Articles | Volume 19, issue 1
Research article
29 Jan 2015
Research article |  | 29 Jan 2015

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

R. G. Anderson, D. Wang, R. Tirado-Corbalá, H. Zhang, and J. E. Ayars

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 (ETEC) 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 (ETr) vegetation versions of the American Society for Civil Engineers (ASCE) equation. The ASCE equations were compared to the Priestley–Taylor ET (ETPT) and ETEC. Reference ET from the ASCE approaches exceeded ETEC during the mid-period (when vegetation coefficients suggest ETEC should exceed reference ET). At the windier tower site, cumulative ETr exceeded ETEC by 854 mm over the course of the mid-period (267 days). At the less windy site, mid-period ETr still exceeded ETEC, but the difference was smaller (443 mm). At both sites, ETPT approximated mid-period ETEC more closely than the ASCE equations ((ETPT-ETEC) < 170 mm). Analysis of applied water and precipitation, soil moisture, leaf stomatal resistance, and canopy cover suggest that the lower observed ETEC 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 ETPT and ETEC 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.

Short summary
Evapotranspiration (ET) was measured and compared to reference ET over irrigated sugarcane in Hawaii, USA: reference ET increasingly diverged from measured ET with higher wind conditions; custom bulk canopy resistance improved reference ET observations; the Priestley-Taylor equation performed better than reference ET to estimate actual ET; bulk canopy resistance was over 150 s/m, but there was no evidence of water stress in the field.