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Water Retention Characteristics (water + retention_characteristic)

Distribution by Scientific Domains

Earth and Environmental Science	75%

Selected Abstracts

Measurement of the size distribution of water-filled pores at different matric potentials by stray field nuclear magnetic resonance

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2005
N. R. A. Bird
Summary The water retention characteristic provides the traditional data set for the derivation of a soil's pore-size distribution. However, the technique employed to achieve this requires that assumptions be made about the way pores interconnect. We explore an alternative approach based on stray field nuclear magnetic resonance (STRAFI-NMR) to probe the water-filled pores of both saturated and unsaturated soils, which does not require information relating to pore connectivity. We report the relative size distributions of water-occupied pores in saturated and unsaturated samples of two sets of glass beads of known particle size, two sands, and three soils (a silty loam, a sandy loam and a loamy sand), using measurements of the NMR T1 proton relaxation time of water. The T1 values are linearly related to pore size and consequently measured T1 distributions provide a measure of the pore-size distribution. For both the sands and the glass beads at saturation the T1 distributions are unimodal, and the samples with small particle sizes show a shift to small T1 values indicating smaller voids relative to the samples with larger particles. Different matric potentials were used to reveal how the water-occupied pore-size distribution changes during drainage. These changes are inconsistent with, and demonstrate the inadequacies of, the commonly employed parallel-capillary tube model of a soil pore space. We find that not all pores of the same size drain at the same matric potential. Further, we observe that the T1 distribution is shifted to smaller values beyond the distribution at saturation. This shift is explained by a change in the weighted average of the relaxation rates as the proportion of water in the centre of water-filled pores decreases. This is evidence for the presence of pendular structures resulting from incomplete drainage of pores. For the soils the results are similar except that at saturation the T1 distributions are bimodal or asymmetrical, indicative of inter-aggregate and intra-aggregate pore spaces. We conclude that the NMR method provides a characterization of the water-filled pore space which complements that derived from the water retention characteristic and which can provide insight into the way pore connectivity impacts on drainage. [source]

A porous-matrix sensor to measure the matric potential of soil water in the field

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2007
W. R. Whalley
Summary The matric potential of soil water is probably the most useful assessment of soil water status. However, the water-filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to ,85 kPa. In this paper, we report the development of a porous-matrix sensor to measure matric potential in the approximate range ,50 to ,300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water-filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than ,80 kPa. The porous-matrix sensors appeared to give reliable readings of matric potential in soil down to ,300 kPa and also responded appropriately to repeated wetting and drying. This porous-matrix sensor has considerable potential to help understand plant responses to drying soil. [source]

ACMEG-TS: A constitutive model for unsaturated soils under non-isothermal conditions

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 16 2008
Bertrand François
Abstract This paper introduces an unconventional constitutive model for soils, which deals with a unified thermo-mechanical modelling for unsaturated soils. The relevant temperature and suction effects are studied in light of elasto-plasticity. A generalized effective stress framework is adopted, which includes a number of intrinsic thermo-hydro-mechanical connections, to represent the stress state in the soil. Two coupled constitutive aspects are used to fully describe the non-isothermal behaviour. The mechanical constitutive part is built on the concepts of bounding surface theory and multi-mechanism plasticity, whereas water retention characteristics are described using elasto-plasticity to reproduce the hysteretic response and the effect of temperature and dry density on retention properties. The theoretical formulation is supported by comparisons with experimental results on two compacted clays. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Estimating water retention curves of forest soils from soil texture and bulk density

JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 1 2003
Robert Teepe
Abstract Forest soils differ significantly from the arable land in their distribution of the soil bulk density and humus content, but the water retention parameters are primarily derived from the data of agricultural soils. Thus, there is a need to relate physical parameters of forest soils with their water retention characteristics and compare them with those of agricultural soils. Using 1850 water retention curves from forest soils, we related the following soil physical parameters to soil texture, bulk density, and C content: air capacity (AC), available water capacity (AWC), and the permanent wilting point (PWP). The ACs of forest soils were significantly higher than those of agricultural soils which were related to the low bulk densities of the forest soils, whereas differences in AWCs were small. Therefore, for a proper evaluation of the water retention curves (WRCs) and the parameters derived from them, further subdivisions of the lowest (< 1.45 g cm -3) of the three bulk density classes was undertaken to the wide range of low soil densities in forest soils (giving a total of 5 bulk density classes). In Germany, 31 soil texture classes are used for the estimation of soil physical parameters. Such a detailed classification is not required because of insignificant differences in WRCs for a large number of these classes. Based on cluster analysis of AC, AWC, and PWP parameters, 10 texture collectives were obtained. Using 5 classes of bulk densities, we further calculated the ACs, AWCs, and the PWPs for these 10 classes. Furthermore, "van Genuchten parameters" (, r, , s, ,, and n) were derived which described the average WRC for each designated class. In a second approach using multiple regression analysis, regression functions for AC, AWC, and PWP and for the van Genuchten parameter were calculated. Abschätzung der Wasser-Retentionskurven von Waldböden anhand der Bodenart und Bodendichte Obwohl sich Waldböden in der Verteilung der Bodendichte und Humusgehalte deutlich von Ackerböden unterscheiden, basiert die Ableitung ihrer bodenphysikalischen Kenngrößen in der Kartieranleitung auf Erhebungen landwirtschaftlich genutzter Böden. Die Abschätzung physikalischer Eigenschaften von Waldböden ist daher weiterhin als unzureichend anzusehen. In dieser Arbeit wurde auf Grundlage von 1850 an Waldböden ermittelten Wasser-Retentionskurven die Luftkapazität, die nutzbare Wasserspeicherkapazität und der Wassergehalt am permanenten Welkepunkt aus der Bodenart, der Bodendichte und dem C-Gehalt hergeleitet. Im Vergleich zu Ackerböden lagen die berechneten Luftkapazitäten aufgrund der unterschiedlichen vertikalen Verteilung der Bodendichten und Humusgehalte von Wald- und Ackerböden in Waldböden deutlich höher, Unterschiede in der nutzbaren Wasserspeicherkapazität hingegen waren gering. Die Ergebnisse zeigen, dass für Waldböden eine differenziertere Unterteilung der Dichteklassen notwendig ist, um die große Streuung im Bereich der unteren Bodendichte adäquat zu berücksichtigen. Andererseits basiert in Deutschland die Abschätzung physikalischer Bodeneigenschaften auf einer detaillierten Einteilung von 31 Texturklassen (Kartieranleitung und Forstliche Standortaufnahme). Da die Unterschiede zwischen vielen Texturklassen häufig sehr gering und statistisch nicht zu trennen sind, wurde unser Datensatz mit Hilfe einer Clusteranalyse auf 10 Texturklassen reduziert. Für diese Texturklassen wurden, unterteilt in jeweils 5 Dichteklassen, die Luftkapazitäten, die nutzbaren Wasserspeicherkapazitäten und der permanente Welkepunkt sowie die van Genuchten Parameter , r, , s, ,, und n berechnet. In einem zweiten Ansatz wurde eine Abschätzung dieser Kenngrößen mit Hilfe der multiplen Regression vorgenommen. [source]