Why is WaPOR overestimating ETIa in irrigated fields?
ETIa-WPR is overestimating ETa dry, hot, non-water-stressed conditions
(e.g., irrigated fields). These errors might lie in the FAO-PM method’s
and may be associated with local advection effects. Local advection may
increase ETa over a water-limited field by up to 30% (De Bruin, Trigo,
Bosveld, & Meirink, 2016; Trigo et al. , 2018). There is an
underlying assumption of no advection in the RET definition for a
reference grass field (Allen et al. , 1998). However, in small
fields, under arid conditions with high temperatures, local advection
effects may occur when warm, dry air formed over an upwind, adjacent
field is advected horizontally over the well-watered fields (De Bruin &
Trigo, 2019). This horizontal advection of sensible heat increases the
evapotranspiration of water from well-watered areas but will result in
the overestimation of evapotranspiration in water-limited fields or
areas. The Zankalon irrigated area, where EG-ZAN is located, has small
fields, ~0.2ha (Table 4), as does the EG-SAA and EG-SAB.
Therefore these sites may be particularly influenced by this effect as
0.2ha is 3% of an L1 -250m pixel, 20% of an L2 -100m pixel and 200%
of an L3 -30m pixel (e.g., see Figure 10).