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: