Estimating the wetting branch of the soil water retention curve from grain-size fractions

The development of a theoretical method for estimating the wetting branch of the soil water retention curve (SWRC) is required for knowledge of the soil infiltration process. The aim of this study was to derive the theoretical functions to represent the wetting branch of the SWRC based on the Jensen method, and then compare the Jensen method and Kool & Parker (KP87) model for estimating the wetting branch of the SWRC. Fifteen soil samples with varying basic properties (e.g., grain-size distribution and bulk density (BD)) were selected from the Unsaturated Soil Hydraulic Database (UNSODA) to test these two methods. Results showed that the Jensen method (root mean squared error (RMSE) = 0.057 cm³ cm⁻³) produced a substantially better performance in predicting the wetting branch of the SWRC than the KP87 (RMSE = 0.089 cm³ cm⁻³) for the 15 samples. The range of the scaled mean bias error (SMBE) between the Jensen method-predicted and measured soil water contents at all pressure heads was −0.529 to 0.402. A positive linear relationship between the SMBE and silt content was observed for the Jensen method. The findings of this study have practical significance for simulating the soil infiltration in the unsaturated zone.     

Temporal Variability of Metal Contamination in Urban Road-deposited Sediment in Manchester, UK: Implications for Urban Pollution Monitoring

An important component of monitoring pollution of urban road-deposited sediment (RDS) is an understanding of the temporal variability in its composition and physical characteristics. This study set out to determine what the monthly variability in metal concentrations, organic matter content, grain-size and grain-size fraction metal-loadings are in inner city sites in Manchester, northwest England. The results show that there is significant temporal variability in metal (Pb, Zn, Fe, Mn) concentration in RDS from inner city Manchester. There was no significant temporal variability in grain-size characteristics or organic matter content, indicting that these metal variations were the result of variation in sources and accumulation processes. Pb and Zn displayed local variability, suggesting local controls on variability, whereas Fe and Mn displayed consistent variability across all sites, suggesting a common, larger-scale control on variability. The finest grain-size fraction (<63 μm) contained the highest Pb, Cu and Zn concentrations, but for the case of Fe and Mn, the coarser fractions (>300 μm) commonly contained the highest concentrations, again suggesting differing controls. For all metals, due to the weight percent dominance of the coarser fractions, the dominant loading of metals is in the coarser fractions. This has implications on management strategies, via street sweeping and the subsequent waste disposal, and on the modelling of the input of RDS and associated metals into surface waterways. The recognition of significant temporal variability of metal concentrations in RDS, independent of grain-size changes, implies that the monitoring of urban road sediment pollution will require not just consideration of spatial variability, but the design of schemes that will capture temporal variability also.     

末次间冰期-冰期旋回黄土环境敏感粒度组分的提取及意义

采用粒级-标准偏差方法对黄土高原西部、河西走廊及河南西部等6个剖面末次间冰期-冰期旋回的黄土-古土壤粒度进行了系统分析，提取了对环境变化敏感的粒度组分。各剖面的环境敏感粒度组分主要分为粗细两个组分，并显示出一定的区域性差异和不同的沉积学意义。敏感组分主要受物源和沉积后成壤作用的影响：冰沟和罗家湾剖面可能受源区的影响更大而使敏感组分偏粗，暖和湾和扣马剖面可能由于后期风化的影响较大而使敏感组分偏细。对扣马剖面末次冰期旋回堆积的L1和S1的粒级一标准偏差分析表明，L1的敏感组分与全剖面相似；而S1的敏感组分与全剖面存在较大差异，可能与成壤作用有关。     

Investigating the effects of wettability and pore size distribution on aggregate stability: the role of soil organic matter and the humic fraction

Soil organic matter (SOM) is an important factor influencing aggregate stability. Interactions between SOM and soil structure are widely studied, although the subtle relationship between SOM content, pore size distribution and aggregate stability is not fully known. Here we investigate such a relationship by means of a long‐term experiment established in 1962 in northeastern Italy, which considers different fertilizer practices (organic, mineral and mixed) applied to a continuous maize crop rotation. We measured wet stability of 1–2 mm aggregates subjected to different pretreatments. Both soil physical properties (such as pore size distribution and hydrophobicity) and chemical properties (soil organic and humic carbon content) affecting aggregate stability were considered. The chemical structure of humic substances was characterized by thermal and spectroscopic analyses (TG‐DTA, DRIFT and ¹H HR MAS NMR). The Pore‐Cor network model was then applied to evaluate the contribution of hydrophobicity and porosity to aggregate wetting. Our study suggests that SOM and its humic fraction can affect aggregate wetting and consequently slaking by modifying the pore size distribution with a shift from micropores (5–30 µm) and mesopores (30–75 µm) to ultramicropores (0.1–5 µm); hydrophobicity was also increased as a result of different humic composition. Spectroscopic analysis showed that hydrophobic compounds were mostly associated with complex humic molecules. Models of fast wetting dynamics, however, suggest that the contribution that hydrophobicity makes to aggregate stability, especially to soils with large carbon inputs, may not be the most significant factor.     

城市地表灰尘的分形特性分析

城市地表灰尘在城市环境中无处不在,其对人体和环境造成危害的程度与灰尘颗粒物的粒径有着直接关系.本文运用分形和多重分形的方法探讨了采样区地表灰尘粒径分布特征,发现城市地表灰尘颗粒分布较分散,但也存在颗粒分布相对集中且较粗的特点;地表灰尘粒径分布、pH值以及有机质含量受城市功能区划影响;熵维数D1和熵维数/容量维数D1/D0与城市地表灰尘的pH值成负相关,与有机质含量成正相关,分维数D与pH值及有机质含量之间无相关性.多重分形参数比简单的分维数更能表征地表灰尘颗粒的分布特征,能够提供更多的信息.     

Hysteresis in the capillary-sorption water potential as dependent on the soil water content

The capillary-sorption hysteresis within the entire range of the soil water contents has been studied in zonal soil types of the European part of Russia. The degree of hysteresis in the sorption and capillary-sorption ranges has been estimated. The main factors of the hysteresis are established. In the area of water sorption, this phenomenon is due to different wetting angles in the course of the soil wetting-drying cycles because of the heterogeneous hydrophilic and hydrophobic surfaces of the soil particles. In the area of capillary moistening, the pore-size distribution as a function of the soil macro-and microstructure is important, as this characteristic is closely related to shrinking and swelling processes.     

土壤初始含水率对膜孔灌湿润体特征的影响

为了研究膜孔灌中土壤初始含水率对湿润体特征及累积入渗量的影响,首先通过室内试验验证HYDRUS模拟西安粉壤土膜孔灌湿润体形状以及含水率分布的可靠性,然后基于HYDRUS模型模拟在不同初始含水率条件下膜孔灌湿润体的变化过程。结果表明:基于HYDRUS模型模拟的累积入渗量和湿润锋运移距离与室内试验结果的R~2均接近1,标准偏差绝对值均小于10%,拟合良好,表明HYDRUS模型模拟入渗过程的可靠性。膜孔灌湿润锋形状可采用椭圆方程表示。当初始含水率较小(不大于0.1 cm~3/cm~3)时,湿润体半径的含水率分布可采用椭圆方程表示;从膜孔中心到湿润锋表面,随着初始含水率的增大,湿润体内的含水率梯度减小,湿润体半径的含水率分布曲线由椭圆曲线逐渐转变为平缓曲线。基于湿润体含水率分布规律建立了考虑初始含水率的累积入渗量模型,累积入渗量与湿润体半径的三次方呈正比,湿润体半径可表示为湿润锋水平运移距离和垂向运移距离的几何平均值;对于不同的膜孔半径(1～5cm),模型计算累积入渗量与HYDRUS模拟值的R~2为0.99,标准偏差绝对值小于10%;对于粉土、粉壤土和壤土,当初始体积含水率不大于0.25 cm~3/cm~3算累积入渗量与HYDRUS模拟值的R~2为0.99,标准偏差绝对值小于10%,结果表明该模型对不同土壤质地和膜孔半径的适用性良好;该模型在计算作物灌水需求量方面优于Kostiacov模型等传统的经验模型。该研究揭示了不同初始含水率下的膜孔灌湿润体特征,并建立了累积入渗量模型,可为膜孔灌灌溉水量的计算提供参考依据。     

渭河咸阳段非饱和层状沉积物中水分分布特征

层状结构是河流相松散沉积物中最常见的沉积结构,通常表现出一些特有的水力学性质,对水分运动过程及沉积物的持水性能产生影响.本文通过对渭河陕西咸阳段河漫滩一个层状沉积物剖面的含水率分布特征进行原位观测,讨论层状沉积物中含水率的分布特征,及其与粒度和毛细壁垒效应的关系.结果表明:在以水平层理和交错层理为主的粗细相间的层状河流沉积物中,天然含水率也呈干润相间的层状分布特征,各层含水率的大小主要与粒度成负相关;沉积物粗细相间的层状分布为毛细壁垒的形成提供条件,在降雨入渗和土壤水蒸发过程中,毛细壁垒效应对水分运动的控制会导致细粒层中含水率高于粗粒层.层状松散沉积物中水分的这种分布规律,为理解土壤持水性能提供了新的思路,对研究土壤含水率与降雨入渗、蒸发等水文过程的关系具有重要意义.     

Organic matter and wetting capacity distribution in aggregates of Chilean soils

The wetting capacity of Hapludands and Palehumults under grassland and forest in southern Chile was assessed by measuring contact angles of solid, water and air interfaces. Microaggregates obtained from natural aggregates of two sizes (4–6.3 and 10–12 mm) were spread onto microscope slides and drops of saturated aqueous KCl solution were placed on the surface of the aggregates. The wetting angles of the drops were measured with a microscope with a horizontal viewing direction and an ocular containing a goniometer. The wetting capacity was determined on layers peeled progressively from the walls of aggregates toward their centres, and the organic matter content of the different aggregate layers was determined. The water stability of the aggregates was measured by dry and wet sieving. Hapludands always showed greater hydrophobicity than Palehumults. The wetting resistance of the soil matrix samples was less than that of the single layers of aggregates. The wetting resistance of aggregates generally increased from the exterior to their centre. The increase was strongest in the topsoil under grassland. Smaller aggregates under both grassland and forest showed a greater hydrophobicity than larger ones. Organic matter content increased towards the aggregate interiors, especially in the fine aggregates. Aggregates with greater water repellence were more stable to dry and wet sieving, especially under forest. Therefore, intensive management decreases wetting resistance and makes the soil less stable.     

深层坑渗灌不同开孔方式单点土壤入渗特性研究

为了探讨深层坑渗灌灌水器不同开孔方式对湿润体的影响,通过室内机理试验研究了砂壤土条件下底面开孔、侧面开孔以及底面和侧面同时开孔的3种灌水器的入渗特性,试验结果表明:开孔方式对累积入渗量,空间3个方向最大湿润锋运移距离以及土壤含水率均产生影响,采用幂函数拟合各方向湿润锋与时间的变化关系,在对3种灌水器条件下的湿润体形状分析基础上,建立了湿润体体积计算模型,验证了该模型精度。在实际应用过程中,对于根系分布较浅的作物,可选取侧面开孔灌水器;对根系分布较深的作物,则选择底部开孔的灌水器。研究结果为深层坑渗灌技术的发展奠定了基础。     

Organische Substanz und Benetzbarkeit von Bodenproben ‐ eine Interpretation

Organic matter and wetting properties of soil samples — an interpretation Samples were taken from A horizons of Udands and Humults at four 4 sites under forest and pasture vegetation in Southern Chile. Aggregates of 4—6 and 10—12 mm diameter were obtained by sieving and separated into aggregate surface and core fraction by peeling. The fraction 38—63 μm was separated by sieving. Organic matter was determined in all samples. Wetting angles were determined by direct reading at drops of saturated KCl solution on a layer of particles fixed on microscope glass slides by double‐sided adhesive tape. Values were correlated with contents of organic matter. The results showed an increase of wetting angles with increasing percentage of organic matter in the overall samples as well as in all separate groups (depth, vegetation, size, and parts of aggregates). The correlations, however, were not close (r ～ 0.5), irrespective of the significance level. Wetting angles in soils are caused by combined effect of different relative amounts and properties of free organic matter particles and organic films on mineral particles.     

Die Messung des Benetzungswinkels von Bodenfilmen nach einem Plattenkontaktverfahren

Measurement of wetting angles of soils using films grown on glas plates Glas plates (microscope mounts) were bedded into 5 soils (A_h-horizons) and incubated for 3 months. Wetting angles against water were measured with a special microscope on the films that developed on the mounts. Results showed higher values than those, obtained on films that were prepared by drying water- or NaOH-extracts on the mounts. Values were considerably lower however than those obtained at earlier measurements or reported by other investigators using indirect methods. The reason for this difference might be inadequacy of the indirect methods, different angle conditions (wetting or transient state) or low age of incubation films.     

Contact angle of soils as affected by depth, texture, and land management

Water repellency can be a significant factor in soil physical behaviour, but little is known about the depth dependence of the contact angle of field soils. We investigated contact angles and wetting properties as a function of depth for a wide range of agricultural and forest soils in Germany. The agricultural soils ranged from silty to sandy texture (six profiles), and the forest soils ranged from sandy to loamy texture (eight profiles). Contact angles (CA) were measured with the Wilhelmy plate method (WPM). In most of the soils, advancing WPM contact angles were considerably greater than 0° and they varied irregularly with depth. In general, sandy soils had larger WPM contact angles than silty soils. From the relation of the contact angle with texture and pH the quality of soil organic matter (SOM) was considered as more important for the wetting properties than the total amount of soil organic carbon (SOC). Finally, it was found that for soils with intermediate sand contents either under agricultural or forest use, the kind of land use seemed not to influence CA. Coarse‐textured sandy soils that were used only as forest sites were more hydrophobic than silty soils which were exclusively used as agricultural soils. We conclude that a coarse texture favours, in combination with other factors (mainly pH), hydrophobic SOM.     

泥石流堆积物粒度分维的讨论

在泥石流堆积物粒度分维原计算方法的基础上,改变研究对象和应用条件,进一步完善了计算方法,并对两种方法下求得的粒度分维进行比较.实例证明,利用改进后方法得到的粒度分维与泥石流堆积物土体的机械组成更相吻合,同时也反映了泥石流的搬运能力和破坏能力,体现了泥石流更多的特征.该研究是对泥石流堆积物土体机械组成和分形特征研究的补充,对泥石流堆积物特征和分形理论的研究都具有一定意义.     

Bimodal grain-size distribution of Chinese loess, and its palaeoclimatic implications

Grain-size analysis indicates that Chinese loess generally shows a bimodal distribution with a coarse and a fine component. The coarse component, comprising the main part of the loess, has pronounced kurtosis and is well sorted, which is interpreted to be the product of dust storms generated by low-altitude northwesterly winds. Its grain-size reflects the strength of the low-altitude circulation in the dust seasons of the year, and its percentage provides an indicator of the source area aridity and the frequency of dust storms. Conversely, the fine component has a wide grain-size range and is poorly sorted. Sedimentary illustrations based on the grain-size distribution characteristics of bulk samples and of detrital quartz suggest that the fine component probably represents the background dust load of the atmosphere and is mainly transported by high-altitude westerly airstreams. Its grain-size provides an estimate of the westerly air stream intensity. The coarse and fine components of a loess sample can be mathematically separated by fitting a designated mathematical distribution function to the measured grain-size data, and this procedure constitutes an approach for reconstructing the palaeowind system of Northern China.     

膜下滴灌土壤湿润区对田间棉花根系分布及植株生长的影响

滴灌条件下的土壤湿润区与作物根系分布和植株生长之间的关系是确定滴灌湿润比的理论依据。以大田膜下滴灌试验为基础,在灌水量相同的情况下,通过调控滴水流量得到不同的土壤湿润区,对不同土壤湿润区下的田间棉花根系分布及株高和叶面积生长均匀性进行了试验研究。结果表明,小的滴水流量形成窄深型土壤湿润区,其土壤水分水平分布均匀性较差,使田间棉花根系分布和植株生长均匀性低;而大的滴水流量形成宽浅型土壤湿润区,其土壤水分水平分布均匀性较好,使田间棉花根系的分布和植株生长均匀性较高;膜下滴灌条件下,宜采用宽浅型土壤湿润区。     

Slaking characteristics of some Australian and British soils

Aggregates (9.5–12.7 mm) from ten soils were equilibrated at a range of matric suctions (Ψ_a) between 1 kPa and 100 MPa before immersion in water or wetting on a porous plate at zero suction. The soils were from cultivated and grassland sites and included hardsetting and non-hardsetting Australian and British soils as well as a Vertisol. The initial rate of wetting of each aggregate, and the composition and size distribution of the slaked fragments were measured. There was a significant inverse linear relation between the amount of slaking produced by plate wetting air-dry soil (Ψ_a=100 MPa) and its organic carbon content (r= 0.82***). The three cultivated hardsetting soils shared several common features. Their slaking was the most pronounced after plate wetting and occurred at the smallest Ψ_a(10 kPa). Their slaking also increased linearly with rate of wetting and the particle-size distribution of their slaked fragments varied significantly and considerably with Ψ_a. This last observation demonstrates that it is not always helpful to call the fragments produced by slaking, microaggregates. Possible explanations for our results and their agricultural implications are discussed.     

Bestimmung der Benetzungseigenschaften von Moorbodenlösungen durch Kontaktwinkelmessung

Determination of the wetting characteristics of peat soil extracts by contact angle measurements A simple method for determination of contact angles of soil extracts and of coats of dried extracts is described. Results of the wetting characteristics of the single horizons of a high bog, an acid and a calcareous low bog are given. Influences of pH, salt content, degree of soil dryness and of degree of decomposition are existing.     

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

¹⁵NO^?₃ was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-¹⁵N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of ¹⁵N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized ¹⁵N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period.Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-¹⁵N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic ¹⁵N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic-¹⁵N of the silt and fine clay fractions.Drying and rewetting of soils hastened or magnified changes occurring in the organic-¹⁵N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions.The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.     

渭河咸阳段近代洪水沉积物粒度特征分析

粒度是沉积物中各种粗细颗粒的机械组成,可以用于洪水沉积环境和物质运动方式的判定.通过对渭河咸阳段河漫滩沉积状况野外调查和53个样品的粒度资料分析,研究了渭河咸阳段的洪水沉积物粒度特征,分析了粒度组成、粒度参数和粒度频率曲线等粒度特征资料.探讨了渭河咸阳段洪水变化和沉积环境.研究结果表明,渭河咸阳段河漫滩北堡子剖面近30 a来洪水沉积厚度为2.6m,主要粒度组成中砂粒级的含量最高,颗粒较粗,分选较差.根据粒度特征资料分析,渭河咸阳段河漫滩沉积物发育较好,变化明显,能够较清楚地指示渭河近30 a来发生的约13次洪水的变化.