Measurement and evaluation of the soil thermal parameters on the surface energy balance in an agricultural plot

A precise estimation of the surface energy budget is a challenge to better understand, model and forecast both weather and climate, and their dependence on external constraints. However, in the micrometeorology community, it is already well known that the energy balance closure is an issue over many surfaces (Foken, 2008). Previous studies often focused on turbulent fluxes, effects of advection or time averaging for the flux calculation. This study is dedicated to evaluating the accuracy of the ground heat flux, thanks to a field experiment carried out on the ICOS site of Lamasquère (FR-Lam). Both the soil heat flux and storage are often difficult to precisely measure because thermal conductivity and heat capacity are strongly dependent on the very local soil texture (% clay, % sand, % organic matter) and water content. 

The experimental campaign is conducted over a wheat field from autumn 2021 to summer 2022, and heat pulse sensors are installed in various pits to measure thermal parameters (conductivity and heat capacity) at different depths within the ploughing crust. This device currently complements the operational measurements of temperature and soil water content profiles (sensors at 5, 10, 30, 50 and 100 cm), meteorological measurements and turbulent fluxes from an Eddy Covariance set-up.

 In a first step, we will present the results of the sensor measurements for the soil thermal parameters, their local dynamics in relation to the soil parameters, the vegetation development and the weather. Then, using these measurements and the temperature profiles in the soil, we will evaluate the accuracy of the ground heat flux in order to estimate its impact on the closure of the energy balance of vegetated surfaces. These results will be also compared to measurements from self-calibrated heat flux plate and the heat storage in the soil layer above the plate.