Heat and water transfer at the bare soil surface: aspects affecting thermal imagery.

Surface temperature as assessed by means of thermal infra red remote sensing is affected by a number of soil properties. The sensitivity of surface temperature and surface energy fluxes to variations in physical soil properties is studied by means of a numerical simulation model. The model developed here takes into account the mutual interdependence between soil and atmosphere. The transportprocesses in the soil are discussed in detail, and are partially revised. Developments in the atmospheric boundary layer are based on the production rates of turbulent kinetic energy. Many physical data on soil hydraulic, thermal and radiative properties are summarized. The ranges over which these properties appear to vary are used to set extreme parameter values for studying model sensitivity. Several field experiments were conducted to study in detail the behaviour of topsoil moisture and temperature; a total of four different experimental datasets is used for model validation. Sensitivity to basic soil physical properties is determined for the three classical stages of drying. The 'falling rate' stage is discussed in more detail in terms of two new soil parameters, characterizing the matric flux potential curve. These appear to have a strong influence on dry layer development, and hence an the sensitivity of surface temperature to soil physical. properties. The possibilities to assess bare soil conditions and surface fluxes by means of thermal infra red imagery appear, from the presented results, to be very limited, even when detailed wind and global radiation data for the site under study are known. A review of methods, used in thermal imagery interpretation, is included; also, most of the existing simulation models an this topic are summarized.