Field evaluation and improvement of the plate method for measuring soil heat flux density

Soil heat flux is an important component of the energy balance at the land surface. Heat flux plates have been used widely to measure soil heat flux, but suffer from errors such as heat flow distortion and soil-plate contact resistance. The Philip correction and self-calibrating heat flux plates have been applied to minimize measurement errors. The objectives of this study were to evaluate the effectiveness of heat flux plate correction methods and to introduce improved approaches for applying these methods under field conditions. Soil heat flux at a depth (z) below the surface (Gz) was measured with conventional and self-calibrating plates buried at 2, 6, and 10 cm in a bare soil. Adjacent to the soil heat flux plates, soil thermal conductivity (λs) and temperature gradients were measured simultaneously with heat-pulse sensors, allowing Gz to be determined with the gradient method. The gradient method values were used as a standard to evaluate the performance of the heat flux plates. Temporal λs values were also estimated from soil sand content, bulk density and water content using a thermal conductivity model. At the 6- and 10-cm depths, the conventional plates underestimated Gz by 4.3-10.2 W m−2 due to heat flow distortion errors resulting from a mismatch between λs and plate thermal conductivity (λp). When the Philip correction was applied, both the measured and modeled λs values improved the accuracy of conventional heat flux plates. However, the modeling approach simplified the procedure for obtaining λs. The self-calibrating plate effectively corrected Gz errors associated with heat flow distortion (accurate to within 6.4 W m−2) at the 6 cm and 10 cm depths. At the 2 cm depth, both types of plates produced erroneous Gz data, which were attributed to alterations in the thermal field and heat flux pattern around the plates due to blocking convective heat and water transfer. We also demonstrated that the heating process of the self-calibrating plate could bias Gz data by disturbing the heat flow field around the plate. Voltage signals during and shortly after self-calibration should be discarded from data analysis. With these corrections, heat flux plates can provide an effective method for measuring soil heat flux.