Evapotranspiration (ET) is the sum of water transfers from the earth surface caused by evaporation from water bodies or soil and the transpiration process of vegetation. It plays an important role in the water cycle and therefore the understanding of ET is a major issue in science questions. ET or also called latent heat flux (λE) is a key climate variable uniquely linking the water, energy, and carbon cycles (Garcia et al., 2013; Yan et al., 2013; Jung et al., 2010). Moisture deficiency in the vegetation root zone leads to the closure of stomata, a reduction in transpiration and elevated canopy temperatures (Anderson et al., 2011; Anderson & Kustas, 2008). In addition, the depletion of water from the soil surface layer leads to the rapid heating of the soil surface and results in reduced evaporation (Anderson et al., 2011; Kustas & Anderson, 2009). Measurements of the radiometric surface temperature (TR) by thermal infrared (TIR) radiometers provide direct information on the land surface moisture status and surface energy balance partitioning (Norman et al., 1995; Kustas & Anderson, 2009; Mallick et al., 2009a). It sets the lower boundary conditions for the transfer of latent and sensible heat through soil, vegetation and atmosphere and can be used for diagnostic assessments of ET without the need for additional information on precipitation or soil pedotransfer functions as commonly required in hydrological models (Anderson et al., 2011; Anderson & Kustas, 2008). The derivation of ET is therefore a key variable for early drought detection and the support of irrigation plans reducing the water amount to the necessary plant needs and can be supported by frequent satellite observations of the Copernicus satellites.