Partition of sensible heat fluxes into bare soil and atmosphere

Abstract : Energy exchanges between a bare soil and the atmosphere depend on both soil parameters and meteorological conditions. The present work has been designed to describe the sensible heat partition between soil and the atmosphere, and to develop simple methods for calculating soil heat flux from soil properties and meteorological data. The relative influences of soil properties and meteorological conditions on the soil energy balance were determined in three fields during sugar beet establishment (April–May) in Northern France in 1991. The three soils differed in their texture (sandy loam, loamy and chalky soils), and consequently in albedo, thermal and hydraulic properties. Soil heat fluxes were estimated by the heat storage method using soil temperature, water content and bulk density measured between soil surface and 0.5 m. Soil heat capacity was estimated from the soil water content and bulk density. Soil thermal conductivity was determined from measurements of soil heat flux and temperature gradients. Atmospheric sensible heat flux was estimated by a two levels aerodynamic method. Solar and net radiations were measured continuously. Meteorological conditions and soil properties variations induced different magnitude of soil and atmospheric sensible heat fluxes. Diurnal soil heat fluxes varied with soil thermal conductivity, and atmospheric heat fluxes varied with albedo. The daily ratios of soil heat flux and atmospheric sensible heat flux to net radiation, like that of soil to atmospheric sensible heat flux, were not constant. They were a function of wind speed at daily time scale. To calculate hourly soil heat fluxes, a simple sinusoidal function was introduced to account for the phase shift between soil heat flux and net radiation or atmospheric sensible heat flux. The hourly soil heat flux was then calculated as the product of its diumal ratio to net radiation, that can be estimated from wind speed measurements, by the sinusoidal function. This provided calculations of the soil heat flux with a satisfactory accuracy for soils where the surface is dry, and where there is little evaporation.