where ETr is the reference ET at the time of the image. For natural forest areas, ETinst/ETr, denominated evapotranspiration faction (ETfrac), is similar to the product between crop coefficient (Kc, dimensionless) and water stress coefficient (Ks, dimensionless). \(\text{ET}_{r_{\text{day}}}\) is the cumulative 24-hour ETr for the day of the image. Both ETr and \(\text{ET}_{r_{\text{day}}}\) were obtained with the program REF-ET 4.1 (Allen, 2016) (mm h-1) by using the Penman-Monteith FAO equation (PM-FAO: Allen, Pereira, Raes, & Smith, 1998). The actual annual evapotranspiration to be used in trend analyses was derived from ETfrac data.
2.4 Validation and climate change trend analysis
In order to evaluate the performance of SEBAL, its ETaresults were compared with those calculated by Eq. 13.