SOIL HEAT-FLUX, THERMAL-CONDUCTIVITY, AND THE NULL-ALIGNMENT METHOD

The null-alignment method of estimating soil heat flux density (q) and thermal conductivity ( lambda ) 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 lambda -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 lambda . Since errors tend to be large and negative in daytime, and small and positive at night, 24-h avg are likely to be too small.