Divergence of actual and reference evapotranspiration observations for irrigated sugarcane with windy tropical conditions

Abstract. Standardized reference evapotranspiration (ET) and ecosystem-specific vegetation coefficients are frequently used to estimate actual ET. However, equations for calculating reference ET have not been well validated in tropical environments. We measured ET (ET_EC) using eddy covariance (EC) towers at two irrigated sugarcane fields on the leeward (dry) side of Maui, Hawaii, USA in contrasting climates. We calculated reference ET at the fields using the short (ET₀) and tall (ET_r) vegetation versions of the American Society for Civil Engineers (ASCE) equation. The ASCE equations were compared to the Priestley–Taylor ET (ET_PT) and ET_EC. Reference ET from the ASCE approaches exceeded ET_EC during the mid-period (when vegetation coefficients suggest ET_EC should exceed reference ET). At the windier tower site, cumulative 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 sites, ET_PT approximated mid-period ET_EC more closely than the ASCE equations ((ET_PT-ET_EC) < 170 mm). Analysis of applied water and precipitation, soil moisture, leaf stomatal resistance, and canopy cover suggest that the lower observed ET_EC was not the result of water stress or reduced vegetation cover. Use of a custom-calibrated bulk canopy resistance improved the reference ET estimate and reduced seasonal ET discrepancy relative to ET_PT and ET_EC in the less windy field and had mixed performance in the windier field. These divergences suggest that modifications to reference ET equations may be warranted in some tropical regions.