where ρ is air density, CP the specific heat capacity of air for which 1004 Jkg-1k-1 was used, ΔT is the temperature difference from height Z1 to Z2 (K), and rah the aerodynamic resistance to heat transport (s m-1). Since there are two unknown variables (ΔT and rah) in Eq. 10, acquiring a sensible heat flux can be complex. This is why the SEBAL process utilizes two “anchor” pixels, “hot” and “cold”, to fix boundary conditions for the energy balance. The selected “cold” pixel is a wet, well-irrigated crop surface with full vegetation ground cover. The surface and near-surface air temperatures are assumed to be similar at this pixel. The selected “hot” pixel is a dry, bare agricultural field with ET assumed to be zero (Allen et al., 2002). Afterwards, the variables ΔT and rah are calculated in the first loop, subsequently current values are implemented in the next loop as new initial values, and variables are updated in every loop. This process continues until the variables converge, which enables the algorithm to assess the sensible heat flux (Losgedaragh & Rahimzadegan, 2018).
The SEBAL model was applied in GRASS GIS 7.6, the Geographic Information System called Geographic Resources Analysis Support System; a script developed for each Landsat platform was used to facilitate the loop procedure to obtain ΔT, rah and, consequently, H.
The instantaneous ET value (mm h-1) at the time of image acquisition and in an equivalent evaporation depth is computed as in Eq.11: