Abstract
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.
Highlights
Without clear evidence of water stress or lack of canopy cover over the study sites, we examine some explanations for the overestimation of the American Society for Civil Engineers (ASCE) ET0 and ETr compared to ETEC and ETPT
We investigated discrepancies between two standardized reference ET equations and eddy covariance (EC)-measured ET at two field sites over irrigated sugarcane on Maui
At both fields, measured daily ET during the mid-period should have approached the tall reference ET equation and exceeded the short reference ET equation. Both ASCE reference ET equations significantly overestimated mid-period ET compared to EC observations of ET
Summary
Accurate estimates of evapotranspiration (ET) are needed for numerous purposes, including efficient irrigation scheduling (Davis and Dukes, 2010), parameterizing and running different classes of biogeochemical and hydrologic models (Fisher et al, 2005; Zhao et al, 2013), assessing changes in regional hydrology under different cultivation systems (Ferguson and Maxwell, 2011; Holwerda et al, 2013; Waterloo et al, 1999), and evaluating the impacts of agricultural production on regional and global climate (Kueppers et al, 2007; Lo and Famiglietti, 2013; Puma and Cook, 2010) and hydrology (Anderson et al, 2012; Vörösmarty et al, 1998). Numerous approaches have been developed to measure or estimate ET, including lysimeters (Meissner et al, 2010), micrometeorological methods (Anderson and Goulden, 2009; Baldocchi, 2003; Hemakumara et al, 2003), satellite remote sensing (Bastiaanssen et al, 2005; Tang et al, 2009), and water balance methods. While these approaches vary in their spatial/temporal scale and methodological assumptions and accuracy, most require significant. Anderson et al.: Divergence of actual and reference evapotranspiration observations observational costs, technical expertise, or have operational difficulties that are too high for most farmers
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