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Moisture content-matric potential relationship and water flow properties of wood at high moisture contents

University of British Columbia

The water potential concept and generalized Darcy's law for unsaturated flow were proposed as a basis for the study of the energy state and flow of water in wood at high moisture contents. A series of experiments was conducted to investigate wood properties that are crucial to the application of these theories. Much attention was devoted to the measurement of the moisture content-matric potential (M-Ψm) relationship by the porous plate methods. Both the boundary drainage and imbibition curves and the drainage curve starting from the green condition were determined. In addition, independent Ψm measurements were made in some of the specimens equilibrated on the porous plates using Wescor PT51-10 thermocouple psychrometers (TCP). The osmotic potential was then assumed to be negligible. Transient flow experiments were also conducted for determining the water conductivity (K) as a function of both M and Ψm, and the water diffusivity (D) as a function of M using an instantaneous profile method and the one-step method. The boundary drainage and imbibition curves for the longitudinal direction of flow were thus obtained. The saturated conductivity had to be measured separately. Western hemlock sapwood was used for all determinations which were made at a temperature of 21°C. The results of the porous plate tests confirmed the presence of a considerable hysteresis in the M-Ψm relationship at high moisture contents, with M at a given Ψm greater in drainage than in imbibition. The ink-bottle effect appears to be the primary cause of this phenomenon. Both types of drainage curves obtained showed a tendency to exhibit a plateau at intermediate M's and were found highly dependent on the initial moisture content. A fairly good agreement was found between the Ψm values imposed on the specimens on the porous plates and those measured by the TCP's for the Ψm range from -1 to -7 bars in drainage. These two types of data differed markedly at lower Ψm 's in drainage and in imbibition. However, the differences observed in imbibition were readily explainable and the results of the TCP tests clearly demonstrated the strong hysteretic behavior of the M-Ψm relationship in the wet state. The reproducibility of the TCP method during the Ψm measurements in wood was rather poor. The K-M, K- Ψm, and D-M curves obtained by the two transient methods used were very similar in shape. The K(M) function exhibited a considerable hysteresis with K at a given M greater in imbibition than in drainage. A change in K of several orders of magnitude was recorded near full saturation. The K(Ψm) and D(M) functions displayed only a partial hysteresis. Disproportionality between flux and gradient was observed in imbibition above 75% M. The non-uniqueness of the M- Ψm relationship with respect to the state of flow was apparently the main cause of this anomaly.

Text

2010-03-23

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http://hdl.handle.net/2429/22404

Soil Science

University of British Columbia

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