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ARS Home » Pacific West Area » Parlier, California » San Joaquin Valley Agricultural Sciences Center » Water Management Research » Research » Publications at this Location » Publication #289903

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

Author
item Anderson, Raymond - Ray
item Wang, Dong
item Tirado-Corbala, Rebecca
item Zhang, Huihui
item Ayars, James

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2014
Publication Date: 1/28/2015
Publication URL: http://handle.nal.usda.gov/10113/60268
Citation: Anderson, R.G., Wang, D., Tirado-Corbala, R., Zhang, H., Ayars, J.E. 2015. Divergence of actual and reference evapotranspiration observations for irrigated sugarcane with windy tropical conditions. Hydrology and Earth System Sciences. 19:583-599.

Interpretive Summary: Reference evapotranspiration equations are commonly used to predict the meteorological demand for water used by agricultural crops through plant transpiration and soil evaporation. Reference evapotranspiration, in conjunction, with crop coefficients, can help farmers and irrigation districts with scheduling proper irrigation and optimizing water use. In this study, we measured evapotranspiration from sugarcane at two fields in an irrigated tropical region with high relative humidity, and we compared measured evapotranspiration to calculated reference evapotranspiration at the two fields. Crop management and meteorological conditions were similar between the two fields with the major exception of wind speed, which was consistently high at one of the fields. The standardized equations significantly over-predicted measured evapotranspiration at both fields when standard crop coefficients were used, with a much larger difference at the windier field. We also used the Priestley-Taylor equation, which does not have an explicit wind term, to estimate evapotranspiration. The Priestley-Taylor equation matched measured evapotranspiration much more closely than the standardized equations. Subsequently we applied a correction to the standardized equations to reduce the impact of wind on the estimation of reference evapotranspiration. These results are important for efficient water use in regions with higher humidity because of recent expansion in supplemental irrigation to reduce periodic crop water stress and maximize crop yield.

Technical 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 more humid 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. We compared the ASCE equations 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 discount the likelihood that the lower ETEC was the result of water stress or reduced vegetation cover. Use of a custom bulk canopy resistance improved the ASCE reference ET estimate and reduced seasonal ET discrepancy relative to ETPT and ETEC for the less windy field and had mixed performance at the windier field. These divergences suggest that modifications to reference ET equations may be warranted in some tropical regions