Divergence of Reference Evapotranspiration Estimates Under Advective Tropical Conditions.

Standardized reference evapotranspiration (ET) and crop specific coefficients are frequently used to assess crop water use in irrigated agriculture.  However, equations for calculating reference ET have not been well validated in more humid environments where optimal crop yields can depend on supplemental irrigation. We measured ET (ET_EC) using Eddy Covariance (EC) towers at two well-irrigated sugarcane fields on the leeward (dry) side of Maui, Hawai’i, USA in contrasting climates.  We calculated reference ET at the tower sites using the daily short crop (ET₀) and tall crop (ET_r) versions of the American Society for Civil Engineers (ASCE) reference equation.  We compared the ASCE equations to the Priestley-Taylor potential ET (ET_PT) and ET_EC.  Reference ET from the ASCE approaches exceeded ET_EC during the mid-period (when crop coefficients suggest ET_EC should exceed reference ET).  At the windier tower site, ET_r exceeded ET_EC by 854 mm over the course of the mid-period (267 days).  At the less windy site, mid-period ET_r still exceeded ET_EC, but the difference was smaller (443 mm).  At both fields, ET_PT approximated mid-period ET_EC more accurately than the ASCE equations ((ET_PT-ET_EC) <170 mm).  Analysis of applied water and precipitation, soil moisture, leaf stomatal resistance, canopy cover, and LAI discount the likelihood that the lower ET_EC in the sugarcane fields was the result of water stress or reduced crop cover.  Use of a custom, constant bulk canopy resistance improved the ASCE reference ET estimate and reduced seasonal ET discrepancy relative to ET_PT for the less windy field and had mixed performance at the windier field.  The divergence between the various reference ET equations suggests that modifications to the standard ASCE/FAO reference ET equations may be warranted for crops in at least some tropical regions.