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Soil Heat Flux, Thermal Conductivity, and the Null-alignment Method¹

ABSTRACT

The null-alignment method of estimating soil heat flux density(q) and thermal conductivity () depends on calorimetry, and ignores convection of sensible heat by mass flow and processes involving latent heat. Study of the energy balance of drying bare soils indicates that the major errors of null-point calorimetry are likely to arise from net evaporation in upper soil layers. A net evaporation rate of at least 1 mm/d probably occurred in the example used to introduce null-alignment. This small evaporation rate changes the q profile so radically that null-alignment may give -values averaging about 52% of the correct ones. Diurnal variations of latent heat sink strengths, null-points, and temperature gradients, combine to produce diurnal variation of the error in null-aligned 's, thereby explaining the reported diurnal variation of null-aligned . Since errors tend to be large and negative in daytime, and small and positive at night, 24h avg are likely to be too small. Null-alignment does not provide the precise measures of q and its exponents supposed, so that various null-alignment-based critiques of the Philip-De Vries theory of heat and moisture transfer in soils, and of the De Vries model of , cannot be sustained. Of course there is much scope for improving both through incisive theory and precise experiment. All calorimetric methods of estimating q that ignore latent heat, not only null-alignment, must involve serious errors in the presence of significant net evaporation in the soil.

¹ Contribution from CSIRO, Australia.

² Distinguished Visiting Fellow, and Chief, Div. of Environmental Mechanics, GPO Box 821, Canberra, ACT 2601, Australia, D. A. de Vries' permanent address is: Rooiven 4, 5737PW, Lieshout, The Netherlands.

Received for publication May 21, 1985. Accepted for publication September 16, 1985.

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