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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 #215318
Title: Evapotranspiration: Progress in measurement and modeling in agriculture

Author
item FARAHANI, H - ICARDI, SYRIA
item Howell, Terry
item SHUTTLEWORTH, W - UNIV. OF ARIZONA, TUCSON
item Bausch, Walter

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2007
Publication Date: 10/1/2007
Citation: Farahani, H.J., Howell, T.A., Shuttleworth, W.J., Bausch, W.C. 2007. Evapotranspiration: Progress in measurement and modeling in agriculture. Transactions of the ASABE. 50(5):1627-1638.

Interpretive Summary: Estimating crop water use is fundamental for irrigation and hydrology. This paper reviewed the past progress and the current and future trends in estimating crop water use. It focused on both models and measurements techniques. Lysimeters and meteorological methods for measuring the energy balance were reviewed as well as soil water balance methods. Models based on the Penman equation and the subsequent Penman-Monteith equations were reviewed. The development of the crop coefficient approach and more advanced crop models were summarized. The paper concludes by emphasizing the need to improving crop water use efficiency by increasing transpiration and reducing evaporation and, in this context, the potential value of using more improved methods and models to partition ET.

Technical Abstract: As the water resources available for agriculture become limiting due to population growth, competition from other water users, drought and water quality degradation, the importance of evapotranspiration (ET) as a major component of water use in agriculture grows. This paper provides a focused survey of past progress and current and future trends in ET measurement and modeling, with emphasis on aspects of interest to the irrigation profession. The significant advances in ET modeling during the latter part of the 20th century are largely due to our increased ability to measure near-surface climate variables and surface energy exchanges. The rapid developments over the last three decades in instrumentation, data acquisition and remote access, computer control and automation, and the off-the-shelf availability of measurement tools have enhanced the reliability and affordability of data for use in ET research and practice. Advances in lysimetry and eddy correlation methods to measure, and Bowen ratio-energy balance method to estimate, ET measurements have been complemented by progress in soil resistance sensors, time domain reflectometry, capacitance, and neutron probes to measure soil water, and by progress in porometry, infrared thermometry, and sap-flow heat gauges to monitor transpiration and crop water stress. However, ET measurement is not common in practice, and modeling is mostly preferred. Much theoretical progress in ET modeling originated with the 1948 work of Penman with its subsequent modification to the Penman-Monteith (P-M) equation and to multi-layer and sparse canopy models. These advances strengthened confidence in using combination equation and encouraged a significant step forward through the adaptation of the P-M equation to provide a standard estimate of reference crop ET for use in the long-established two-step, crop coefficient (Kc) methodology. Recently there has been a recommendation for continued progress in this field via the one-step application of the P-M equation to estimate crop water use directly using effective stomatal resistance rather than Kc. The paper concludes by drawing attention for the need to improving crop water use efficiency by increasing transpiration and reducing evaporation and, in this context, the potential value of using more improved methods and models to partition ET.