Evaluating evapotranspiration rates and surface conditions using Landsat TM to estimate atmospheric resistance and surface resistance

Abstract

A new method for a composite evaluation of atmospheric resistance, surface resistance, and evapotranspiration rate ( λ E ) is applied to Landsat-5 TM. The method uses three equations to solve for three variables: the atmospheric resistance between the surface and the air ( r ae); the surface resistance ( r s); and the vapour pressure at the surface ( e s). The novelty of this approach is the estimation of e s, which is assessed using the decoupling coefficient ( Ω) by Jarvis and McNaughton [Adv. Ecol. Res. 15 (1986) 1]. The input parameters are: surface temperature ( T s), net radiation ( R n), soil heat flux ( G ), air temperature ( T a), and air humidity ( e a). A time series (100 days) of field data collected for a wheat crop is used to illustrate the method, which is validated using latent heat fluxes recorded by the eddy covariance technique. The control of r s on λ E is expressed through the Surface Control Coefficient (SCC=1− Ω), which is compared to soil moisture data. The application of the technique in a remote sensing monitoring context is demonstrated for a Danish agricultural landscape containing crops at different stages of development. For the satellite-based estimation of λ E and SCC, the variables T s, R n, and G are calculated on the basis of Landsat-5 TM, which leaves solar irradiance (for computing R n), T a, and e a as the only field data required. The method is directly applicable without any calibration when the soil surface is moist or when the vegetation cover is dense. Only for a dry bare soil surface, where the effective source area of water vapour is below the surface, is the modification of a surface humidity parameter ( h s,max) required.