Coupled heat and water transport in soils has enjoyed extensive focus in soil physics and hydrology and yet, until recently,
there has never been a satisfactory comparison of water vapor ¯uxes measured in the ®eld with theory. At least two factors
have led to this, ®rst, most of the experimental work has been laboratory oriented with steady state boundary conditions
imposed and second, there have been relatively few ®eld experiments to test the existing theory. In this paper we review a new
theoretical development which explains ®eld observations of water vapor movement. The diurnal warming at the land surface
leads to an expansion and contraction of the soil air as it warms and cools resulting in a convective (or ``advective'') transport
of water vapor. This mechanism has important consequences for the transport of any vapor in the soil air near the landatmosphere
interface.