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

A new method for a composite evaluation of atmospheric resistance, surface resistance, and evapotranspiration rate (λE) 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 (rae); the surface resistance (rs); and the vapour pressure at the surface (es).

The novelty of this approach is the estimation of es, which is assessed using the decoupling coefficient (Ω) by Jarvis and McNaughton [Adv. Ecol. Res. 15 (1986) 1]. The input parameters are: surface temperature (Ts), net radiation (Rn), soil heat flux (G), air temperature (Ta), and air humidity (ea).

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 rs on λE 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 λE and SCC, the variables Ts, Rn, and G are calculated on the basis of Landsat-5 TM, which leaves solar irradiance (for computing Rn), Ta, and ea 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 (hs,max) required.