A novel approach to evaluate soil heat flux calculation: An analytical review of nine methods

There are no direct methods to evaluate calculated soil heat flux (SHF) at the surface (G(0)). Instead, validation and cross evaluation of methods for calculating G(0) usually rely on the conventional calorimetric method or the degree of the surface energy balance closure. However, there is uncertainty in the calorimetric method itself, and factors apart from G(0) also contribute to nonclosure of the surface energy balance. Here we used a novel approach to evaluate nine different methods for calculating SHF, including the calorimetric method and methods based on analytical solutions of the heat diffusion equation. The SHF (Gz) measured by a self-calibrating SHF plate at a depth of z = 5 cm below the surface (hereafter Gm_5cm) was deployed as a reference. Each SHF calculation method was assessed by comparing the calculated Gz at the same depth (hereafter G(c_5cm)) with G(m_5cm). The calorimetric method and simple measurement method performed best in determining G(c_5cm) but still underestimated G(m_5cm) by 19% during the daytime. Possible causes for this underestimation include errors and uncertainties in SHF measurements and soil thermal properties, as well as the phase lag between G(c_5cm) and G(m_5cm). Our results indicate that the calorimetric method achieves the most accurate SHF estimates if self-calibrating SHF plates are deployed at two depths (e. g., 5 cm and 10 cm), soil temperature and water content measurements are made in a few depths between the two plates, and soil thermal properties are accurately quantified.