:压砂条件下,土壤水分累计入渗量与入渗时间可以用正相关幂函数拟合.累计入渗量随着压砂厚度的增加呈增加趋势,入渗速率增加值的临界点是压砂厚度为15 cm;当...     

土壤初始含水率对涌泉根灌土壤水分及氮素运移特性的影响

为了探究涌泉根灌水肥一体化灌溉在不同土壤初始含水率下水氮运移特性,通过室内肥液入渗试验,研究了不同土壤初始含水率(4.13%,7.21%,8.77%,11.06%,14.01%)条件下入渗特性、湿润锋运移、土壤水分以及铵态氮和硝态氮的运移特性,建立了涌泉根灌累积入渗量、各向湿润锋运移距离与不同土壤初始含水率之间的关系,提出了不同初始含水率下涌泉根灌累积入渗量、各向湿润锋运移距离的经验模型。结果表明:累积入渗量、各向湿润锋运移距离以及湿润体内水分和氮素的分布、转化等均不同程度地受到土壤初始含水率的影响。同一时刻条件下,累积入渗量随着土壤初始含水率的增大而减小,而湿润锋运移距离却呈现出增大的趋势;土壤初始含水率越大,湿润体体积越大,湿润体内水分、铵态氮和硝态氮的分布范围越广泛;距离灌水器出水孔越近,土壤中的铵态氮和硝态氮含量越高。入渗系数K随着土壤初始含水率的增大而减小,入渗指数α随着土壤初始含水率的增大而增大;水平湿润锋拟合参数a、b均随土壤初始含水率的增大而增大,竖直向下湿润锋运移指数c随着土壤初始含水率的增大而增大,入渗指数d随着土壤初始含水率的增大而减小。随着土壤水分再分布的持续进行,湿润体内水分分布越加均匀,采用克里斯琴森均匀系数Cu评价灌水结束、再分布1,3天条件下湿润体内水分分布均匀度依次为61.99%,74.27%和83.60%;湿润体内铵态氮含量逐渐减小,但铵态氮的分布区域基本无变化;湿润体内硝态氮分布区域变大,平均值呈增大,最值区域有下移趋势。研究成果为进一步研究涌泉根灌水氮高效利用技术奠定了基础。     

非饱和土壤导水参数的推求

本文根据一维非饱和土壤水分的垂直入渗再分布和水平扩散再分布,采用土壤湿润剖面平均湿度和湿润锋湿度之间函数关系的三种形式,分别推导出非饱和土壤导水率,水分扩散率,比水容量的解析表达式,解析表达式中仅有四个独立参数,均可通过实验数据的简单拟合而得到.与其它方法相比,这种新的推求方法具有花费少、准确度高和测定范围大等特点.     

The effect of initial moisture content and rain (sprinkler) intensity on wetting front advance during rain (sprinkler) infiltration

A laboratory experiment was carried out on a 5 cm inside diameter by 70 cm long column of silty clay soil under five initial moisture contents, and five sprinkler intensities using a sprinkler simulator. A general equation was derived taking into account the initial moisture content and water application rate, to compute the wetting front advance during sprinkler infiltration. The experimental results strongly supported the derived equation, and the wetting front advance could be predicted under any combination of initial moisture content and sprinkler intensity. The study also revealed that the advance of wetting front increases either by increasing the sprinkler intensity or initial moisture content.The numerical solution of the partial differential equation of water flow offered by HANKS et al. (1969) was applied to compute the wetting front advance during the sprinkler infiltration. A good agreement was found between the two methods of computations; the derived equation and the differential equation. However, the solution of the partial differential equation needs very accurate estimation of K − Θ and ψ − Θ relation-ships to achieve reliable results.     

基于颗粒模型的风积砂层降雨入渗深度计算及验证

为了研究沙漠干旱区降雨对砂层非饱和带水分的贡献,该文对风积砂层水分入渗过程中的水分存在形式和运移机理进行了理论和力学分析。通过以触点水为主要储水单元的立方布局颗粒模型推导出水分入渗过程中湿润深度的解析表达式,并进行验证。物理和力学分析表明,砂颗粒表面很难形成较厚的薄膜水,砂层非饱和带水分大部分以触点水的形式存在。模型计算结果表明,水分入渗的湿润深度与入渗水量和触点水湿润角有关。模型验证表明,水分入渗湿润深度的解析表达式在风积砂层入渗深度计算中具有一定的适用性,但触点水湿润角与砂颗粒粒径的函数关系还有待进一步研究当设定触点水湿润角为π/4时,比例常数为11.5。该研究可为干旱区农业规划提供依据。     

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。     

THE PERCOLATION OF RAINWATER THROUGH SOIL DEMONSTRATED BY FLUORESCENT DYES

Twelve fluorescent dyes were tested in the laboratory for their stability under conditions of alternate wetting and drying, and persistence of fluorescence in the presence of an acid soil. The strongly fluorescing Pyranine cone, was most suitable for tracing water percolating through soils. In the field this dye could be detected in the soil after several months and couldbe photographed in ultraviolet light. The greater percolation of water against tree trunks and lateral movement in the humus layer were demonstrated.     

竖管地表滴灌与普通地表滴灌土壤水分运移特性对比试验研究

为探究竖管地表滴灌和普通地表滴灌土壤水分运动规律及区别,在室外同步进行2种滴灌模式下风沙土入渗和蒸发试验,对比分析了土壤水分分布、蒸发规律和土壤湿润锋运移特性。结果表明:(1)灌水量为2 L,2种滴灌模式下,随着滴头流量的增大,湿润体体积和灌水均匀度逐渐减小,湿润体含水率平均值逐渐增大,当滴头流量一定,竖管地表滴灌的湿润体体积大于普通地表滴灌,而灌水均匀系数小于普通地表滴灌;(2)不同滴头流量处理(0.3,0.4,0.6 L/h)蒸发7天结束后,普通地表滴灌土壤蒸发量分别占灌水量的32.5%,35.0%和40.0%,而竖管地表滴灌土壤蒸发量仅占灌水量的22.5%,说明竖管地表滴灌对土壤蒸发有明显的抑制作用;(3)相同灌水量(2 L)时,普通地表滴灌水平和垂直湿润锋运移距离均随滴头流量的增大而略有减小,竖管地表滴灌垂直向下湿润锋运移规律与普通地表滴灌相同,而水平和垂直向上方向运移规律相反;随着时间的延长,普通地表滴灌与竖管地表滴灌水平和垂直方向湿润锋比值均呈不断减小趋势,最后趋于稳定;(4)构建了包括滴头流量和灌水时间在内的普通地表滴灌湿润锋运移距离经验公式,验证所建经验公式的可靠性,均方根误差介于0.24~0.27 cm,纳什效率系数均大于0.985。研究结果可为竖管地表滴灌技术应用提供理论参考。     

Characteristics of Water Infiltration in Layered Water-Repellent Soils

Water-repellent(WR) soil greatly influences infiltration behavior. This research determined the impacts of WR levels of silt loam soil layer during infiltration. Three column scenarios were utilized, including homogeneous wettable silt loam or sand, silt loam over sand(silt loam/sand), and sand over silt loam(sand/silt loam). A 5-cm thick silt loam soil layer was placed either at the soil surface or 5 cm below the soil surface. The silt loam soil used had been treated to produce different WR levels, wettable, slightly WR, strongly WR, and severely WR. As the WR level increased from wettable to severely WR, the cumulative infiltration decreased. Traditional wetting front-related equations did not adequately describe the infiltration rate and time relationships for layered WR soils. The Kostiakov equation provided a good fit for the first infiltration stage. Average infiltration rates for wettable, slightly WR, strongly WR, and severely WR during the 2 nd infiltration stage were 0.126, 0.021, 0.002, and 0.001 mm min~(-1) for the silt loam/sand scenario,respectively, and 0.112, 0.003, 0.002, and 0.000 5 mm min~(-1) for the sand/silt loam scenario, respectively. Pseudo-saturation phenomena occurred when visually examining the wetting fronts and from the apparent changes in water content(?θ_(AP)) at the slightly WR,strongly WR, and severely WR levels for the silt loam/sand scenario. Much larger ?θAPvalues indicated the possible existence of finger flow. Delayed water penetration into the surface soil for the strongly WR level in the silt loam/sand scenario suggested negative water heads with infiltration times longer than 10 min. The silt loam/sand soil layers produced sharp transition zones of water content. The WR level of the silt loam soil layer had greater effects on infiltration than the layer position in the column.