Sensible heat flux and radiometric surface temperature over sparse Sahelian vegetation II. A model for the kB−1 parameter

Abstract To estimate sensible heat flux from radiometric surface temperature over sparse vegetation, it is necessary to add an excess resistance to the aerodynamic resistance calculated between the canopy source height and the reference height. This excess resistance is classically expressed as a function of the kB −1 parameter. By using the Shuttleworth-Wallace two-layer model, together with the linearity hypothesis on radiometric temperature, an analytical expression of the kB −1 parameter has been obtained. This expression and the numerical simulations show that kB −1 is not a constant parameter: it varies as a function of the structural characteristics of the vegetation, the level of water stress and the climatic conditions. The model predictions have been confronted with experimental data, obtained on a fallow savannah during HAPEX-Sahel. The model performs fairly well, with a slight under-estimation which has been explained. From these results, it emerges that the kB −1 parameter does not seem to be an appropriate and accurate tool to estimate sensible heat flux over sparse vegetation. Since it does not depend only on structural characteristics, it is not a constant parameter for a given vegetation, and thus, it can not be used in an operational way.

[1]  M. S. Moran,et al.  Use of ground‐based remotely sensed data for surface energy balance evaluation of a semiarid rangeland , 1994 .

[2]  M. S. Moran,et al.  Determination of sensible heat flux over sparse canopy using thermal infrared data , 1989 .

[3]  A. S. Thom,et al.  Momentum, mass and heat exchange of vegetation , 1972 .

[4]  Bruno Monteny,et al.  Estimating sensible heat flux from radiometric temperature over sparse millet , 1994 .

[5]  J. Lhomme,et al.  Estimating sensible heat flux from radiometric temperature over crop canopy , 1992 .

[6]  John L. Monteith,et al.  A four-layer model for the heat budget of homogeneous land surfaces , 1988 .

[7]  Jetse D. Kalma,et al.  Estimating evaporation from pasture using infrared thermometry: evaluation of a one-layer resistance model. , 1990 .

[8]  J.-P. Goutorbe,et al.  HAPEX-Sahel: a large-scale study of land-atmosphere interactions in the semi-arid tropics , 1994 .

[9]  A. Chamberlain Transport of gases to and from surfaces with bluff and wave‐like roughness elements , 1968 .

[10]  R. Shaw,et al.  Aerodynamic roughness of a plant canopy: A numerical experiment , 1982 .

[11]  Robert J. Gurney,et al.  The theoretical relationship between foliage temperature and canopy resistance in sparse crops , 1990 .

[12]  M. S. Moran,et al.  Sensible heat flux - Radiometric surface temperature relationship for eight semiarid areas , 1994 .

[13]  P. R. Owen,et al.  Heat transfer across rough surfaces , 1963, Journal of Fluid Mechanics.

[14]  William P. Kustas,et al.  Estimates of Evapotranspiration with a One- and Two-Layer Model of Heat Transfer over Partial Canopy Cover , 1990 .

[15]  A. Chehbouni,et al.  Determination of sensible heat flux over Sahelian fallow savannah using infra-red thermometry , 1994 .

[16]  J. Lhomme,et al.  Radiative surface temperature and convective flux calculation over crop canopies , 1988 .

[17]  Bruno Monteny,et al.  Sensible heat flux and radiometric surface temperature over sparse Sahelian vegetation. I. An experimental analysis of the kB−1 parameter , 1997 .

[18]  A. J. Dolman,et al.  The Roughness Length for Heat of Sparse Vegetation , 1995 .

[19]  M. S. Moran,et al.  Surface energy balance estimates at local and regional scales using optical remote sensing from an aircraft platform and atmospheric data collected over semiarid rangelands , 1994 .