A new parameterisation scheme of ground heat flux for land surface flux retrieval from remote sensing information

The objective of the study was to assess the performance of a new parameterisation scheme of ground heat flux (G) for retrieving surface fluxes from remote sensing data (MODIS-Terra). Formulae that are based on empirical relationships relating G to net radiation, Rn (G = αRn, α being a function of a vegetation index, VI) are currently used, but presented drawbacks, especially in bare or sparse vegetation areas because of the poor adequacy of VI-based relationships to account for changes in soil moisture. In this study, we proposed to link α to the evaporative fraction, EF. In a first step, using a non-dimensional form of the surface energy balance, we demonstrated that α is functionally related to EF and to the ratio γ = G/H (H = sensible heat flux). In a second step, we proposed an EF-based parameterisation of α, using ground fluxes data sets collected throughout the years 2005, 2006 and 2007 at four flux-tower sites in West African countries (Mali, Benin, Niger) that differ in surface conditions and Monsoon influence. The analysis indicated that the average site-specific values of α and EF were well described by a linear relationship of the type α = a EF + b, with a = −0.22 and b = 0.23. In a third stage, we investigated whether ET-retrieval from remote sensing information (MODIS-Terra) using the new parameterisation of α perform better than the classical formulation through VI-based relationships. We found that the retrieved values of H using the new parameterisation supplied the best agreement with the observed ground data and significant improvement with respect to estimates from α–VI relationships. Advantages and limitations of the proposed parameterisation scheme were discussed.