Determining the Inception and Magnitude of Subsurface Evaporation

Evaporation from an initially wet soil occurs at the soil surface but further drying of surface soil with time results in the formation of a dry surface layer. At this stage, the evaporation front moves from the surface to the subsurface. This phenomenon occurs in a highly dynamic near-surface zone making it very challenging to know the location/depth of the evaporation front and to quantify the subsurface evaporation rate. Recent studies show that subsurface evaporation can be measured using a sensible heat balance approach by accounting for the latent heat flux originating below soil surface which is not taken into account in the traditional surface energy balance equation. The soil sensible energy balance approach has been successfully tested against mass balance for estimating evaporation under steady-state controlled lab conditions, as well as to a limited extent in the field. Limitations of the approach for field conditions include inability of instrumentation to quantify evaporation during the initial shift between surface and subsurface evaporation (i.e. when evaporation occurs at depths shallower than approximately 3 mm). The objectives of this study are to 1) find indicators of the change in the location of the evaporation front from surface to subsurface, and 2) test the sensible heat balance approach for quantifying evaporation from the inception of the subsurface evaporation zone. Recently introduced multi-needle heat pulse probes were used to make continuous soil temperature and thermal property measurements in the near-surface zone at the mm scale in a bare surface soil. Preliminary results from this investigation will be presented.