摘要:

Steady-state infiltration rates were measured at three instrument scales within a 4.0 x 4.0-m field plot. The three scales were 4.0 x 4.0 m (scale SL,n= 1), 1.0 x 1.0 m (scale SL/4,n= 16), and 0.25 x 0.25 m (scale SL/16,n= 256). After a 30-day ponding period under a constant hydraulic head, infiltration measurements were made at each measurement scale. Even though the entire area was sampled at each instrument scale, the average infiltration rate decreased with decreasing size of infiltrometer. The infiltration rate measurements were adjusted assuming an “apparent” stagnation zone of 4 cm along the boundaries of each infiltrometer. This adjustment produced average infiltration rates of 1.96 cm/h for each of the smaller measurement scales. This value was in excellent agreement with the final value measured at scale SLupon completion of the experiment (1.97 cm/h). However, the value for the “apparent” stagnation zone was found to be valid only in an average sense, because the correlation between the adjusted average value of the SL/16measurements and the adjusted value of the SL/4measurements was not significant. The 256 measurements at scale SL/16were neither normal nor lognormally (natural log (ln)) distributed, although the latter provided a somewhat better representation. Too few measurements were available to determine the probability distribution for scale SL/4. The regularized semivariogram (range of 0.7 m) for scale SL/16was decon-voluted to provide estimates of the point semivariograms for both the actual and adjusted measurement scales, leading to spatial ranges of 0.21 and 0.30 m for actual and adjusted, respectively. With or without adjustment, dispersion variance analyses illustrated that the spatial structure estimate obtained from measurement scale SL/16was inconsistent with that obtained from measurement scale SL/4. Thus, we conclude that the infiltrometer instrument fundamentally alters the infiltration process in such a way that measurements are only meaningful in a relative sense.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Simple deterministic models are useful tools for evaluating the relative environmental impact of chemicals. Analytical solutions that assume all soil parameters to be constant in time and depth have been used for screening purposes. In this study, two recently presented numerical models, the moving mean slope method for solving the unsteady water flow equations and the moving concentration slope method for solving the convective dispersion equation, were combined to calculate unsteady one-dimensional transport and volatilization of toxic organic compounds in soils. An expression for the combined gaseous and liquid flux (fluid flux) of chemicals was derived by integrating the fluid flux equation with respect to soil depth. The resulting model for evaluating the relative impact of chemicals at transient state (RIOCATS) is numerically simple, easy to program, and accurate if given criteria for time and depth increments are observed. Typical variations of soil parameters with depth and time, such as soil-water content, organic matter, and zone of microbial activity, resulted in a significant change in the ranking of volatilization of 18 different chemicals in a clay soil compared with a screening based on constant, average parameters. Therefore, the RIOCATS model appears to offer advantages for predicting the relative fate of toxic chemicals in soils.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Numerically induced dispersion is an important, but often ignored, source of calculation errors in transport simulations. General correction terms for removing numerical dispersion from the applied calculation schemes would be valuable in improving the accuracy of the simulation results before they are compared with measured soil data. In this study, a general transport equation for unsaturated water and solute transport is obtained by casting the one-dimensional Richards and convection-dispersion equations into general form. Correction terms for removing numerical dispersion from four commonly used finite difference (FD) calculation schemes used on the general transport equation are derived using Taylor series. The correction terms are given both in case of constant and variable depth increments. The derived terms are validated by method of moments analysis and tests against analytical solutions. The use of the correction terms in cases where the transport equations are extended with sink-/source terms is discussed. It is shown that a variable calculation grid should be chosen with care because the use of variable depth increments creates additional numerical dispersion and skewness and, in some cases, numerical oscillations in depth. The suggested procedure for deriving and validating correction terms for numerical dispersion can easily be extended to other FD schemes.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

This paper deals with transport of nutrients in soil by mass flow and diffusion towards plant roots. The root system is assumed to consist of uniformly distributed cylindrical vertical parallel roots, all taking up nutrients at the same constant rate. Each root thus can be thought to be surrounded by a separate soil cylinder. Steady-state conditions with respect to flow of water to the root are assumed. Two situations with respect to the steady-state water flow are distinguished: one where replenishment of the water taken up by the root takes place at the outer boundary of the soil cylinder, and one where replenishment takes place uniformly over the soil cylinder. Analytical solutions to the transport problem are derived. The constant uptake condition leads to concentration distributions converging eventually to a steady-rate solution, where the decrease in concentration is independent of time. With these, the period of unconstrained uptakeTu, i.e., the period during which transport in the soil allows the required uptake, is calculated. It is shown that transport by mass flow is more important the stronger the nutrients are adsorbed by the soil. The solutions for the two situations of steady-state water flow are shown to differ only slightly.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

This paper deals with a model for zero-sink uptake of nutrients by a plant root. Transport to the root is by diffusion and mass flow. A solution for the transport equation is derived where the initial distribution of the concentration around the root is that of steady-rate as was obtained in the first of this set of papers. Also, relatively simple approximations of the analytical solutions obtained are derived. It is shown that a good approximation to the uptake rate is proportional to the average concentration. With the help of these approximations, macroscopic uptake by a root system can be calculated.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Organic matter content is ascertained to be the major factor for alachlor adsorption in soils. Limited knowledge exists, however, as to the effects of structural and chemical properties of soil organic matter on the mechanisms and extent of alachlor adsorption. The binding mechanisms and adsorption isotherms for alachlor have been measured onto three humic acids (HA) originating from a municipal sewage sludge, a sludge-amended soil, and a control soil. Spectroscopic techniques, including Fourier-transform infrared, fluorescence and electron spin resonance analyses, and high-pressure liquid chromatography have been used. The magnitude of alachlor adsorption and the type of binding mechanism involved depended on the nature and chemical properties of the HA. Soil HA exhibited linear isotherms and higher alachlor adsorption at low concentration, whereas sludge humic acid featured an S-shaped isotherm and increasing adsorption at higher alachlor concentration. Multifunctional hydrogen bonds and charge-transfer bonds were preferentially involved in the adsorption of alachlor at low concentration, especially onto well humidified, highly aromatic soil humic acids rich in O-containing groups. Hydrophobic bonding appeared to predominate at higher alachlor concentration, especially onto low-humified, highly aliphatic sludge HA. A reliable quantitative evaluation of the extent of adsorption of alachlor to HA in the experimental range of concentration used could be provided by the use of both the Freundlich K-values and the distribution coefficients, Kd.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

In southwest France, thick humic acid soils have developed from Quaternary silty alluvial deposits. The objective of this study was to determine if specific patterns of carbon profiles occurred in these soils in order to evaluate total carbon storage using mathematical modeling and laboratory analysis on a small number of samples. Soil samples were collected from an oceanic zone of the French Pyrenean piedmont, ancient terraces of Pyrenean streams (southwest France), and from seven mature forests. The results demonstrated that these soils have accumulated large amounts of organic carbon. The pattern of C accumulation showed an exponential decrease with depth and appeared to be characteristic of these soils. This typical pattern allowed modeling of the carbon storage profile and reduction of the number of samples from 10 to 3 without modifying significantly the fitted model and the evaluation of the total amounts of C. This study also demonstrated that sampling to shallow depths fails to include a large proportion of the total C content in these soils. Therefore, studies restricted to shallow depths appear not to be relevant for studying the role of these soils in carbon storage. A regression predicting bulk density from organic carbon was used to calculate total C amounts down to a depth of 1 meter. The analysis used simple determinations of C concentration in three specific horizons.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID