Numerical Evaluation of Heat Pulse Technology for Estimation of Evaporation Rates from a Subsurface Drying Front

Soil water evaporation plays a crucial role for both the soil surface energy balance and the hydrologic cycle. Recently introduced heat pulse probes (HPP) allow in-situ measurements of subsurface soil water evaporation. The sensible heat component is calculated from soil heat flux densities measured at two depths and the change in sensible heat storage between these depths is measured by the HPP. The latent heat component detectable during stage 2 evaporation is then estimated from the heat balance residual. Although the accuracy of the estimated evaporation rate depends on many factors (i.e., location of sensor needles, soil texture), the theoretical limits of the HPP method have not been thoroughly evaluated. In the study presented, numerical simulations of the soil water evaporation process were conducted for a heat pulse line source to evaluate the capabilities of the HPP method using a high resolution grid (i.e., mm scale). Calculated temperatures at the depths of virtual sensor needles were used for the HPP method and measured subsurface evaporation rates were compared with simulated ones. The impacts of sensor needle depths were also evaluated for tri-needle (THPP) and penta-needle (PHPP) heat pulse probe configurations. Furthermore, the impact of soil texture was evaluated using coarse- and fine-textured soils. Numerical simulations including liquid water, soil water vapor flux and heat transport were conducted using the HYDRUS-1D code.