Locating the dry layer interface during soil water evaporation by using numerical inversion-based heat pulse method

The location of dry layer interface (LDI), which varies during soi drying process, is a key parameter for characterizing soil water evaporation process. Recently the heat pulse (HP) technique has been applied to estimate the LDI indirectly. However, errors may occur with the HP method when analytical solutions are used because of soil heterogeneity across the measurement plane, especially when a heterogeneous interface lies between the two probes. In this study, we propose a numerical inversion-based heat pulse method for estimating the LDI under five configurations. The inputs are thermal properties of dry and wet soils, and the temperature rise-by-time curves at two locations from the heat source. The heat source was positioned at different distances from the LDI. The inversion method was evaluated with temperature rise-by-time curves from 17 scenarios obtained from numerical simulation and 14 scenarios obtained from laboratory measurements. Results demonstrated that the new approach produced reasonable LDI estimates within the range of 0.1 to 5 cm to the soil surface, with relative errors (REs) less than 0.30, except for the situation that the LDI was close to the heat source. The proposed method has significant implications in groundwater management and modeling hydrological processes in unsaturated soils.