Jean-Paul Lhomme

Sensible heat flux and radiometric surface temperature over sparse Sahelian vegetation. I. An experimental analysis of the kB−1 parameter

Estimating the sensible heat flux H over sparse vegetation from thermal infrared temperature requires an estimate of the excess resistance rr to the aerodynamic resistance. This excess resistance has been commonly expressed as a function of the dimensionless parameter kB−1. Experimental analysis of this parameter over diverse sparse vegetation in semi-arid areas showed a large and unexpected range of variation. The flux and ground-based thermal infrared data of the East Central Site of HAPEX-Sahel were used in this paper to study the behaviour of the kB−1 parameter and to compare these results with the conclusions of previous studies. At first, the global accuracy of the estimation of H as a function of the kB−1 value was analysed; it was found that overestimating kB−1 has less serious consequences than underestimating it. Then, optimal values of kB−1, obtained for each site (one fallow savannah site and two millet sites), ranged from 5.1 to 10.2, with significantly different values for the two millet sites. The corresponding Root Mean Square Error (RMSE) between estimated and measured H ranged from 43 to 67 W m−2. Finally, the instantaneous values of kB−1 proved to be highly variable and the parameterization of kB−1 proposed by Kustas et al. (1989) as a function of u(Tr − Ta) appeared valid only for high H values. For lower values, the slope of this relationship must be adjusted for each class of H. All the previous results confirmed that a constant value can neither be assumed for semi-arid areas nor for a given type of vegetation and that further studies are needed to understand its behaviour. Considering the difficulty in predicting the kB−1 values, a different approach based on the relationship observed between aerodynamic and radiometric temperatures was investigated. This approach gave more accurate estimate of H (RMSE between 35 and 48 W m−2), but remains at this stage purely empirical. Further investigation is now needed to predict the parameterization of this approach.

A THEORETICAL BASIS FOR THE PRIESTLEY-TAYLOR COEFFICIENT

AbstractThe relationship between potential evaporation and arealevaporation is assessed using a closed-box model of the convectiveboundary layer (CBL). Potential evaporation is defined as theevaporation that would occur from a hypothetical saturated surface,with radiative properties similar to those of the whole area, and smallenough that the excess moisture flux does not modify thecharacteristics of the CBL. It is shown that the equilibrium rate ofpotential evaporation is given by Ep0=αE0,where E0 is the equilibrium evaporation (radiative termof the Penman formula), and α is a coefficient similar to thePriestley-Taylor coefficient. Its expression is

Effective parameters of surface energy balance in heterogeneous landscape

\alpha = 1 + \left[ {1/(\varepsilon + 1)} \right]\left( {\left\langle {r_s } \right\rangle /r_a } \right)

The role of the Sahelian biosphere on the water and the CO2 cycle during the HAPEX-Sahel experiment

, where

A simple modelling of crop water balance for agrometeorological applications

\left\langle {r_s } \right\rangle

Estimating sensible heat flux from radiometric temperature over crop canopy

is the areal surface resistance, ra is the localaerodynamic resistance, and ε is the dimensionless slope of thesaturation specific humidity at the temperature of the air. Itscalculated value is around 1 for any saturated surface surrounded bywater, about 1.3 for saturated grass surrounded by well-watered grassand can be greater than 3 over saturated forest surrounded by forest.The formulation obtained provides a theoretical basis to the overallmean value of 1.26, empirically found by Priestley and Taylor for thecoefficient α. Examining, at the light of this formulation, thecomplementary relationship between potential and actual evaporation(as proposed by Bouchet and Morton), it appears that the sum ofthese two magnitudes is not a constant at equilibrium, but depends onthe value of the areal surface resistance.