Determination of soil ground heat flux through heat pulse and plate methods: Effects of subsurface latent heat on surface energy balance closure

Abstract

The soil heat flux plate method is popularly applied in surface energy balance studies. Previous studies have shown that impervious plate blocks the flow of water and vapor within soil. Soil heat flux is generally commonly measured below surface, and its exact constitution is required in calculating surface energy balance. When subsurface evaporation occurs, subsurface latent heat sink constitutes an important proportion of the apparent ground heat flux. However, the plate method fails to detect such occurrence. In aboveground meteorological measurement, evaporated vapor moving out of soil profile is also being detected and the subsurface latent heat sink is recognized as part of turbulent latent heat flux. Thus, caution should be exercised when excluding the potential error from double counting of subsurface latent heat sink in surface energy balance evaluation. In this study, two common combination methods were used to determine the ground heat flux without latent heat sink (G0). One method is a combination of gradient-based heat pulse measurements and calorimetric method (GradC), and the other method is a combination of plate measurements at shallow depths and calorimetric method (PlateC). Results demonstrated that, in contrast to the PlateC method, the GradC method minimized the disturbance in soil structure and reduced the disruption in heat and water flow. Furthermore, the estimated G0 from the PlateC method was only 49.2% of that of the GradC method during daytime. Moreover, surface energy balance closure (EBC) was evaluated using the estimated G0 and aboveground turbulent energy flux data. In comparison with the PlateC method, the GradC method improved the surface EBC from 79.3% to 87.7% during daytime. In summary, accurate knowledge on the composition of ground heat flux and the location of water evaporation is necessary to calculate surface energy balance during micro-meteorology measurements.