Design and Operability of a Large Weighable Fen Lysimeter

In principle, conventional lysimeters are suitable for the investigation of vertical water and solute fluxes. Lateral fluxes in water-saturated fen sites are characterized by heterogeneities and abnormities due to anisotropic layering. But due to lack of adequate monitoring techniques, these fluxes have been insufficiently analyzed. The newly developed large weighable fen lysimeter (LWFL) overcomes the limitations of conventional lysimetry and enables the measurement of vertical and horizontal transport processes in undisturbed large volume soil monoliths. The LWFL has a volume of 6 m³ (4 m length, 1 m width and 1.5 m depth) and was tested by filling the lysimeter with an undisturbed fen monolith. A special extraction procedure for the horizontal sliding of the lysimeter vessel through the natural fen was developed. In front of the vessel a converted cutting tool assisted in carving the soil monolith out of the peat, both vertically and horizontally. Inlet and outlet of the LWFL was constructed to allow the adjustment of a wide range of hydraulic gradients to depict natural occurring lateral transport processes. The LWFL including the measurement techniques was tested successfully for 3 years. On the basis of these tests, we conclude that complex physical and biogeochemical research problems involving lateral flows can be tackled now with multiphase observations and measurements at high spatial and temporal resolution, transdisciplinary data evaluation and numerical modelling approaches.     

Hydraulic and transport properties of the plant–soil system estimated by inverse modelling

Modelling soil water flow and solute transport under field conditions requires the knowledge of many parameters that are difficult to determine directly. Values determined on small isolated samples in the laboratory are often not representative of field situations. We investigated the applicability of inverse modelling to a soil–plant system in lysimeter experiments. We also tested whether parameters obtained from one experiment could be applied to another with the same soil. In a lysimeter planted with young trees, we first did a multistep drainage experiment and then a long‐term bromide tracer experiment with atmospheric boundary conditions at the soil surface. To estimate the unsaturated hydraulic properties, we linked the inverse program SUFI (Sequential Uncertainty FItting) to the flow and transport model HYDRUS5. A comparison of several scenarios showed that the resulting values of parameters depended strongly on the data used for calibration and the formulation of the objective function. The results suggested that inverse modelling could be used to identify important processes. Inversely obtained parameters gave better predictions for a second experiment when more variables were considered in the objective function and when the range of hydraulic conditions was wider. Furthermore, with retention curves directly fitted to measured water retention data we achieved acceptable results. Despite some limitations, the inverse approach was found to be a sound and useful procedure for estimating parameters of a complex system involving water uptake by roots, solute transport and unsaturated flow.     

A comparison of water flux measurements: passive wick‐samplers versus drainage lysimeters

Quantification of soil water flow is a prerequisite to accurate prediction of solute transfer within the unsaturated zone. The monitoring of these fluxes is challenging because the results are required to answer both scientific and practical questions regarding protection of groundwater, sustainable management of agricultural, forestry, mining or set‐aside industrial areas, reducing leachate loss from landfills or explaining the fate of environmentally harmful substances. Both indirect and direct methods exist for estimating water‐flux rates and have been used with varying success. In Europe, the use of direct lysimetry methods for measuring water and solute fluxes in soils has increased in recent years. This technique ensures reliable drainage data, but requires relatively large investment and maintenance expenses. Other research groups, especially in the USA, have developed alternative techniques. In this paper we compare the functioning of a passive‐wick sampler, especially the deep‐drainage meter type (DDM), with two different types of drainage lysimeters (weighing and non‐weighing) under field conditions in Germany for the measurement period from May 2004 until April 2009. The study showed that under sandy soil conditions no significant differences occurred between the measurements from DDM and both drainage lysimeter types. Only in periods with increased precipitation was there a tendency of drainage over‐estimation by the DDM in comparison with the lysimeters tested. For longer periods, no significant differences in the amount of drainage or the pattern of drainage formation were found between weighing and non‐weighing gravitation lysimeters. The practical use of DDMs is restricted because the groundwater level must be >2 m from the soil surface. Suggestions are made for the technical improvement of the DDM as well as the testing of the device with more cohesive soils.     

Water and Bromide Recovery in Wick and Pan Lysimeters under Conventional and Zero Tillage

Abstract

Quantifying in situ solute transport through soils and the landscape has been widely acknowledged as important and yet challenging. The objective of this study was to evaluate water and bromide movement in no‐tilled (NT) and conventionally tilled (CT) corn using two different types of in situ lysimeters—pan and capillary wick—for single rainfall events. Four zero‐tension pan and four capillary‐wick lysimeters were installed 1.2 m deep on opposite sides of four soil pits. Two were under NT corn, and two were under CT corn. Bromide (Br) was either surface applied or applied with an initial 25 to 27 mm of irrigation (33 to 34 g Br m²). A total of 120 to 147 mm of irrigation was applied continuously at 8.8 mm h^?1. Leachate was collected on 15 min intervals for 24 h and on greater intervals for up to 350 h. Lysimeter discharge and Br concentration were determined for each interval. After drainage began and until rainfall was discontinued, the water drainage rate was, on average, greater in NT (7.2 mm h^?1) than in CT (5.6 mm h^?1) based on results from the pan lysimeters. By contrast, the water drainage rate for the wick lysimeters was, on average, greater in CT (7.3 mm h^?1) than in NT (3.0 mm h^?1). The wick lysimeter appears to have behaved as a sink under the CT conditions, likely representing water flow in smaller channels. Under NT conditions, greater discharge observed with the pan lysimeter implicates the response from larger channels as the conduit for water flow. Flow‐weighted mean Br concentration was less when Br was applied on the soil surface (17.9 mg L^?1) than when Br was applied with the irrigation water (50.6 mg L^?1). Implications from preferential flow studies are often determined based on a single method of evaluation for solute transport, which are likely subject to the limitation of the method used. This study illustrates that contrary to the conventional understanding about preferential flow in NT, water flow and Br transport to the 1.2-m depth was as great as or greater with CT than with NT based on the results from the wick lysimeters for single rainfall events.     

The role of lysimeters in the development of our understanding of soil water and nutrient dynamics in ecosystems

This paper considers the development of lysimeters and their role in the evolution of our understanding of the dynamics of water and plant nutrients in ecosystems. Lysimeters are delineated volumes of soil. They can be divided into those filled with repacked soil, and those enclosing an undisturbed monolith. The original repacked lysimeter was developed to investigate the concept that all life stems from water, and is considered to be the first quantitative experiment in history. It focussed on the growth of a willow tree and how much of the increment was derived from the soil solids. From this start some 360 years ago lysimeters quickly contributed to the quantification of the transpiration stream and the differentiation of water loss by evaporation from the soil from loss via the leaves of plants. Chronologically, further development began about 210 years ago with the exploration of whether precipitation could account for all the water moving from the land to the oceans, and was the origin of springs. In part, this required a careful quantification of soil evaporation, runoff and deep drainage. This in turn led to the quantification of the soil water balance. As a result, we are able to predict indices, such as crop water use efficiency, drainage and irrigation requirements, contributions to stream flow, groundwater recharge and nutrient loss by leaching. Recognition that the quantification of drainage and leaching required soils of natural structure and profile integrity resulted in the building of the first monolith lysimeter and the development of ‘pan’ or ‘Ebermayer’ lysimeters. Improved technology allowed a better understanding of the role of soil in the regional water balance through the development of small diameter lysimeters that could be transported to a central location subject to the same climatic variables. In contrast, other technological changes allowed the impact of typical soil management operations carried out using regular machinery to be applied on field‐scale lysimeters. The contribution of the different types of lysimeter to the development of our understanding of soil use and management is considered.     

COLLECTION AND EVALUATION OF LARGE SOIL MONOLITHS FOR SOIL AND CROP STUDIES

A technique is described which allows collection and transportation of undisturbed soil monoliths in glass fibre casings 80 cm in diameter and 135 cm deep. The technique has been used to obtain 150 monoliths from a range of soil types in England and Wales, including soils with compact or chalky horizons. Measurements from lysimeters containing a non-swelling sandy loam and a swelling clay showed that the hydraulic properties of both soils were not affected by encasement of the profiles, provided that supplementary drainage outlets at the depth of field mole drains were provided in the lysimeters containing the clay soil. Aeration of the clay monoliths was comparable with that of the same soil in the field. When winter wheat plants growing on the lysimeters were surrounded by a similar guard crop, yields were equivalent to those obtained in the field. Edge effects were not significant; plants grown adjacent to the lysimeter wall yielded the same weight of grain per unit soil area as those in the central area of the monolith.     

Comparison of soil solution chemistry assessment using zero-tension lysimeters or centrifugation

The composition of soil solutions obtained from the field varies with the method of extraction. Variations in sampling methods and the difficulties in extracting representative samples from soils in space and time, can explain divergent results. In this study we compared soil solutions from a forest soil in northern Sweden obtained by a centrifuge drainage technique and by zero-tension monolith lysimeters. Zero-tension lysimeters were destructively sampled, and centrifuge solutions from this soil were compared with that from soil outside. In our study we found three major differences in the solute composition between the centrifugate and the lysimeter leachate: (i) larger concentrations of most solutes in the mor layer centrifugate than in the mor layer leachate, (ii) accumulation of nitrate in the lysimeters, and (iii) larger concentrations of base cations in the zero-tension lysimeters below 0.3 m depth. Water contents within the lysimeters were up to 3.5 times greater than under natural conditions and the water yields from the lysimeters indicate that water residence time ranged from < 1 to >5 years. This study shows that differences in results from the two methods are due to inherent differences in the methods themselves and not just to the collection of different soil waters. The hydrological anomaly and disturbance induced by the zero-tension lysimeters affects the solute chemistry and thus the applicability of the results to field conditions.     

Novel lysimeter techniques — a basis for the improved investigation of water,gas, and solute transport in soils

For many years lysimeters have been proven to be effective tools in assessing and predicting the effects of current land use and future land use changes in catchment areas on both water and solute balances. Although due to the diverse aspects of mass transport modelling, many different types of lysimeters exist, water and solute balances are by no means fully understood, especially in post‐mining areas. To tackle this problem, a new piece of equipment has been developed which enables the actual weight of a lysimeter to be precisely measured. The newly designed device, which has been used for one of the experiments described in this paper, permits the weighing of for example a 2 m³ lysimeter vessel with an accuracy down to 30 g. The second newly developed appliance presented here is the GAMS (Gas‐Migration‐Simulator). Basically comparable to a lysimeter, the difference is that the GAMS allows the detailed investigation of soil‐gas migration processes and their dependence on parameters like the diffusion coefficient and the gas permeability of the soil, alterations of the groundwater level and on various external influences such as changes of the actual meteorological conditions. These two newly developed techniques are described in this paper, and their respective suitability is demonstrated on the basis of data sets recorded during initial experiments.     

An experimental and numerical study on flow and transport in a field soil using zero-tension lysimeters and suction plates

Zero-tension lysimeters are widely applied to study the fate of chemicals in the subsurface environment. However, conditions in lysimeters differ from the field situation, because local saturation is required at the lower boundary to collect leachate. The objective was to characterize the influence of the lower boundary on the flow and transport behaviour of bromide observed in six 1.2-m-long lysimeters and in the field by 30 suction plates installed at 1.2-m depth, which were operated with a time-variable suction equal to the ambient soil water potential. A bromide pulse was applied at the bare surface of a silty soil in autumn 1997 and monitored for 2.5 years. The mean leachate flux was 0.98 mm day⁻¹ for the lysimeters versus 0.66 mm day⁻¹ for the suction plates. The lysimeters had a slightly slower effective mean pore-water velocity, expressed as transport distance per unit of leaching depth, and exhibited more solute spreading than the suction plates. Numerical simulations revealed that the amount of water collected with the suction plates was sensitive to the hydraulic conductivity of the plates. The spatial variability in hydraulic properties in the model explained the observed variability in cumulative leachate, at least qualitatively. The arrival time and spreading of the breakthrough curves (BTCs) were well described by the simulations in the lysimeters, but were underestimated in the suction plates. Preferential flow through macropores, which is not an effective carrier for bromide, might be the reason for this discrepancy. Molecular diffusion contributed significantly to solute spreading and enhanced lateral mixing. Both the experiments and the simulations revealed that the dispersivity derived from BTCs is significantly influenced by the observation method and experimental conditions.     

Modelling water and solute transport in macroporous soil. I. Model description and sensitivity analysis

A detailed mechanistic model of water movement and transport of non-reactive solute in a macroporous soil is described. One important feature of the model is that it may be run in either one or two flow domains using the same values for the hydraulic properties characterizing the soil. Water and solute movement in the micropores is calculated with the Richards and convection-dispersion equations and, in two domains, this is coupled to fluxes of water and solute in the macropores by empirical interaction terms. These interaction terms are redundant in the one-domain model, which simply reduces to the non-steady state convection-dispersion equation. A sensitivity analysis is presented showing how it is possible to identify conditions under which a macropore flow domain may need to be considered. In part II (Jarvis et al., 1991), the model is evaluated under field conditions in chloride breakthrough experiments in soil monolith lysimeters.     

Comparisons of methods for measuring the leaching of mineral nitrogen from arable land

Comparisons were made between 1988 and 1991 to evaluate three methods of estimating the leaching of mineral nitrogen (N) from unstructured freely draining sandy loam and loamy sand soils. The studies compared the drainage patterns and quantities of N (almost exclusively nitrate) leached from monolith lysimeters with those estimated from ceramic suction cups and soil core extracts. The latter two methods gave direct measurements of the mineral N concentrations in drainage, but required an estimate of the drainage volume calculated from meteorological observations and evapotranspiration equations to give total N leached. A bromide tracer was also used to confirm conclusions from nitrate leaching studies. There was a delay in the onset of drainage from free draining lysimeters because they lack the subsoil matric potential of field soils. However, total annual drainage measured by lysimeters or calculated from meteorological observations was similar, providing that return to field capacity was correctly identified in the field soil. During the first year there were discrepancies between methods which were attributed to soil disturbance during lysimeter and/or ceramic cup installation. In the second and third years of the experiment, estimates of N leaching losses using the lysimeters and ceramic cups were in good agreement. Nitrate concentrations in soil solution at a depth of 130 cm measured from soil core extracts were smaller than found by the other methods during the second year and the peak concentrations were significantly different (P<0.05). However, total overwinter N leached was not significantly different. Thus, while lysimeters and cups can be used to quantify leaching losses on unstructured, free draining soils if used correctly, the use of soil core extracts is questionable.     

Model calculations of water dynamics in lysimeters filled with granular industrial wastes

A three‐year long lysimeter experiment with a fine‐grained aluminum (Al) recycling by‐product and a mixture of this by‐product and a coal combustion waste was conducted. The wastes were proposed as possible soil substitutes in an engineered surface barrier covering a potash mining residue mount. To evaluate the suitability of the wastes as surface barrier material, their hydrological behavior under field conditions must be known. Lysimeter experiments provide one means to study the hydrological behavior of soils or soil‐like materials. However, it is difficult to estimate the long‐term hydrological behavior from short‐term lysimeter studies. The present study was conducted therefore to derive from short‐term lysimeter observations the long‐term hydrological behavior of the two waste materials. The lysimeter data were used to calibrate the one‐dimensional soil water flow model HYDRUS‐1D. With the calibrated model, hydrological simulations for the site of the residue mount were carried out for a period of 31 yr. Calculated long‐term annual seepage from the lysimeters was 237 mm for the pure Al waste and 186 mm for the mixture, or 39% and 24% of the average annual precipitation (764 mm). The average discharge of the bare mount is 482 mm or 63%. We conclude that a soil cover could considerably reduce the discharge and that the mixture is better suited as surface barrier than the pure Al waste.     

Development and practical test of a weighable groundwater lysimeter for floodplain sites

Presently, the soil water balance of flood‐influenced soils in fluvial plains is insufficiently described. The new development of a weighable groundwater lysimeter is the basis for recording the water‐balance components precipitation, evapotranspiration, groundwater recharge, capillary rise, and interaction with the water course. Soil‐hydrologic measuring setups at two floodplain sites of the Elbe river serve for direct comparability of lysimeter measurements with data obtained on site. A groundwater control was designed for lysimeters that automatically adjusts the current groundwater level at the floodplain measuring setups and quantifies inflow into or outflow from the lysimeter. It turned out that the lysimeter developed is capable of identifying the individual water‐balance quantities at high accuracy. Contrary to previous assumptions, it was possible to prove groundwater recharge for the floodplain sites.     

Modelling water and solute transport in macroporous soil. II. Chloride breakthrough under non-steady flow

A model of water and solute transport in macroporous soils (Jarvis et al., 1991) has been evaluated in column breakthrough experiments under field conditions. Hydraulic properties were first measured in replicate soil monolith lysimeters sampled from grass ley and continuous barley treatments in a clay soil. A pulse input of 0.05 M KCl was then supplied by drip irrigation and measurements made of the water discharge and chloride leaching resulting from the natural rainfall over a 1-month period. The results showed that the macropores constituted the dominant flow pathway (accounting for 80% of the total water outflow) and that diffusive exchange of chloride between the two flow domains was the main factor causing variability in leaching. Larger hydraulic conductivities and macroporosities in the lower topsoil and at plough depth in the grass ley monoliths were taken as evidence of structural amelioration. Less of the applied chloride was leached in the grass monoliths than in the barley (means of 20% and 31% respectively). This was mainly due to a smaller effective aggregate size and thus a more efficient diffusion-controlled retention.     

A semi‐analytical model for solute transport in layered dual‐porosity media

The vulnerability of groundwater from chemical leaching through soil is a concern at some locations. Because measurements are laborious, time‐consuming, and expensive, simulation models are frequently used to assess leaching risks. But the significance of simulated solute movement through a layered soil is questionable if vertical homogeneity of physical soil properties has been assumed. In the present study, a semi‐analytical model for solute leaching in soils is presented. The model is relatively simple, but it does account for soil layers having different physical properties. The model includes the mobile‐immobile model (MIM) to describe one‐dimensional (1‐D) nonequilibrium, transient solute transport under steady‐state flow conditions. The MIM is rewritten as a second‐order differential equation and solved by a numerical scheme. Differing from fully analytical or fully numerical solutions, the new approach solves the differential equation numerically with respect to time and analytically with respect to distance. Numerical experiments for a single layered soil profile show that the semi‐analytical solution (SA‐MIM) is numerically stable for a wide range of parameter values. The accuracy of SA‐MIM predictions is comparable to that of analytical solutions. Numerical experiments for a multilayered profile indicate that the model correctly predicts effluent curves from finite layered soil profiles under steady‐state flow conditions. The SA‐MIM simulations with typical parameter values suggest that neglecting vertical heterogeneity of flow paths in a layered soil can lead to inaccurate prediction of soil‐solute leaching. The quality of predictions is generally improved if parameter estimates for the different soil layers are considered. However, the mobile‐immobile‐parameter estimates obtained in a number of previous studies may not be transferable to a field situation that is characterized by a slow and steady flow of water. Further field experiments to determine mobile‐immobile parameters under such conditions are desirable.     

地面灌溉技术参数对氮素运移分布影响的研究进展

地面灌溉是应用最广泛的灌水方法，通过优化地面灌溉设计和管理减少水氮淋失，正在成为研究的热点问题，该文对有关成果进行了综述。由于地面灌溉的尺度较大，加之土壤水力参数和溶质运移参数的空间变异，使得地面灌溉施肥条件下溶质(N)运移分布的田间试验研究，无论在技术上还是在投入上都具有一定难度。有关沟灌、畦灌和水平畦田灌溉施肥时水氮分布的田间试验结果指出，肥料(N)分布没有入渗水深分布均匀，且与土壤初始含水率、流量、土壤入渗参数、田面糙率和施肥时机等因素密切相关。在模拟研究方面，文献中报道的模型有纯对流模型、对流-弥散模型、基于活塞流的水平衡模型和传递函数模型等。鉴于地面灌溉条件下水氮运移的复杂性，在水氮运移分布的数学模拟、灌水技术参数和施肥方式对水氮淋失的影响以及灌溉施肥质量评价指标体系等方面尚需要进一步研究。     

The solubility of aluminium in two Swedish acidified forest soils: An evaluation of lysimeter measurements using batch titration data

This paper presents aluminium (Al)-solubility data for two acid forest soils (Inceptisol and Spodosol), obtained in connection with lysimeter measurements (tension-cup and zero-tension lysimeters) and batch equilibrium experiments. The solubility of Al obtained in the batch experiments was used as a reference to test whether Al³⁺in soil solutions collected by the lysimeters was in equilibrium with secondary forms of solid-phase Al (Al(OH)₃or organically bound Al). The relation between pH and Al³⁺activity found for the zero-tension lysimeter solutions collected from the Inceptisol agreed well with that obtained in the batch experiment. This suggests that Al³⁺in the lysimeter solutions were in, or close to, equilibrium with the solid phase, whether this was organically bound Al (A horizon) or an Al(OH)₃phase (B horizon). For the tension-cup lysimeters, solutions obtained from the Inceptisol B and Spodosol Bs1 horizons were generally close to equilibrium with respect to secondary solid-phase Al (apparently Al(OH)₃; average ion activity product was 10^9.3and 10^8.8, respectively), whereas the Inceptisol A and Spodosol Bh solutions were not. The Al solubility in Inceptisol A and Spodosol Bh horizons was consistently higher than that obtained in the batch equilibrium experiment, indicating that the sampled solution partly originated from the underlying horizons. Thus, tension-cup lysimeters should be used with care in soils (or in parts of soil profiles) having steep solute concentration gradients because the soil volume from which the sample is drawn with this lysimeter type seems to be poorly defined.     

田间尺度下污染物在土壤中的迁移: 目前的认识和存在的问题

Agro-chemical transport processes at different scales are discussed and relevant opening question are identified by literature review to make some suggestions concerning the improvement of research methods for filed scale solute transport by aid of evaluation of existing models,and examining transport behaviors of solute in vadose zones on different scales.The results indicate that present research progess and understanding on field scale solute transport have not yet been enough to guarantee the use of our models for the management of field soulte movement.Much more research work needs to be done,particularly,in aspects of high resolution of spatial structures relevant to the hydraulic and transport properties,explicit numerical simulation of actual structure on field scale and field measurement corroborated with model development.