ESTIMATING SENSIBLE HEAT-FLUX FROM RADIOMETRIC TEMPERATURE OVER SPARSE MILLET

A two-layer model was developed and used to estimate sensible heat flux over a sparse millet croP froM Surface radiometric temperature. The millet crop was grown in farming conditions on the central site of the HAPEX-Sahel experiment in southern Niger. Surface temperature was measured with a nadir-looking radiometer. Measurements of the convective fluxes of sensible and latent heat were made simultaneously by means of the energy balance-Bowen ratio method. It is assumed that infra-red surface temperature can be represented by a weighted sum of foliage and soil surface temperatures, the weighting factors being the fractional areas of foliage and soil surface. With this assumption, the basic equations of two-layer models lead to an expression of sensible heat flux H close in form to the Ohm's law type formulation obtained from a one-layer approach, but in which the temperature difference between the surface and the air T(r) - T(a) has to be corrected by a factor proportional to the temperature difference deltaT between the foliage and the substrate. DeltaT being not available in our experiment, it was assumed that a statistical relationship linking deltaT to T(r) - T(a) of the type deltaT = a(T(r) - T(a))m could be used. Using one part of the data set, m and a were statistically determined by adjusting H estimated by the model to H observed by the Bowen ratio method. The best adjustment gave m = 2 and a = 0.10. For the other part of the data set (different from the one employed to calibrate this relationship) it was found that H estimated using the two-layer model with this empirical relationship compared fairly well with the values of H observed.