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A Field Comparison of Null-Aligned and Mechanistic Soil Heat Flux¹

ABSTRACT

Soil heat fluxes using the mechanistic soil heat flux model of Philip and de Vries were compared with those experimentally determined in the field using a null-alignment method. Comparisons at 20 depths in the top 60 cm of soil during midsummer showed better agreement of theory and measurement for the wet than for the dry Walla Walla silt loam. Average water content in the top 11 cm was 0.22 and 0.12 m³/m³ for the wet and dry plots, respectively, during the 24-h measurement period. Measured daily soil heat energy at 0.25 cm was 935 and 910 kJ/m² for the wet and dry plots, respectively. Computed energy totals at this same depth overestimated measured totals 10-fold. At 11 cm, the approximate seeding depth of wheat, measured daily soil heat energy was 562 and 863 kJ/m² for the wet and dry treatments, respectively; respective computed energy totals were 616 and 1345 kJ/m². The greatest disparity between measured and computed heat flux density occurred at shallow depths (<2 cm) during midday. Computed heat flux density in the top 35 cm generally was overestimated by 20 and 35%, as compared to measured fluxes for the wet and dry plots, respectively. Water content changes at depths equivalent to the seedzone (5–11 cm) in the dry plot were predicted within a factor of two by the vapor flux components of the heat flux model. Computed conductive heat fluxes were more seriously overestimated than were the vapor flux components.

¹ Contribution from the USDA-ARS, Columbia Plateau Conservation Research Center, Pendleton, OR 97801.

² Soil Scientists, USDA-ARS, Columbia Plateau Conservation Research Center, Pendleton, OR 97801.

Received for publication December 12, 1983. Accepted for publication May 14, 1984.