摘要:

Simple one-parameter models for predicting changes in unsaturated soil hydraulic properties with changing soil-water content are useful, considering the great uncertainty and variability in the hydraulic parameters, and the models are often used for stochastic simulations of water and solute transport under field conditions. Two general one-parameter models for predicting relative hydraulic conductivity between water saturation and −350 cm H2O of soil-water potential are presented. The two new hydraulic conductivity models were labeled the Double Log Conductivity (DLC) and the Single Log Conductivity (SLC) models and represent modifications of the Campbell and the Libardi et al. conductivity models, respectively. The DLC and SLC model constants were optimized based on conductivity and retention data from a broad texture group of 40 sieved soils, but they can be calibrated easily to specific series of soils. The main parameter in both models (the Campbell soil-water retention parameter, b) can be estimated readily from soil texture or water retention data. Both models gave improved predictions of relative hydraulic conductivity in sieved soils compared with traditionally used one-parameter models. DLC and SLC model predictions were further compared with conductivity data from 10 undisturbed soils. The results suggested that the new models are also useful for predicting both relative and absolute hydraulic conductivity in undisturbed soil, but in this case, a calibration of the model constants based on a larger undisturbed soil data set is recommended.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

We present here a method for determining the hydraulic properties of unsaturated soil via inverse modeling. A modified cone penetrometer, called a cone permeameter, has been designed to inject water into the soil through a screen and to measure the progress of the wetting front with two tensiometer rings positioned above the screen. Cumulative inflow and pressure head readings are analyzed to obtain estimates of the hydraulic parameters describing the hydraulic conductivity curveK(h) and the soil-moisture characteristic curve, &thetas;(h). Two sets of cone permeameter tests, which differ in the method of placement in the soil, are discussed. In the first case, the cone permeameter was buried; in the second case, it was placed by direct push to the testing depth. Optimizations for various sets of unknown parameters, with and without additional soil moisture information, were performed. The effects of applied pressure head at the source and of initial pressure readings on the resulting hydraulic parameters are investigated. The optimization results are compared with the results of standard laboratory and field methods. The saturated hydraulic conductivity was well estimated. For cases in which the saturated moisture content was estimated or fixed near the laboratory-derived value, the soil-moisture characteristic curves were between the wetting and drying curves obtained from other standard methods.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Theoretical quantity/intensity (Q/I) analysis reveals that addition of a third cation (e.g., NH + [over] 4) to a heterovalent cation exchange system (e.g., K-Ca) under constant CEC and cation exchange selectivity coefficients decreases the apparent linear potential buffering capacity for K+(PBCK) whereas labile K+(ExK°) remains unaffected. However, ternary cation exchange reactions, K-NH4-Ca, in soils cannot always be predicted from binary exchange data. This study deals with Q/I relationships of K-Ca exchange, with and without added NH + [over] 4, and NH4-Ca exchange, with and without added K+in the Apand Bthorizons of a Greek soil (Typic Rhodoxeralf) amended with 0, 1%, 5%, and 10% of zeolite (clinoptilolite) on a weight basis. The experimental approach was based on the Q/I concept. In the case of K-Ca exchange in the Ap horizon, addition of solution NH + [over] 4 to soil samples with increasing quantities of zeolite increased PBCK, whereas in the case of NH4-Ca exchange, addition of solution K+or zeolite had no particular influence on PBCNH4. In the case of K-Ca exchange in the Bthorizon, solution NH + [over] 4, as well as zeolite, had no apparent consistent influence on PBCK, whereas in the case of solution K+and zeolite, PBCNH4increased significantly. Labile K+(ExK°) increased with addition of NH + [over] 4 and/or zeolite in K-Ca exchange, whereas ExNH4° behavior showed no particular pattern, perhaps because of the small quantity of residual ExNH4in the systems. The above demonstrated an ion-selective sieving effect by soil and zeolite that was not consistent with theoretical ternary cation exchange behavior.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The objective of this study was to provide a contribution to a database on the heavy metal status of tropical soils. We assessed the partitioning of Al, Cd, Cu, Fe, Mn, Pb, and Zn in A horizons of Costa Rican soils used for coffee cultivation. These soils are subject to heavy metal inputs as a result of the regular use of fungicides and fertilizers and, possibly, because of atmospheric depositions as well. A horizons of 16 soils in coffee plantations and two under natural forest (eight Oxisols, six Andisols, two Mollisols, two Alfisols) were sampled and analyzed for pH, ECEC, BS, Corg, Ntot, oxalate-extractable Al and Fe (Alo, Feo), dithionite-citrate-extractable Fe (Fed), and texture. Al, Cd, Cu, Fe, Mn, Pb, and Zn were extracted with a seven-step sequential extraction procedure. Because of advanced desilication, total Al and Fe concentrations were high in Mollisols, Alfisols, and particularly in Oxisols (45-134 and 21-115 g kg−1, respectively). In Andisols, total Al concentrations were high as a result of the occurrence of allophanes (up to 76 g kg−1). Cd, Pb, and Zn concentrations in all soils were low or comparable to background concentrations in temperate soils (Cd: 0.03-0.33; Pb: 4-57; Zn: 31-141 mg kg−1). Mn concentrations were highly variable (Mn: 156-4965 mg kg−1). Total Cu concentrations were high (generally 100-330 mg kg−1). Average metal partitioning was element-specific, and in Mollisols, Alfisols, and Oxisols it was similar to that in temperate soils, but it was markedly different in Andisols, which contained high EDTA- and oxalate-extractable proportions and low metal proportions that are extractable only with strong acids.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Sieving is a routine measurement that may provide additional information about soil physical properties such as particle shape. The usefulness of sieving for this purpose is restricted by limited empirical descriptions of the time dependence of the sieving process and the unclear influence of diluent particles. The purpose of this research was to (i) evaluate existing models to describe sieving curves, (ii) describe and evaluate a modified proportional model, and (iii) determine the effects of material mass, material type, and diluent particles on the sieving curves and model parameters. Glass beads and sand-sized particles were sieved, with the material passing being collected on a balance whose output was captured electronically. Five initial masses were used. Additional studies examined the influence of oversized particles (i.e., larger than the sieve opening) on the sieving process. Existing models were inadequate, but a modified proportional model gave superior fit, and its parameters (c and n) can be interpreted. The parameter n is insensitive to loading but is dependent on particle type and shape. Increasing oversized particles changes the time dependence of the sieving curves such that n decreases, showing a decreased effectiveness of passage at later times.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID