Estimation of ground heat flux from soil temperature over a bare soil

Ground soil heat flux, G(0), is a difficult-to-measure but important component of the surface energy budget. Over the past years, many methods were proposed to estimate G(0); however, the application of these methods was seldom validated and assessed under different weather conditions. In this study, three popular models (force-restore, conduction-convection, and harmonic) and one widely used method (plate calorimetric), which had well performance in publications, were investigated using field data to estimate daily G(0) on clear, cloudy, and rainy days, while the gradient calorimetric method was regarded as the reference for assessing the accuracy. The results showed that harmonic model was well reproducing the G(0) curve for clear days, but it yielded large errors on cloudy and rainy days. The force-restore model worked well only under rainfall condition, but it was poor to estimate G(0) under rain-free conditions. On the contrary, the conduction-convection model was acceptable to determine G(0) under rain-free conditions, but it generated large errors on rainfall days. More importantly, the plate calorimetric method was the best to estimate G(0) under different weather conditions compared with the three models, but the performance of this method is affected by the placement depth of the heat flux plate. As a result, the heat flux plate was recommended to be buried as close as possible to the surface under clear condition. But under cloudy and rainy conditions, the plate placed at depth of around 0.075 m yielded G(0) well. Overall, the findings of this paper provide guidelines to acquire more accurate estimation of G(0) under different weather conditions, which could improve the surface energy balance in field.