Sensible heat measurements indicating depth and magnitude of subsurface soil water evaporation

Abstract

Most measurement approaches for determining evaporation assume that the latent heat flux originates from the soil surface. Here, a new method is described for determining in situ soil water evaporation dynamics from fine‐scale measurements of soil temperature and thermal properties with heat pulse sensors. A sensible heat balance is computed using soil heat flux density at two depths and change in sensible heat storage in between; the sensible heat balance residual is attributed to latent heat from evaporation of soil water. Comparisons between near‐surface soil heat flux density and Bowen ratio energy balance measurements suggest that evaporation originates below the soil surface several days after rainfall. The sensible heat balance accounts for this evaporation dynamic in millimeter‐scale depth increments within the soil. Comparisons of sensible heat balance daily evaporation estimates to Bowen ratio and mass balance estimates indicate strong agreement (r2 = 0.96, root‐mean‐square error = 0.20 mm). Potential applications of this technique include location of the depth and magnitude of subsurface evaporation fluxes and estimation of stage 2–3 daily evaporation without requirements for large fetch. These applications represent new contributions to vadose zone hydrology.