Ground Heat Flux Determination Based on Near-Surface Soil Thermodynamics Estimated from in Situ Measurements

Near-surface soil thermodynamics plays an important role in heat and water transfer between land and atmosphere. However, it is difficult to quantify its processes in field campaigns, such as soil-water movement and evaporation, etc. This has affected the determination of ground heat flux, a key component of surface energy budget. This study aims to quantify soil heat and water processes utilizing in situ measurements, and then to improve the estimation of ground heat flux and surface energy imbalance. A new model has been proposed and tested at a grassland site based on three main physical processes, including thermal conduction, convection of heat by moving water, and convection of latent heat by evaporation within soil. The input includes near-surface soil temperature profiles (constructed by the harmonic method), soil thermal and hydraulic properties, and soil moisture measured at a shallow depth (5 cm in this study). The calculation of ground heat flux considers vertical variation of soil thermal conductivity in the 0-5 cm layer and the important contribution of vapor, which is usually ignored in previous studies. Results show that the model can capture high values of soil water content during rain events, low values under intense solar radiation, and high-frequency fluctuations under intermittent cloudy conditions. Also, the model produces reasonable vertical velocity and the shift of evaporation zone. Compared with the traditional plate and calorimetric method, the model obtains lower ground heat flux. Although the surface energy balance closure has not been improved, the new model provides a trial in estimating near-surface soil thermodynamics and ground heat flux.