利用热脉冲技术测量土壤表层含水率

准确测定表层土壤水分对陆地-大气间水热交换研究具有重要意义。由于对土壤结构影响轻微,热脉冲技术在原位监测含水率方面具有较大优越性,但目前田间应用集中在5 cm以下土层。该研究利用多针热脉冲传感器测定土壤容积热容量,然后基于热脉冲含水率法和热脉冲含水率变化法分别得到了3、9、21和39 mm的土壤含水率。结果表明,与烘干法含水率比较,热脉冲含水率变化法含水率在4个深度的均方根误差分别为0.022、0.006、0.004和0.006 m3/m3,均小于相应深度上热脉冲含水率法含水率的均方根误差。另外,热脉冲含水率变化法也降低了4个热脉冲传感器测定含水率的变异性。因此,热脉冲技术能够监测表层的土壤水分动态,表层土壤含水率的均方根误差在0.022 m3/m3以内。     

Influence of gravimetric water content and bulk density on the dielectric properties of soil

Moisture content and bulk density largely characterize physical and mechanical soil status and behaviour. A nondestructive determination of these soil properties is essential. Time domain reflectometry (TDR), although widely accepted for determination of volumetric water content, θ, has its limitations, and recently a frequency domain (FD) sensor has been developed and tested. An equation relating relative permittivity, ?′, to gravimetric water content, w, and bulk density, p, was established for three soil types (sand, sandy loam and clay). If ?′ and w are known, our model can be used to calculate bulk density and associated volumetric water content, θ, keeping in mind that θ= pw. Utilization is found in long-term monitoring of moisture fluctuations or short-term detection of traffic-induced soil compaction.     

超声波土壤含水量检测装置的模型建立与验证

为探究利用超声波脉冲速度检测土壤体积含水量的可行性,以广东省红壤、赤红壤、水稻土为研究对象,设计了一种超声波土壤含水量检测装置,并利用ZBL-U510型非金属超声波检测仪在3种不同温度环境下(10、20、30℃)对不同含水量的土壤样本进行声速测定,构建了土壤体积含水量与超声波差值声速的温度效应数学模型。结果表明:超声波在水稻土中的传播速度比红壤、赤红壤快,且温度对超声波声速随土壤体积含水量变化节律的影响不同。20℃环境下超声波在土壤中的传播速度最快,10℃其次,30℃最慢。采用Richards模型表征土壤体积含水量与超声波差值声速关系的预测误差在3%左右,采用分段结构温度效应模型的预测误差在5%以内,证明该文提出的超声波脉冲速度-土壤体积含水量的温度效应模型可用于动态温度条件下的土壤含水量预测。该研究可为超声波技术在土壤水分检测领域的应用研究提供参考。     

Dual probe heat pulse technique for measuring soil water content and sunflower water uptake

Soil water content is the most sought-after soil physical parameter. Recent experiments have shown that dual probe heat pulse (DPHP) sensors can be used to determine volumetric water content of soil without roots. Little work has been done to document the performance of DPHP sensors in the presence of roots, and no work has been done with a taprooted plant. Thus, the objective of this experiment was to determine the accuracy of DPHP sensors in measuring volumetric water content (θ_v) and changes in volumetric soil water content (Δθ_v) in soil with a branched taproot system. Another objective was to determine plant water use. A sunflower plant (Helianthus annuus L. ‘Hysun 354') was grown in a column (0.20 m in diameter and height) with Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents; FAO-Eutric Fluvisols) containing 11 DPHP sensors. Results from the sensors were compared with those from the gravimetric method. Discrepancies between measurements of soil volumetric water content and changes of soil volumetric water by the DPHP and gravimetric methods were small (within 0.018 and 0.01 m³ m⁻³, respectively). The sunflower had a small amount of nocturnal transpiration, and roots took up water at a higher rate near the surface of the soil than at deeper depths. The results showed that the DPHP technique can monitor volumetric soil water content in the presence of a taproot.     

Measurement of volumetric water content by TDR in saline soils

Time-domain reflectometry (TDR) evaluates the bulk dielectric constant, K, of the soil by measuring the travel time of an electromagnetic pulse through a sensor, and through it estimates the volumetric water content. We show that for saline soils the effects of conductivity and frequency on the travel time cannot be neglected and that, as a result, TDR systematically overestimates the water content in saline soils. Simultaneously the bulk electrical conductivity of soils can be estimated by TDR. The equivalent impedance after multiple reflections is related to the bulk electrical conductivity, σ This relation differs from sensor to sensor and requires calibration for each individual sensor. A method is proposed for correcting the volumetric water content in saline soils. First, the bulk electrical conductivity, o, is estimated from the equivalent impedance at a specific equivalent distance of cable, several times the actual length of the sensor. The zero-salinity dielectric constant, K_O, of this soil is obtained by correcting the apparent K as a function of the measured bulk electrical conductivity. The volumetric water content is estimated from K_o. The correction of K is a function of the equivalent frequency of the electromagnetic pulse. The imaginary part of the dielectric constant is primarily due to ohmic losses. The model, which calculates the velocity of propagation of the electromagnetic pulse and which takes into consideration the imaginary part, performs reasonably well. An empirical approach based on calibration gave slightly better results.     

喀斯特地区洼地剖面土壤含水率的动态变化规律

本文基于连续2年土壤水分的定位监测数据,分析探讨了喀斯特地区不同地质背景(纯灰岩与白云质灰岩)洼地剖面(0～90 cm)土壤含水率的动态变化规律。结果表明:洼地剖面土壤含水率总体较高,且从表层到深层表现为增长型;2009年和2010年土壤含水率的变化均具有明显的分层现象,从上到下依次为活跃层、次活跃层、相对稳定层,但均无速变层,不同地质背景的具体分层略有差异;活跃层和次活跃层集中分布在浅层土壤层,相对稳定层较厚,对应着较差的水文调蓄功能,洼地土壤的水分调蓄功能可能会因其相对较深厚(80～100 cm)的土层而被高估。受降雨、蒸发及植物蒸腾等因素的影响,土壤储水量具有明显的动态变化特征,一年中可分为相对稳定期、消耗期和补给期3个阶段,而土壤水分亏缺的补偿和恢复,主要依靠强度适中、历时较长且雨量较大的降雨,微雨和暴雨的作用较小。     

A method for predicting soil susceptibility to the compaction of surface layers as a function of water content and bulk density

Identifying the vulnerability of soils to compaction damage is becoming an increasingly important issue when planning and performing farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require knowledge of the stress/strain relationship and of mechanical parameters and their variations as a function of different physical properties. Since soil compaction depends on the soil's water content, bulk density and texture, good understanding of the relations between them is essential to define suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 representative French soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties, easily measurable soil properties, water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remolded samples allowed us to examine a wide range of initial conditions with low measurement variability. Good linear regression was obtained between soil precompression stress, the compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil's capacity to bear greater stresses at higher initial water contents without severe compaction. Initial water content plays an important role in clayey and loamy soils. In contrast, for sandy soils, mechanical parameters were less dependent on initial water content but more related to initial bulk density. These pedotransfer functions are expected to hold for the soils of tilled surface layers, but further measurements on intact samples are needed to test their validity.     

Some problems associated with the use of the neutron probe in swelling/shrinkling clay soils

Comparison of neutron probe count rate ratios measured at 10 cm depth, with bulk soil water contents measured 3–5m away from the access tubes, confirmed that in swelling/ shrinking clay soils, recharge may be over estimated. This is due to the drying soil shrinking form the access tube, creating an air gap and radial cracks which allow preferential re-writing of the soil near the access tube. Installation of access tubes into dry soil was only a short-term answer.
Comparison of results obtained for bulk soil and for small colds shows that calibrating the neutron probe using small diameter cores may be lead to serious under estimates of changes in water content in shrinking clay soils.     

玻璃纤维水泥土集雨面物理性能的试验研究

为了提高集雨—灌溉农业中雨水收集的效益，开发经济、适用的材料用于建造集雨面非常重要。为确定新开发的一种集雨面材料—玻璃纤维水泥土在恶劣环境下（如冻融、干湿变化）其强度和抗渗透性的变化，在实验室内进行了控制条件下的性能试验研究。用玻璃纤维水泥土制成的集雨面立方体试块，研究了集雨面在干湿循环、冻融循环物理作用下的强度、抗渗性能和体积冻胀变形特性。定量确定了玻璃纤维水泥土的性能变化规律。结果表明，玻璃纤维水泥土集雨面的抗压强度在浸水饱和后比初始抗压强度降低26%,而在冻结融化和干燥后恢复并比初始抗压强度增加41     

Field calibration of an impedance soil water probe for the shallow seedbed across field topography

Impedance soil water probes enable frequent and non‐destructive determination of soil water status in situations where gravimetric soil sampling is too demanding of time and sampling space. The ThetaProbe is an impedance soil water probe requiring calibration for local soil conditions, because measurement accuracy can be affected by properties of the soil. Often, only a single calibration is performed for an experimental site. An experiment investigating the seedbed to 75‐mm depth across a field topography with variable soil properties was examined to determine which soil properties affected the calibration of the ThetaProbe, and if soil‐specific calibration was required to derive suitable estimates of the water status in the experiment. Experimental factors examined included hillslope aspect, hillslope position, crop residue and soil depth. Soil properties, other than volumetric water content, significantly affecting the probe measurements were bulk density, electrical conductivity and temperature. The probe underestimated soil water at very low water contents, and overestimated soil water at contents greater than 11 m³ m^?3, compared with gravimetric measurements. A single calibration, not corrected for hillslope position at a water content of 20 m³ m^?3, overestimated water content by 0.02 m³ m^?3 in the summit hillslope position and underestimated water content by 0.04 m³ m^?3 in the toeslope position. A single calibration, not corrected for soil depth at a water content of 20 m³ m^?3, overestimated water content by 0.02 m³ m^?3 in the 0‐ to 25‐mm soil layer and underestimated water content by 0.03 m³ m^?3 in the 50‐ to 75‐mm layer. The complexity of microsites in a shallow seedbed requires soil‐specific calibration in field experiments containing heterogeneous soil properties.     

Simplified method for quantifying the hydraulic properties of shrinking soils

In general, soils and their pore size systems are assumed to be rigid during the loss of water on drying. In reality, it is not the case for most soils, especially for soils with high quantities of clay or organic matter. As a result of shrinking, there are changes in the bulk density, the porosity, the pore size distribution, and the hydraulic properties of these soils. Currently, only a few methods enable the shrinkage behavior of soil samples to be determined while simultaneously quantifying the corresponding soil hydraulic properties. Either the methods need proprietary software for data processing, the equipment used is expensive or the calculation of the hydraulic properties is executed by inverse modelling. The aim of this study was to develop an alternative, simplified method for the simultaneous and automatic determination of the soil hydraulic properties, taking shrinkage into account. The HYPROP® evaporative device was combined with a circumference meter. A preliminary investigation found that the diameter of the cylindrical samples used for the HYPROP decreased linearly during evaporation from the bottom to the top. To sum up, recording the perimeter change in the middle position of the sample during drying‐out, together with the corresponding tension and water content, was sufficient to determine the hydraulic functions taking shrinkage into account. Measurements are presented for 6 samples which are different in texture and geological origin. The maximum shrinkage (19.5% by vol. between saturation and 5,000 hPa) was measured in the peat samples. The minimum shrinkage was quantified at 0.68% by vol. for the silty loam samples from Chile. The advantages of the method presented are: (1) the water retention curve and the hydraulic conductivity function can be determined simultaneously in the range between saturation and close to the wilting point, at a high resolution and taking into consideration shrinkage; (2) the method and device are simple and robust to use; (3) little time is required for measurement, between 3 and at most 10 d; (4) the functions are described over the whole tension range, using more than 100 user‐defined data points; (5) the evaluation of the volumetric soil water content measurement in shrinking soils is improved; and (6) common data models can be fitted to the hydraulic data as well as to the shrinkage data.     

Glycerol‐extraction refractometry for determination of gravimetric water content of soil samples

Abstract

We developed a simple and rapid method suitable for on‐site determination of gravimetric water content of soil samples. The proposed procedure was as follows; i) take 10–20 g of a soil sample in a tared mortar and weigh with a portable electronic balance, ii) add glycerol at a soil:glycerol ratio of 1 to 2 and weigh again, iii) mix the soil and glycerol thoroughly to ‘extract’ soil water into the added glycerol, iv) measure the refractive index of the ‘extract’ passed trough a filter placed on a refractometer, and vi) calculate the water content of the soil sample from a calibration curve relating the refractive index to water content of glycerol and the weights of the soil sample and glycerol. We tested the method on four soils at various moisture levels and found that the water contents from the proposed method were close to those from the standard oven‐drying method with slight downward deviations.     

Does transport of water to roots limit water uptake of field crops?

The question was examined under which conditions the water extraction rate of plant roots in the field can be limited by water transport to the roots. For this purpose we used a numerical solution of the single root model. Scenario calculations were carried out in order to investigate the general model behaviour. A sensitivity analysis showed that initial volumetric water content and root length density are of greater importance than root diameter in determining the maximum water transport rate to the roots. Data from a field experiment were taken, describing root length density, volumetric water content and water uptake rates under oats (Avena saliva L.) and faba beans (Vicia faba L.) as model input parameters. With this data the model calculated the water content difference between the bulk soil and the root surface which is necessary to induce a water flow to the roots matching the observed water uptake rate. Root length densities below the grain legume crop faba beans are one order of magnitude lower compared to that of the cereal crop oats. The therefore higher specific water influx rates of faba beans roots resulted in a higher decrease in water content near the root surface. However, water uptake by faba beans was controlled by the water flow towards the roots probably only in deeper soil layers with very low root length density. For the given conditions no transport limitation of water uptake was calculated, when rooting densities were higher than about 0.1 cm.cm^?3.     

The determination of the macroporosity of impregnated blocks of a clay soil and its relation to volumetric water content

The bulk density of slices cut from resin impregnated soil blocks was obtained by immersion in paraffin. Total porosity was then calculated using measured density of soil solids and the density of the resin. Macroporosity (> 350 μm) was obtained by image analysis and microporosity was derived as the difference between total porosity and macroporosity. On four separate sampling occasions throughout the year, there was a good agreement between values of microporosity obtained in this way and volumetric water contents measured at the time of sampling.     

Calibration of the time‐domain reflectometer and determination of the volumetric water content of the soil profile in an ultisol of Costa Rica

Abstract

An experiment was conducted to determine if time‐domain reflectometry (TDR) could be used to measure the water content at different depths in the O‐to‐75 cm soil layer. Probes of three wires (1/8 inch diameter and 30 cm exposed length) were installed in field plots differing in current crop‐fertilization history. Measurements of volumetric water content using bulk density and gravimetric water content were made to calibrate the TDR method. Comparison of water contents determined by TDR with those from gravimetric samples showed that there is a linear relationship (small offset but same slope) of water content with depth, indicating that there is little difference in volumetric water content from the 0 to 75 depth. However, the TDR method gives consistently lower water content values as compared with values obtained by gravimetric determination. Continuous measurements of profile soil water content with TDR in wet and dry periods during the year indicated that the mayor differences in volumetric water content correspond to the first 30 cm depth.     

不同土壤含水率、体积质量及光谱反射率的关系模型

为了准确、快速地检测湿胀干缩型土壤含水率,该文提出了基于近红外光谱技术的土壤含水率分析方法。该文以湖北省黄棕壤、潮土、水稻土为研究对象,利用美国Ocean Optic公司的NIR256-2.5微型光纤光谱仪在暗室环境下对不同含水率下的土壤样本进行光谱反射率的测定和特征分析,并同时测量相应的土壤体积质量,研究土壤含水率、体积质量、光谱反射率之间的相关关系,通过采用2种土壤含水率表示方法与3种土壤光谱反射率表示方法反映土壤含水率与光谱反射率之间关系的对比试验分析,消除土壤其它性状对土壤反射率反演土壤含水率的影响,得到较适宜地进行土壤光谱反射率反演土壤含水率的匹配表示方法,构建三者之间关系的曲面模型和体积质量变化的土壤体积含水率与土壤光谱反射率的关系指数模型。研究结果表明,构建的3种土壤的曲面回归模型,决定系数均大于0.977,F值均达极显著水平,光谱反射率与体积质量的偏回归系数检验亦达显著或极显著水平。采用指数模型表述1400、1900nm波长处的归一化减土光谱反射率与体积质量变化的土壤体积含水率的关系,其决定系数均在0.9以上,对模型进行验证,其预测误差在0.3左右,精度较高,所建模型拟合效果好。该研究可为用近红外光谱检测体积质量变化的土壤含水率提供科学资料。     

黄土丘陵区深层干化土壤中节水型修剪枣树生长及耗水

黄土丘陵区人工林地深层土壤干层是否影响后续植物的生长是众多学者关心的热点。该文在砍伐23 a生旱作山地苹果园地后休闲4 a又栽植枣树,连续3 a观测干化土壤中枣树的生长及土壤水分变化,研究采用节水型修剪的再植枣林的生长及耗水情况。结果表明,前期23 a生苹果园地已使0~1 000 cm深土壤干化,休闲4 a后0~300 cm土层水分得到恢复,300~500 cm范围为中度偏重亏缺,500~700 cm为中度亏缺,700~1 000 cm为轻度亏缺;3龄枣树时开始采取节水型修剪,0~300 cm土层有效水分被消耗34.97%,至4龄时0~300 cm范围内前期恢复的土壤水分已消耗殆尽;在此情况下采取节水型修剪的枣树仍可保持良好生长,产量及其水分利用效率均高于相同水分条件下的常规修剪枣树,产量可达正常水分条件下枣树的1.39倍以上,产量水分利用效率可达1.52倍以上。研究结果证明节水型修剪是半干旱区深层干化土壤中枣树克服雨量不足和土壤水分亏缺的一条有效途径。     

Maximum compactibility of Argentine soils from the Proctor test; The relationship with organic carbon and water content

Soil compaction is recognized as an increasingly challenging problem for the agricultural, horticultural and forest production in many climatic regions. The Proctor test provides a standardized method to study compactibility of disturbed soils over a range of soil water contents. The objectives of our study were: (a) to determine values of the critical water content for compaction and maximum bulk density from Proctor compaction curves for soils different in their properties; (b) to study the correlation between the maximum bulk density and readily available soil properties. Thirty soil samples were taken from six different locations in Argentina between 58 and 64°W and 34 and 38°S. The degree of saturation at maximum bulk density varied from 73.2 to 96.8%. Comparison of our data with data of two studies in USA showed that relationships between the maximum bulk density and the critical water content were similar to these studies. However, the slope of the relationship between the maximum bulk density and the organic carbon content was 50% less in our study as compared with the two others. The maximum bulk density was highly correlated with the organic carbon content and the silt content, the determination coefficient of the multiple linear regression, r², was 0.88.     

电介质型水分传感器测定栽培基质含水率的标定模型

土壤与基质的理化特性相差较大,土壤水分传感器测定基质含水率时有较大误差,不能直接用于基质含水率测定。为实现栽培基质水分快速检测,在不同配比的基质中采用电介质型EC-5土壤水分传感器进行了适应性测试。试验研究了温度、体积质量和电导率对传感器输出值的影响,采用多项式和线性回归处理方法,建立了基于温度、体积质量影响下的基质含水率标定模型。试验表明,经标定后,EC-5电介质型土壤水分传感器的测定含水率与实际含水率之间有较好的线性关系（R2＞0.9791）,且最大误差小于13%,因此,EC-5电介质型土壤水分传感器经标定后可作为基质的快速检测设备。     

The effect of bulk density, water content and soil type on the diffusion of chloride in soil

The relative importance of soil bulk density, water content and potential on the self-diffusion and impedance factors of ³⁶C1 in a sandy loam and loamy clay were studied. The soil bulk densities used represented a range of conditions from freshly tilled seedbeds to compacted soils. The volumetric water contents and pF were the main factors controlling the soil impedance factors, with bulk density making a small but significant contribution.
Soil type affects the impedance factors through differences in anion exclusion volumes, the water contents of poorly connected pores that contribute little to the diffusion process, and tortuosity of the diffusion pathways.