An amended functional leaching model applicable to structured soils. II. Model application

The performance of a functional leaching model (Hall, 1993) is compared with leaching data from two lysimeter experiments with soils of contrasting texture using sodium bromide and potassium chloride as the non-reactive solutes. The model parameters are optimized using the solute elution curves as standards and compared with the physical properties of the soil. A good match with the measured discharge of both water and solute was achieved for both soils using the moisture release characteristics to define the pore volume available for mobile and immobile water. The results indicate that preferential flow takes place through even coarse-textured soils but that there is negligible diffusive exchange of solute between water passing through the macropores and the rest of the soil.     

An amended functional leaching model applicable to structured soils. I. Model description

A functional model designed to simulate the transport of non-interactive solutes through macroporous soil is described. The concept of mobile and immobile water is used but the pore volume available for mobile water is partitioned to allow for flow through smaller water-filled pores and rapid preferential flow through larger macropores and fissures. The general performance of the model under steady infiltration following an injection of solute is presented. The sensitivity of the output to variations in the model parameters is also discussed. A second paper compares the model with bromide and chloride leaching data on two texturally contrasting soils.     

Simulation of water flow and solute transport in free-drainage lysimeters and field soils with heterogeneous structures

Lysimeters are valuable for studying the fate and transport of chemicals in soil. Large‐scale field lysimeters are used to assess pesticide behaviour and radionuclide transport, and are assumed to represent natural field conditions better than laboratory columns. Field lysimeters are usually characterized by a free‐draining lower boundary. As a result, the hydraulic gradient is disrupted, and leachate cannot be collected until the bottom of the lysimeter becomes saturated. We compared heterogeneously structured, free‐drainage lysimeters and field soils with respect to water flow and solute transport. Numerical simulations were carried out in a two‐dimensional heterogeneous sandy soil under unsaturated water flow conditions with the CHAIN_2D code. Three different soil structures (isotropic, horizontal, and vertical) were generated, and Miller–Miller similitude was used to scale the hydraulic properties of the soil. The results showed that ponding occurs at the bottom of the lysimeter for the three soil structures and that it occurred faster and was more pronounced with the vertical structure (preferential flow effect). Breakthrough curves of a conservative solute (bromide) showed that solutes are moving faster in the field than in the lysimeters. Fewer differences between lysimeters and field soils were found with the horizontal soil structure than with the isotropic and vertical structures.     

Aspects of transport processes in aggregated soils

The way in which water and solutes move in aggregated soils depends on the mode of saturation of the pore space that is made up of the micropore region within the aggregates and the macropores surrounding them. When both regions are saturated, a hydraulic-head gradient causes water to flow preferentially in the macropores with little flow within the aggregates, so that movement of solutes into or out of the aggregates is mainly by diffusion caused by the difference between the solute concentrations of the water in the two regions. When macropores full of water surround unsaturated aggregates, water is imbibed by the aggregates giving rise to convective movement of solutes with the moving water. When the macropores are empty, the aggregates become almost isolated so that redistribution of water and solutes occurs only within the aggregates with very little transport of water and solutes between them. The movement of water and solutes in the micropore region within the aggregates can be considered to behave as if in a continuum, and can be described by Darcy's law and the dispersion equation, with boundary conditions imposed by conditions in the macropores. These physical considerations of transport behaviour in aggregated soils can be used to give guidance on soil management practices concerning drainage and leaching.     

阿特拉津在饱和砂质壤土中非平衡运移的模拟

针对农药阿特拉津在稳定流场饱和砂质壤土中的运移 ,根据平衡与非平衡假设条件下对流—弥散方程数学模型的解析解 ,基于易混合置换实验获得的阿特拉津和示踪溶质Br- 的穿透曲线及批量平衡法求得的阻滞因子 ,应用CXTFIT 2 0软件 ,通过拟合土柱实验中溶质的出流浓度变化 ,估算了模型的有关参数 ,在此基础上模拟分析了实验土柱不同埋深处阿特拉津的出流浓度和累积淋溶量动态 ,结果表明 ,化学非平衡的两点模型对本文实验条件下阿特拉津运移的仿真具有较高的精度     

冬小麦/夏玉米轮作中NO3-N在土壤剖面的累积及移动

通过田间试验研究了冬小麦 /夏玉米轮作中NO- 3 N在土壤剖面的累积及移动 ,结果表明 ,尿素施入旱地土壤后 ,硝化作用一般在 7d之内完成 ,NH 4 N只在施肥后的短期内保持较高浓度 ,其它时期NH 4 N含量基本在 1～ 3mgkg- 1 范围内 ,土壤剖面不同层次NH 4 N一般也低于 4mgkg- 1 ,NH 4 N的含量不能反映土壤有效氮的水平。土壤剖面中的NO- 3 N随施氮量的增加而显著升高。在低施氮量条件下 (N <12 0kghm- 2 ) ,NO- 3 N主要在 0～ 40cm土层内移动 ,但当施氮量高于N 2 40kghm- 2 时 ,冬小麦季即有相当数量的氮移出 0～ 10 0cm土体。NO- 3 N在土体中的移动存在着很大的年际变化 ,在干旱年份 ,即使夏玉米季 ,NO- 3 N向深层移动的可能性也很小。试验年份中 ,除 1999年夏玉米季发生了较严重的气体损失以外 (该季节特别干旱 ) ,其余季节损失的肥料氮主要以NO- 3 N的形式在深层土壤剖面中累积 ,这在两个试验点的结果相当一致。     

Mobility of the herbicide pyrithiobac through intact soil columns

The relative mobility of pyrithiobac [sodium 2-chloro-6-(4, 6-dimethoxypyrimidin-2-ylthio)benzoate], a new herbicide used for postemergence control of broadleaf weeds in cotton (Gossypium hirsutum), was evaluated and compared against that of bromide (Br(-)) tracer on four soils representative of cotton-growing regions using intact soil columns under saturated flow conditions. Pyrithiobac breakthrough curves were asymmetrical in shape with significant tailing and displaced to the left of 1 pore volume in the Houston Black clay (fine, montmorillonitic, thermic Udic Pellustert), Orelia fine sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualfs), and Ships silty clay (very-fine, mixed, thermic Udic Chromustert) soils. Breakthrough of pyrithiobac in the Hidalgo sandy loam soil (fine-loamy, mixed, hyperthermic Typic Calciustoll) was delayed and more symmetrical, with peak pyrithiobac concentration reached after 1.2 pore volumes. The immobile pore water (IPW) fractions estimated from the Br(-) breakthrough curves ranged from 20 to 87% of total pore water. The IPW values demonstrated that soils with the greatest amount of IPW (Ships with IPW = 87.3%) exhibited the most rapid movement of pyrithiobac (peak concentration after 0.04 pore volume). The experimentally determined pyrithiobac breakthrough curves confirmed the high mobility of this herbicide in these alkaline and predominantly smectitic soils. These results indicate that pyrithiobac mobility was influenced by soil type and preferential flow processes when leached through intact soil columns.     

阿特拉津在饱和砂质壤土中非平衡运移的模拟

针对农药阿特拉津在稳定流场饱和砂质壤土中的运移 ,根据平衡与非平衡假设条件下对流—弥散方程数学模型的解析解 ,基于易混合置换实验获得的阿特拉津和示踪溶质Br- 的穿透曲线及批量平衡法求得的阻滞因子 ,应用CXTFIT 2 .0软件 ,通过拟合土柱实验中溶质的出流浓度变化 ,估算了模型的有关参数 ,在此基础上模拟分析了实验土柱不同埋深处阿特拉津的出流浓度和累积淋溶量动态 ,结果表明 ,化学非平衡的两点模型对本文实验条件下阿特拉津运移的仿真具有较高的精度     

A new method to quantify how water repellency compromises soils' filtering function

Soil water repellency (SWR) is known to lead to preferential flow and to degrade the soil's filtering efficiency. However, no method is available to quantify directly how SWR affects the transport of reactive solutes. We propose a new method for conducting solute transport experiments in water‐repellent soils. It involves sequentially applying two liquids, one water, the other a reference fully wetting liquid, namely aqueous ethanol, to the same intact soil core with air‐drying between liquids. We applied this approach to quantify the impact of SWR on the filtering of the herbicide 2,4‐Dichlorophenoxyacetic acid (2,4‐D) in two Andosols. In batch experiments conducted prior to the transport experiments, 2,4‐D sorption was not influenced by aqueous ethanol for one soil. However, sorption in the second soil followed the co‐solvency theory, which predicts decreasing sorption with increasing solvent fractions. Thus, sorption experiments are necessary to complement our new method. Breakthrough curves were characterized by preferential flow with large initial concentrations, tailing and a long prevalence of solutes remaining in the soil. In the soil in which 2,4‐D sorption was unaffected by aqueous ethanol, SWR increased 2,4‐D losses by four and 50 times in the first 5‐mm outflow compared with the 2,4‐D losses with water. After 50‐mm outflow, the 2,4‐D losses were similar for one core, but in the other core they were still about four times greater with water than with aqueous ethanol. This method to quantify the reduction of the soil's filtering efficiency by SWR is needed for assessing the increased risk of groundwater contamination by solutes exogenously applied to water‐repellent soils.     

Application of a two-dimensional model to simulate flow and transport in a macroporous agricultural soil with tile drains

It is essential that important field processes are taken into account to model water flow and chemical transport accurately in agricultural fields. Recent field studies indicate that transport through macropores can play a major role in the export of solutes and particulates from drained agricultural land into surface water. Non‐ideal drain behaviour may further modify the flow and transport. We extended an existing two‐dimensional flow and transport model for variably saturated soils (SWMS_2D) by adding a macropore domain and an additional Hooghoudt drain boundary condition. The Hooghoudt boundary condition accounts for an entrance head needed to initiate flow into the drains. This paper presents the application of the new model (M‐2D) to an agricultural field in Switzerland. To understand interactions between macropore flow and drains better we simulated water flow and bromide transport for four different field scenarios. We considered both collector drains only with an ideal drain boundary condition (with and without macropores) and collectors and laterals with a Hooghoudt boundary condition (also with and without macropores). For each scenario, inverse modelling was used to identify model parameters using 150 days of data on observed cumulative discharge, water table depth, and tracer concentration. The models were subsequently tested against a 390‐day validation data set. We found that the two additional components (macropore flow, drain entrance head) of the M‐2D model were essential to describe adequately the flow regime and the tracer transport data in the field.     

Macropore flow estimations under no-till and till systems

The processes associated with water movement through silt loam soils involve both the flow through macropores as preferential flow or macropore flow and flow through the micropore as matrix flow. Macropore and matrix flow components were separated from total flow by a hydrograph-separation technique which used the assumption of dual porosity and a tracer mass balance. A mixture of potassium bromide was applied through a rain simulator to four plots in northern Mississippi in two rain events at 12.7 mm/h lasting 5 and 3 h separated by 6 h. The plots were either tilled or no-tilled with drains installed by two methods at the surface of the fragipan. The magnitude of water and bromide (Br⁻) transported by macropore flow to a drain line were estimated and the resulting hydrographs provided an indication of the potential significance of macropore flow in transporting water and non-reactive chemicals through macropores to the shallow groundwater system. Matrix flow appears to contribute the majority of the water moving to the drains even during the early stages of the drain flow hydrographs. The no-till plots produced more macropore flow than the tilled plots, independent of how the drains were installed. Macropore flow in the drainage at any time was small as compared to the matrix flow; however it contributed a disproportionate amount of Br⁻ tracer. These data support the concept that models used to predict mass balances using only the matrix (Darcian) flow will underestimate those chemicals that move like bromide into the soil profile.     

Simulation of Water and Chloride Transport in Field Lysimeters

In a lysimeter study in the field with soil samples of a Fluvisol the transport of chloride was monitored under high frequency irrigation. The results of this investigation were used for verification of two computer models of different complexity. One was a transient state water and solute movement model the other a simplified steady state model. It was demonstrated that both models predicted nearly equally well the chloride concentrations in the drainage water. For many practical situations, the simple model may be all that is needed to predict the downward movement of solutes in field soils.     

Solute recycling by crops and leaching in a drained arable soil

Preferential flow, as it bypasses the soil matrix, can greatly enhance the leaching of chemicals. When a soil is drained there is the risk that such short‐circuiting results in more or less direct passage of polluting chemicals from the soil to the groundwater. If the groundwater table is shallow the chemicals could be transferred back into the surface soil by hydraulic lift through roots and subsequent release by exudation or from decaying plant residues and again become exposed to leaching by preferential flow, thus strongly enhancing the chance of export via the drains. We investigated the leaching of bromide in a tile‐drained arable field over 2 years of crop rotation. The site was a former wetland, artificially drained a century ago for agriculture. Bromide was applied over 1.6 ha at a dosage of 10 g Br per m² in August 1995 after the harvest of wheat. During the 2 years 18% of the applied bromide was exported via the drainage system, most of it in preferential flow events and more than half of it in a single winter storm 5 months after the application. Within 7 months 56% of the applied tracer was leached out of the main root zone into the groundwater. Subsequently the tracer re‐emerged in water taken up by sugar beet in the following season. The beet accumulated 50% of the initially applied bromide in their leaves and released it again after harvest when the leaves were left as green manure on the field. Our results show that this recycling of solutes to the topsoil can have an important influence on their leaching as the solutes are thus again exposed to preferential transport into drains in the course of preferential flow events.     

紫色土坡耕地土壤大孔隙流的定量评价

为阐明大孔隙丰富且孔径呈两极分化的紫色土坡耕地土壤大孔隙流的运移规律,通过室内土柱试验获取耕作层0~20 cm、非耕作层20~40 cm原状土柱和填装土柱的穿透曲线,分析饱和条件下土壤大孔隙流发生规律,并采用解析法CXTFIT软件拟合了水分优先运移参数,PFSP指标(大孔隙流引起的穿透曲线延展量与水动力弥散作用及两区作用引起的延展量的比值)定量评价土壤大孔隙流的贡献率。研究结果表明:1)以填装土柱水流为平衡基质流计算,耕作层0~20 cm原状土柱中大孔隙流的导水贡献率为66.2%~68.5%,而Br-累积淋出量占总淋出量的62.3%~66.1%。对于非耕作层20~40 cm,土壤大孔隙流导水贡献率为0.2%~1.7%,而Br-随大孔隙流运移的比例却达14.5%~20.5%。说明耕作层土壤中大孔隙流现象远比在非耕作层土壤中更为显著;2)PFSP值结果表明大孔隙流作用对穿透曲线延展量的贡献率最大,两区交换运移作用次之,水动力弥散作用的最小。即PFSP值越大,大孔隙流对总水流通量的贡献率越大。     

Estimating leaching losses from sub-surface drained soils

Abstract. Leaching losses of solutes can be calculated if two variables, the amount of water passing through the soil and the concentration of solute in that water (a flux concentration), are known. Two simple approaches, soil extraction and suction cup sampling, were used to estimate the concentration of solutes in the water moving through a silt loam soil. The results were compared with actual concentrations measured in the drainage water from a sub-surface (mole-pipe) drained soil.
Seasonal leaching losses were calculated as the sum of the products of estimated monthly drainage and the estimated average monthly solute concentration in the soil solution. These results were compared with the leaching losses measured in drainage water from the mole-pipe system. For non-reactive solutes such as bromide (an applied solute) and chloride (a resident solute), the suction cup data provided better estimates of the leaching losses than did the soil extraction data. The leaching losses calculated using volume-averaged soil solution concentrations (obtained by soil extraction) overestimated the loss for the resident solute, but under-estimated the loss for the surface-applied solute. On the other hand, the data for non-reactive solutes suggest that measurements on suction cup samples may be representative of the flux concentration of a solute during leaching. For nitrate, a biologically reactive solute, there was no clear pattern in the differences between the estimated and measured leaching losses. The flux-averaged concentration in the drainage water was about midway between those measured in the suction cup samples and in the soil solution.     

优先流问题研究及其科学意义

 优先流是近年来针对土壤水运动所提出的术语,它是一种较为常见的土壤水分运动形式,与土壤入渗、地表及地下水质密切相关。开展优先流研究,是土壤水运动机制研究由均质走向非均质领域的标志。分析优先流的基本涵义及特征,阐述大孔隙流、环绕流、管流、漏斗流、指流、沟槽流、短路流、部分置换流、地下强径流、非饱和重力流、异质流、摆动流及低洼再蓄满等优先流类型及其表征现象,论述优先流研究的科学意义,并指出了优先流研究存在的问题及未来发展方向。     

Bromidverlagerung an zwei gedränten Standorten in Schleswig-Holstein

Bromide transport at two tile- drained field sites Within comparative studies on solute movement in tile drained agricultural fields in Schleswig-Holstein a bromide field test was conducted at two field sites. The soil of site ‘Bokhorst’ shows both spatial and profile heterogeneity with significant clay and silt contents (loam) whereas site ‘Hohn’ had a homogeneous particle size distribution within the profile with sand being the dominating particle fraction (sand) but with neglible spatial variability. Rainfall amount, drain discharge and bromide concentrations in drain outflow were monitored over a 6 months period. A rapid response of drain discharge on rain events was observed for the loamy site but not for the sand. The simple water balance yielded + 8.7 mm of drain discharge for site Bokhorst (loam) and ?43 mm for the sandy site Hohn (sand). Largest bromide concentrations were detected shortly after chemical application at the loamy site. The bromide loss curve was characterized by simultaneous in- and decrease of flow rate and bromide concentrations. This solute behaviour was named ‘event-dependent’. Bromide occurred in drain outflow at the sandy site not before 30 mm discharge and maximum concentrations were observed at the end of the monitoring period. Mass balance calculations revealed that 70.5% (loam) and 33% (sand) of the applied solute mass were transported from the fields via the tile drains. A two dimensional, advection dispersion equation based simulation model was applied to predict solute behaviour at site Bokhorst. Computed bromide concentrations were not in line with the observed preferential breakthrough curve. Consequences of preferential transport conditions in tile-drained field soils on surface water quality are briefly discussed, especially concerning the displacement of plantprotective agents and methodical difficulties.     

Comparison of three models describing bromide transport affected by different soil structure types

This study was conducted to investigate the effects of soil structure on bromide (Br) transport through three soils with granular, prismatic, and single-grain structures. The breakthrough curve (BTC) of the single-grain structure was sigmoidal, symmetrical and similar to a piston flow, showing the dominance of mass flow. In contrast, the BTCs of the granular and prismatic structures were initially steep, becoming more gradual at high pore volumes (PVs). The stable structure and preferential pathways caused the early breakthrough of Br in the leachate of these columns. The convection–dispersion equation (CDE), mobile–immobile water (MIM), and dual-permeability (DP) models were fitted to observed data using the program HYDRUS-1D. The equilibrium transport model (CDE) was not as successful as non-equilibrium (MIM and DP) models in describing the Br transport in prismatic and granular soil columns, although it was able to describe the Br transport in single-grain column well. Overall, the results demonstrated the importance of soil structure in pollutant transport through soils.     

Short‐term transport of cadmium during a heavy‐rain event simulated by a dual‐continuum approach

The transport of solutes in soils, and its intensification due to preferential flow, plays crucial role when problems related to the groundwater pollution are dealt with. The objective of this study was to examine transport of cadmium (Cd) in response to an extreme rainfall event for three different soils using numerical modeling. The ^115mCd²⁺ concentration profile had been measured in the Bodiky reference site (Danubian Lowland, Slovakia) by the radioactive‐tracer technique and used for the calibration of the dual‐continuum model S1D. The Cd transport during a single rain event was predicted with the S1D model for light, medium‐heavy, and heavy soil in the same region. The Cd transport through the soil profile was simulated by the one‐dimensional dual‐permeability model, which assumes the existence of two pore domains: the soil‐matrix domain and the preferential‐flow domain. The model is based on Richards' equation for water flow and advection‐dispersion equation for solute transport. A modified batch technique enables to distinguish process of adsorption in the matrix domain and the preferential pathways. Modeling with classical single‐permeability approach and dual‐continuum approach without considering the particle‐facilitated transport led to negligible Cd penetration. The rainfall event with extremely high rainfall intensity induced deep penetration of Cd in the medium‐heavy and heavy soil, which may indicate increased vulnerability to shallow groundwater pollution for the respective sites in Danubian Lowland region. The highest Cd leaching was predicted for heavy clay soil, where the preferential particle‐facilitated transport of Cd through the soil profile was significant due to the contrasting properties of the soil‐matrix domain and the preferential‐flow domain. The results of the sensitivity analysis suggested only slight effect of the transfer rate coefficients on simulated Cd leaching.