Understanding heat transfer and vapor transport in the shallow subsurface

Improved understanding of heat transfer and vapor transport in the shallow subsurface is important both for a better general understanding of surface-atmosphere interactions and for satellite based soil moisture observations. We compared soil temperature profiles observed during MicroWex2 to temperature profiles estimated with a thermal diffusion model. We found that, in the shallow subsurface (<10 cm), the thermal diffusion model fails. From this finding, we concluded that conduction is not the dominant heat transfer mechanism in this part of the soil. Instead, a significant heat sink was inferred. Based on order of magnitude analysis of heat transfer mechanisms, we expect the sink to be caused mainly by evaporation within the shallow subsurface. Yet, molecular vapor diffusion is not fast enough to sustain evaporation large enough to explain the sink. Our present working hypothesis is that the vapor diffusion is enhanced by advective and dispersive transport due to atmospheric pressure fluctuations that propagate into the soil. In the presented work, we tested this hypothesis with an experiment in which we monitored the movement of trace gas through a soil column under various pressure conditions.