Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2010
Publication Date: 4/1/2011
Citation: Allen, R.G., Pereira, L.S., Howell, T.A., Jensen, M.E. 2011. Evapotranspiration information reporting: I. Factors governing measurement accuracy. Agricultural Water Management. 98(6):899-920. Interpretive Summary: Measurements and modeling of crop water use and water use from all types of native vegetation are important to understand the natural water balance and are essential in irrigated agriculture. These data are required to be accurate and reliable to have value to water resource planners and the many other users. This paper is a synthesis of the most frequently used methods to measure and estimate water use from vegetation. It describes procedures for the various methods that should be used to obtain reliable water use information as well as many common problems that can result in erroneous or biased data. The paper presents expected limits on crop water use rates to aid researchers in evaluating the quality and reliability of water use data. It also provides a framework to access data quality and its validity for the conditions it is intended to represent.
Technical Abstract: More and more evapotranspiration (ET) models, ET crop coefficients, and associated measurements of ET are being reported in the literature and used to develop, calibrate, and test important ET process models. Evapotranspiration data are derived 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 to the advancement of ET models and in the establishment of irrigation water requirements, and translate into substantial economic losses caused by misinformed water management. Basic principles of ET measuring systems are reviewed and causes of common error and biases endemic to systems are discussed. Recommendations are given for reducing error in ET retrievals. Upper limits on ET measurements and derived crop coefficients are proposed to serve as guidelines. The descriptions of errors common to measurement systems are intended to help practitioners collect better data, as well as to assist reviewers of manuscripts and users of data and derived products in assessing quality, integrity, validity, and representativeness of reported information. This paper is the first part of a two-part series, where the second part describes recommendations for documentation to be associated with published ET data.