On water vapor transport in field soils

Measurements of soil volumetric moisture content and temperature were made at 2, 4, 7, 10, and 15 cm below the surface of a bare field soil, over a 1-week period at 20-min intervals. The conductive heat and liquid moisture fluxes were calculated for the soil layer 7-10 cm below the surface, and the water vapor flux was then determined from both the energy transfer and mass transfer equations. Water vapor flux in this layer transported a significant amount of the total energy flux (up to 50%) and an appreciable amount of the total moisture flux (up to 25%). There was reasonable agreement between the water vapor flux calculated by the mass transfer equation and the vapor flux calculated by the energy equation. For at least 80 years it has been recognized that the move- ment of moisture and heat in the soil are coupled (Boucoyous, 1915). The total heat flux in the soil occurs not only from simple conduction but also from water movement in both the vapor and liquid states. Likewise, temperature gradients can drive mass transfer. Conceptually, the coupling of the heat and mass transfer equations can be seen as largely resulting from the water vapor flux. The movement of moisture from one location in the soil to another by evaporation and the subse- quent recondensation can contribute significantly to the net moisture movement in the soil. Additionally, because of the large value of the latent energy of vaporization of water, the water vapor transports significant energy when it evaporates and condenses. Various authors have examined the significance and magni- tude of the water vapor flux as it affects either the mass or energy balances in experimental studies (see Table 1 for sum- mary results). One of the first field-scale tests of the coupled effects of soil heat and moisture transport was done by Rose (1968a, b). Rose was interested in water vapor transport driven by temperature gradients and looked only at the mass balance. In his equation for the conservation of mass the only unknown term was the thermally driven vapor flux. Using the measured values of the other fluxes and the net change in moisture content, Rose solved for the thermal vapor flux, and found that the amount of water transported through a soil layer as vapor was on the same order of magnitude as the increase or de- crease of the volumetric moisture content in that layer. An- other study that looked at role of vapor flux in the soil moisture balance was done by Jackson and coworkers (Jackson, 1973; Jackson et al., 1974); they performed a similar experiment of combined measurements of soil temperature and volumetric moisture content in a field soil. Their calculated water vapor fluxes were on the same order as Rose's (1968b). Monji et al. (1990) also found large values of water vapor transport due to temperature gradients. A study which looked at the effect of vapor flux on the energy balance was that of Westcot and Wierenga (1974). In their combined modified-field experiment and computer model they calculated that heat transported by vapor flux was on the same order as heat flux by conduction and accounted for 40 - 60% of total heat flux in the top 2 cm of the soil and up to 20 -25% of total heat flux at a depth of 25 cm. When the heat transport by vapor flow was not included, the soil tem- perature was underestimated at the middle of the day. In the present study we examine the transport of water in a bare field soil (Yolo silt loam) using subsurface measurements of soil temperature and volumetric moisture content. Thermal conductivity and liquid water diffusivity are both calculated from well-established previous results for the soil used in the experiment. We show that there exists for these field experi- mental conditions a significant amount (40 - 60%) of heat flux due to vapor transport. The contribution of the water vapor flux to the total moisture flux is less (10 -30%) but still signif- icant. Unlike previous studies, which have looked at either mass or energy transport, we compare the vapor flux computed from the residual of the energy equation to the vapor flux computed from the mass equation. Reasonable agreement be- tween the two calculated time series of water vapor flux is found.