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SOIL PHYSICS

In Situ Monitoring of Soil Thermal Properties and Heat Flux during Freezing and Thawing

Soil and Water Management Research Unit, Agricultural Research Service, St. Paul, MN 55108

^* Corresponding author (ochsner{at}umn.edu).

When soil freezes or thaws, latent heat fluxes occur and conventional methods for monitoring soil heat flux are inaccurate, often wildly so. This prevents the forcing of surface energy balance closure that is used in Bowen ratio flux measurements and the assessment of closure that is used as a check on the accuracy of eddy covariance measurements. We hypothesized that heat pulse sensors could be used to obtain accurate measurements of apparent thermal conductivity (_a) and apparent volumetric heat capacity (C_a), which, together with soil temperature data, would permit accurate monitoring of soil heat flux under freezing and thawing conditions. Wintertime apparent thermal properties were monitored in situ using heat pulse sensors and independently predicted using a theoretical model. The measurements and the model both showed that for temperatures between –5 and 0°C, _a and C_a were strongly temperature dependent, varying more than two orders of magnitude. This temperature dependence is primarily the result of latent heat transfer processes. Good agreement existed between the measured and modeled thermal properties, with mean absolute differences of 20% for C_a and 37% for _a. Measured and modeled soil heat flux during spring thaw and snowmelt were similar, with cumulative totals differing by only 6% during a 7-d period. During that same period, we measured a latent heat flux into the soil of 7.9 MJ m^–2, a sizeable heat flux completely undetectable by previous methods. The results of this study support our hypothesis and indicate that this method may be useful in wintertime surface energy balance studies.