Quantifying Evaporation in the Field From the Inception of Subsurface Evaporation.

Net radiation at the soil surface is partitioned between sensible and latent heat. As soil dries, the location of this partitioning encompasses both surface and subsurface soil. Evaporation occurs below the soil surface during soil-limited evaporation. This can be easily observed in the field when a wet soil is exposed to the sun. The surface soil dries and results in the formation of a dry surface layer (DSL). Vaporization of water takes place at the bottom boundary of DSL. Recent studies have demonstrated the presence of a subsurface latent heat sink and emphasized the need to account for this sink when computing an accurate surface energy balance. Subsurface evaporation can be measured using the sensible heat balance (SHB) approach. There remains need to investigate the onset of soil-limited evaporation and corresponding effects on evaporation rates and the surface energy budget. We conducted field experiments with objectives to 1) find indicators of the change in the location of the evaporation front from the surface to the subsurface, and 2) quantify evaporation rates from the inception of the subsurface evaporation zone. We used the SHB approach together with measurements of albedo and surface temperature of dry and drying soil for bare surface conditions under natural wetting-drying cycles. Results from the field study will be presented.