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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #260641
Title: Recommended documentation of evapotranspiration measurements and associated weather data and a review of requirements for accuracy

Author
item ALLEN, RICHARD - UNIVERSITY OF IDAHO
item PEREIRA, LUIS - UNIVERSITY OF PORTUGAL
item HOWELL, TERRY
item JENSEN, MARVIN - CONSULTANT

Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: 11/15/2010
Publication Date: 12/5/2010
Citation: Allen, R.G., Pereira, L.S., Howell, T.A., Jensen, M.E. 2010. Recommended documentation of evapotranspiration measurements and associated weather data and a review of requirements for accuracy. In: Proceedings of the 5th Decennial National Irrigation Symposium, December 5-8, 2010, Phoenix, Arizona. Paper No:IRR10-9970.2010 CDROM.

Interpretive Summary: Methods for measuring crop water use have increased with newer electronic instruments. These methods each pose difficulties or issues that render the data of questionable value if care is not taken. This article outlines the more common methods used to measure crop water use along with guidelines on use and problems associated with each method. Also discussed are advantages and disadvantages for each method of measurement. The information is intended for scientist and for journal editors and reviewers to improve the peer review process to determine when more supporting information should be presented and tools to determine when reported crop water use data might be suspect. A list of common difficulties is presented based on the extensive experiences of the authors.

Technical Abstract: More and more evapotranspiration (ET) models, ET crop coefficients, and associated measurements of ET are reported in the literature. These measurements base from a range of measurement systems including lysimeters, eddy covariance, Bowen ratio, water balance (gravimetric, neutron meter, other soil water sensing), sap flow, scintillometry and even satellite-based remote sensing and direct modeling. All of these measurement techniques require substantial experimental care and can be prone to substantial biases in reported results. Reporting of data containing measurement biases causes substantial confusion and impedance in the advancement of ET models and also in the establishment of irrigation water requirements and can translate into millions of dollars of economic losses caused by misinformed water management. Journal reviewers and readers would benefit from more complete documentation of field procedures, assumptions, instrumentation, data filtering, and site review in order to discern the likely accuracy and representativeness of the report data and ET parameters, including crop coefficients. Documentation should include a description of the vegetation, its aerodynamic fetch, water management and background soil moisture, types of equipment and calibration checks, photographs of the measured vegetation/equipment combinations, and independent assessments of measured ET using models or other means. Documentation and assessment should include all weather equipment and parameters, including the vegetation and water management environment of the weather station. Suggestions are given for documentation within the primary ET measuring systems including recommended independent testing and filtering. Lists of common biases endemic to the measuring systems are reviewed and upper limits for crop coefficients are proposed.