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.