Triaxial Testing of Unsaturated Agricultural Soils

In this paper, triaxial testing procedures and equipment as they have been applied to unsaturated soils are surveyed. Modifications to conventional triaxial apparatus and procedures for testing unsaturated soils based on principles of “unsaturated soil mechanics” are described in some detail. The modifications accommodate the independent measurement or control of pore-air and pore-water pressures and the resolution of both air and water components of volume change. This makes it possible to study the influence of soil suction on soil strength and volume change behaviour. To illustrate this approach, an agricultural soil was tested in a modified triaxial apparatus using independent stress state variables to describe stresses in the specimen. Matric suction, mean net stress and deviatoric stress were monitored or controlled in a series of constant water content tests. Volume change and shear strength behaviour were evaluated based on unsaturated soil mechanics principles. For low stress regimes, matric suction remained essentially constant under constant water content testing; however, as loading was increased matric suction varied significantly. The relevance of using matric suction as an independent stress state variable in fundamental studies of agricultural soil behaviour, as opposed to a total stress approach, is discussed.     

Estimating field-measured, plant extractable water from soil properties: beyond statistical models

For effective irrigation management we need to know the water storage capacity of the soil reservoir. Though plant extractable water is best measured in the field, sometimes it is useful to estimate it. Laboratory-derived retention curves do not necessarily reflect field conditions. Statistical models to estimate plant extractable water from other soil properties are restricted by assumptions that are difficult to check, and they can look very complicated. We propose to test a physical-based model that exploits the similarity between the particle size distribution curve and the soil water retention curve. A large data set of soil properties from the USA was used. Detailed particle size fraction data enabled the construction of simulated soil water retention curves for 388 samples. The physical-based model was compared against a statistical model that was derived from a subset of the data base. The statistical model fit the data better than the physical-based model. On the other hand, the statistical model overpredicted the soil water limits of those soils that were not used in the derivation of the statistical model. The strength of the physical model is that it represents a cause and effect relationship between particle size distribution and soil water retention. Also, it is conceptually simple and requires few inputs. The physical model may be improved by considering soil structure and type of clay.     

Transpirational response to water availability for winter wheat as affected by soil textures

Soil texture and evaporative demand have been reported to be the main factors which influence the transpirational response to soil water deficits. However, experimental evidences are not enough. The objective of this study was to investigate the transpirational response to soil water availability in soils of different textures under different evaporative demand levels. The three main soils of the Loess Plateau of China (loamy clay, clay loam and sandy loam) were selected and six constant soil water treatments were applied for winter wheat (Triticum aestivum L.) grown in pots. In order to reduce the influence of environmental conditions and plant factors, a normalized daily transpiration rate was used to develop the relationships with volumetric soil water content and soil water suction. Results showed that, under various levels of evaporative demand, a linear-plateau function with a critical value could be used to describe the dynamic change of the normalized transpiration rate with soil drying. Soil texture significantly influenced both the critical and the slope values of the linear-plateau equations, however, evaporative demand significantly affected the critical values of volumetric soil water content and soil suction for the loamy clay and clay loam only. Therefore, for saving water, different strategies are needed for these three soils.     

Crop root system response to irrigation

Summary In the field, root systems develop in response to both endogenous plant design and soil environment. Downward penetration of root systems results primarily from the growth of monocot axes or of dicot taproots; root proliferation at a given depth results from the growth of laterals at that depth. Root length densities generally decline exponentially with depth under well-watered conditions. Root growth rates are partially controlled by soil conditions. Under irrigation, the most critical soil properties for root growth are oxygen diffusion rate, water content and soil strength and all of these properties are inter-related. Under excess irrigation, especially in heavy soils, root growth may be limited by oxygen diffusion rate. Under limited irrigation, root growth may be limited by lack of water or high soil strength. When irrigation maintains wet surface soils, most of the root system is found in the upper part of the profile where the majority of the water is also taken up.     

南北方粘质土壤的水分特征研究

为了研究南北方粘质土壤的水分特征,选择北方塿土和南方红壤典型粘土,测定其持水特征和饱和导水率,采用双环入渗仪进行入渗试验。结果表明:耕作对红壤有较强的压实作用;从持水特征分析,同一吸力下红壤的含水量较高,田间持水量和凋萎系数也高于塿土,而有效水含量略低于塿土,饱和导水率却低于塿土1个数量级;从入渗特征分析,红壤的侧渗能力很强,入渗率比塿土高1个数量级,容易发生大孔隙流。该结果对南北方土壤水分运动的研究具有一定的参考价值。     

Grouping water storage properties of Indian soils for soil water balance model applications

Soil water balance models are useful tools of agricultural and water resources planning and management. Data of water storage limits of soil profiles, field capacity (FC) and permanent wilting point (PWP), are essential for running soil water balance models. A procedure for estimating these parameters for Indian soils is presented. The values of FC and PWP for each textural class are estimated independently for alluvial, black and red soils. The estimated parameters are tested by running a soil water balance model for a case study area. The use of the model in saving irrigation water is also demonstrated. The estimated parameters of Indian soils are also compared with corresponding values of US soils. The comparisons are favourable for FC for all soil texture groups, and for PWP for coarse to medium and medium texture groups. Differences in PWP between US and Indian soils increase at finer soil textures as a result of differences in clay fraction, mineralogy, and organic matter content.     

Soil-dependent wetting from trickle emitters: implications for system design and management

For trickle irrigation systems to deliver improved water- and nutrient-use efficiency, distance between emitters and emitter flow rates must be matched to the soil's wetting characteristics and the amount and timing of water to be supplied to the crop. Broad soil texture ranges (e.g. sand, loam, clay) are usually the only information related to soil wetting used in trickle system designs. In this study, dimensions of wetted soil were calculated from hydraulic properties of 29 soils covering a wide range of textures and soil hydraulic properties to assess the impact of soil texture and/or type on soil wetting patterns. The soils came from two groups that differed in the extent to which hydraulic properties depended on soil texture. Vertical and radial distances to the wetting front from both surface and buried emitters were calculated for conditions commonly associated with daily irrigation applications in a widely spaced row crop (sugarcane) and horticultural crops. In the first group of soils, which had least expression of field structure, the wetted volume became more spherical (i.e. the wetted radius increased relative to the depth of wetting below the emitter) with increasing clay content, as is commonly accepted. However, in the second group of soils in which field structure was preserved, there was no such relationship between wetted dimensions and texture. For example, five soils with the same texture had as great a variation in wetting pattern, as did all 11 soils in the first group, indicating the considerable impact of field structure on wetting patterns. The implications of the results for system design and management were illustrated by comparing current recommendations for trickle irrigation systems in coastal northeastern Australia with the calculated wetted dimensions. The results suggest that (1) emitter spacings recommended for sugarcane are generally too large to allow complete wetting between emitters, and (2) the depth of wetting may be greater than the active root zone for both sugarcane and small crops in many soils, resulting in losses of water and chemicals below the root zone. We conclude that texture is an unreliable predictor of wetting and there is no basis for adopting different dripper spacing in soils of different textures in the absence of site-specific information on soil wetting. Such information is crucial for the design of efficient trickle irrigation systems.Communicated by J. Annandale     

斥水土壤的水力参数及水平吸渗规律

为了对不同斥水程度土壤的水力性质进行分析,对比了van Genuchten和Brooks-Corey模型对于不同斥水程度下的塿土、砂姜黑土、盐碱土和砂土的适用性;进行了一维水平吸渗试验,分别运用Philip模型和Kostiakov公式对入渗规律进行了模拟,并分析了吸渗率和斥水持续时间的关系;采用水平吸渗法推求了土壤非饱和扩散率,并用指数函数拟合了非饱和扩散率和体积含水率的关系.结果表明:van Genuchten和Brooks-Corey模型对亲水和斥水土壤均具有较好的适用性;斥水性土壤的累积入渗量随时间变化曲线在一定时刻发生转折,未转折前Kostiakov公式的模拟结果比Philip模型好;当斥水时间大于40 s时,吸渗率的变化趋于稳定并在0~0.1 cm/min0.5内变化;非饱和扩散率和体积含水率关系的模拟可采用指数关系,且其对亲水性土壤的模拟效果优于斥水性土壤.斥水土壤的水力参数与亲水土壤的有明显差别,且表现出特殊性.     

The Influence of Soil Type,Soil Water and Share Sharpness of a Mouldboard Plough on Energy Consumption,Rate of Work and Tillage Quality

The results of field tests to investigate the influence of soil type and soil water on the wear of soil tillage tools are presented. The soil water had a positive effect for loam and clay soils because the wear decreased as the water content increased. For sandy soils however, wear increased with soil water. As the thickness of the cutting edge of the plough increased, draught force and fuel consumption increased considerably, while rate of work and tillage depth decreased. The quality of the tillage was seriously reduced, that is, the size of the soil clods increased as the thickness of the cutting edge of the plough increased. Also, the percentage of surface residues was greater as the thickness of the cutting edge of the plough increased. The best overall performance of the plough was obtained from the sharpest share cutting edge with a thickness of 1 mm.     

土壤斥水性与有机质质量分数的变化关系研究

土壤有机质质量分数是影响土壤斥水性的最主要因素之一,为研究二者之间的关系,选用以色列3类不同质地斥水性土壤为对象,分别在田间和实验室测定土壤斥水性和有机质质量分数,并绘制土壤斥水性和有机质质量分数的等值线图和关系图。结果表明,对于粘性土壤和沙质土壤,土壤斥水性随有机质质量分数成幂指数关系,但对于壤土,二者之间没有非常明显的线性关系。研究结果可为分析不同土壤的斥水性影响因素和土壤改良提供理论依据。     

水田土壤强度的模糊预测探索

列出了我国南方水稻产区１６种具有代表性的水田土壤及其特征参数,探讨了基于模糊数学原理由土壤含水量和机械组成来预测任一种水田水壤强度（内聚力系数和内摩擦因数）的可能性,得出了预测公式并给出了预测实例,结果表明,该方法具有一定的可行性。     

Treatment of spatially variable groundwater levels in one-dimensional stochastic unsaturated water-flow modelling

A stochastic lower-boundary condition for an unsaturated water-flow domain is presented, which can be used in the numerical solution of the unsaturated water-flow equation for soils with a shallow groundwater-table. The resulting stochastic model can be useful in studying the influence of spatially variable soil hydraulic properties and groundwater levels on soil water regime and plant transpiration. The lower-boundary condition employed assumes an exponential relation between the discharge of a watershed and the groundwater level at a specific location. Since it was found that groundwater levels were influenced by the presence of low-conductive clay in the soil profile, tubes from which groundwater levels were measured are divided into two classes. These classes were based on the presence of clay within the first 1.2 m of the soil profile. Scaling was then used to obtain a reference discharge-groundwater-level relationship for each class and for two years. This relationship in combination with its frequency distribution of the set of scale-factor values for each class and year can be used as the stochastic lower-boundary condition for an unsaturated water-flow model to simulate water transport in a soil with a shallow groundwater-table. An application of such a variable lower-boundary condition is given in two simulation examples.     

初始含水率对土壤垂直线源入渗特征的影响

为解决现有渗灌系统在深根系植物应用中存在的问题,该文设计了一种垂直线源灌水器,并通过室内土箱试验研究了不同初始含水率条件下砂黄土和塿土的垂直线源入渗特征。结果表明：Philip入渗公式的形式能够较好地描述2种土壤的三维入渗过程;入渗时间一定时,砂黄土和塿土的累积入渗量均与初始含水率之间呈二次函数递减关系;2种土壤入渗的最大径向距离、最大垂向距离、宽深比均与初始含水率之间呈二次函数抛物线关系;2种土壤相比,偏砂性的砂黄土累积入渗量在低初始含水率下较塿土大,但随着初始含水率的增加其减小速度较塿土快,偏黏重的塿土入渗临界初始含水率较砂黄土大,但其入渗范围较砂黄土小。这些结果可为不同土壤条件下的垂直线源灌水技术发展提供参考。     

气封闭情况下的非饱和土实用强度试验研究

考虑吸力的非饱和土研究较为精确,但吸力量测起来比较困难。以饱和度作为参数的非饱和土实用化研究更有利于工程的实际应用。根据非饱和土的分类,用饱和度作参数法对不同类型的非饱和土分别进行研究,将提高饱和度方法的精确性。本文针对气封闭情况下的非饱和土,对两种不同类型的土进行不固结不排水剪切试验,发现不同的土样表现出了相同的性质,即粘聚力和内摩擦角随饱和度的变化并没有呈现出相应的规律性,它们的变化主要是由于土的结构性所引起的,并从细观的角度进行分析,验证了试验结果。     

Water requirement for irrigated rice in a semi-arid region in West Africa

The purpose of this study was to determine the water requirement of rice (Oryza sativa L.) in the areas of the Office du Niger (Niono, Republic du Mali, West Africa). Average annual rainfall is 600 mm and evapotranspiration exceeds precipitation in every month except August. Alluvial soils vary in texture from heavy clay to sandy loam and have very low infiltration rates. The water use of rice was measured by establishing the water balance of four irrigation units varying in size from 12 to 26 ha. Evapotranspiration was evaluated with ‘non-weighing’ lysimeters and varied in 1980 from 5.3 mm/day shortly after flooding and 7.1 mm/day during heading to 4.1 mm/day at ripening. The crop coefficients found in this study do not always correspond to those presented by Doorenbos and Pruitt (1977). The crop coefficient depended strongly on the soil cover and was 1.0 for 10–70% soil cover and increased linearly to 1.25 for 100% soil cover. This relationship between soil cover and crop coefficient can be used to improve the estimates of rice evapotranspiration from large irrigation units with incomplete soil cover. The peak evapotranspiration may be as high as 7.8 mm/day during a 10-day period under full soil cover. The peak water requirement during the period of dry land preparation does not exceed 6 mm/day on level fields. The amount of water needed to saturate the soil profile after the dry season varied from 122 to 302 mm. A significant difference was measured between the pre-irrigation gift on a level field (175 mm) and on a non-level field (255 mm), which indicates the importance of levelling.     

滴灌条件下斥水土壤水盐运移试验研究

通过室内滴灌入渗的三维水盐运移试验,分析了斥水和亲水土壤在相同滴头流量下的湿润锋变化规律,研究了轻微斥水土壤的湿润锋随时间变化规律、含水率的三维空间分布特征以及不同斥水度土壤中含盐量和Cl-浓度分布特点。结果表明,在相同的滴头流量(0.07mL/min)下,亲水土壤与轻微斥水土壤的水平、垂直湿润锋与入渗时间具有良好的幂函数关系;亲水土壤与轻微斥水的土壤含盐量与距滴头距离符合良好的二阶多项式关系;虽然定量结果不同,但总体上轻微斥水的土壤在入渗中仍与亲水土壤具有相似的水分、盐分分布特征。在轻微斥水的农田中,滴灌灌水技术仍可为作物创造有利的水盐环境。     

Salt leaching and the effect of gypsum application in a saline-sodic soil

Field leaching studies in representative mixed Hyperthermic Salic Calciorthids showed that for 80% reduction in profile salinity, 0.4 cm leaching water/cm soil depth was required to be passed through the soil. This corresponded to 1.14 pore volume displacement. From four different predictive models, Burns' model, which is theoretically based and requires knowledge of simple parameters, was found to be highly predictive. Dieleman's empirical relationship also adequately described field leaching data. Desalinization of the profile was accompanied by simultaneous reduction in the SAR of the soil solution. When leached in the presence of gypsum, the SAR was reduced to a greater extent in lower depths which would otherwise have been achieved by passing additional quantities of water. Results have been discussed in view of the possible need or otherwise of amendments during reclamation of soils having excess neutral salts and a high SAR of soil solution (saline-sodic soils).     

Determining percolation losses of packed clay soil from tensiometer data

The measurement or prediction of percolation losses in field situations is of great practical significance for efficient irrigation and for determination of the leaching requirement, particularly of clayey soils where impeded percolation occurs. Hydraulic properties and water losses in packed Ashutia clay soil were determined under prevented-evaporation and free-evaporation conditions using lysimeter and tensiometric techniques. Hydraulic conductivity was determined as a function of soil moisture content using percolation flux computed. An exponential relationship between hydraulic conductivity and soil water content K = ae^bθ, was found. The percolation and evaporation-plus-percolation fluxes estimated from tensiometer readings under prevented-and free-evaporation conditions, respectively, matched with profile water losses from lysimeter measurements. The error ranged between 0.01 and 0.82 mm day⁻¹ with high correlation coefficient indicating that water loss from a soil profile can be estimated from tensiometer readings.     

Soil matric potential-based irrigation scheduling to rice (Oryza sativa)

About half of the total fresh water used for irrigation in Asia is used for rice production. Decreasing water resources and increasing water costs necessitates increasing water use efficiency for rice. The most common method of irrigation in northwestern India is through alternate wetting and drying with a fixed irrigation interval, irrespective of soil type and climatic demand resulting in over-irrigation or under-irrigation under different soil and weather situations. Soil matric potential may be an ideal criterion for irrigation, since variable atmospheric evaporativity, soil texture, cultural practices and water management affect rice irrigation water requirements. A 4-year field study was conducted to assess the feasibility of rice irrigation scheduling on the basis of soil matric potential and to determine the optimum matric potential so as to optimize irrigation water without any adverse effect on the yield. The treatments included scheduling irrigation to rice with tensiometers installed at 15–20 cm soil depth at five levels of soil matric suction viz. 80, 120, 160, 200 and 240±20 cm, in addition to the recommended practice of alternate wetting and drying with an interval of 2 days after complete infiltration of ponded water. The grain yield of rice remained unaffected up to soil moisture suction of 160±20 cm each year. Increasing soil matric suction to 200 and 240±20 cm decreased rice grain yield non-significantly by 0–7% and 2–15%, respectively, over different years compared to the recommended practice of the 2-day interval for scheduling irrigation. Irrigation at 160±20 cm soil matric suction helped save 30–35% irrigation water compared to that used with the 2-day interval irrigation. With a soil matric potential irrigation criterion the total amount of irrigation water used was a function of the number of rainy days and evaporation during the rice season.     

离心机法测定土壤水分特征曲线中的收缩特性

为了探明土壤在离心力作用下的收缩规律,开展了离心机法测定土壤水分特征曲线试验.砂壤土和黏壤土分别设定3个初始容重,以离心机法测定的数据为基础,研究了离心力变化下的土壤收缩规律,并通过van Genuchten-Mualem(VG-M)模型对2种情景模式(考虑容重变化和未考虑容重变化)下所测定的土壤水分特征曲线进行拟合,并以此估算所得的土壤水力特性参数对沟灌二维水分运动特性进行了数值模拟,同时结合室内试验对比分析了参数的合理性.结果表明,离心机转速增大,土壤含水率降低,容重随之增大,当吸力为7 000 cm时,砂壤土和黏壤土的容重分别近于1.81和1.79 g/cm³;基于土壤收缩特征曲线,供试土壤收缩过程可采用三直线模型进行表征,但各收缩段的吸力范围存在差异;与未考虑容重变化所得VG-M模型中的参数值相比,考虑土壤容重变化所得的滞留含水率θ_r和进气吸力值倒数a均增大,但形状系数n均减小;以考虑土壤容重变化所得VG-M参数为基础进行沟灌二维水分运动数值模拟,其入渗水量、湿润锋运移距离(垂直和水平)与实测值的误差绝对值均值分别为5.8%,3.0%和2.6%,较未考虑容重变化时精度分别提高了39.2%,57.2%和52.9%.因此离心机法测定土壤水分特征曲线的过程中需考虑土壤容重的变化,且以此获得的参数能够较为显著地提高数值模拟精度.