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United States Department of Agriculture

Agricultural Research Service

Title: The Penman-Monteith Method

Authors
item Howell, Terry
item Evett, Steven

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 5, 2004
Publication Date: April 5, 2004
Citation: Howell, T.A., Evett, S.R. 2004. The Penman-Monteith method. Section 3 in Evapotranspiration: Determination of Consumptive Use in Water Rights Proceedings. Continuing Legal Education in Colorado, Inc. Denver, CO.

Interpretive Summary: The Penman-Monteith equation is a widely used method to estimate reference crop water use in predicting actual crop water use. It has become a focal point in various legal cases in the Western U.S. on water rights transfers. This presentation discussed the fundamentals of the Penman equation and then the three main forms of the Penman-Monteith equation now in use in engineering and legal cases. An example from Bushland, Texas of alfalfa water use during an advective period (one where heat and dry air was imported into the region from large-scale weather patterns during a drought) that had very high water use rates were accurately predicted by various equations. The Penman-Monteith equation was demonstrated, even under these extreme conditions, to function well and accurately.

Technical Abstract: The Penman equation was derived in 1948 from a combination of the energy balance and an aerodynamic equation. It has become a milestone in evapotranspiration theory. It was reviewed along with its advances and its constraints. Then the advancements to more recent forms of the Penman-Monteith equation, first presented in 1965, were discussed. The ASCE (American Society of Civil Engineers) version (from 1990), the FAO (Food and Agriculture Organization version (from 1998) and the EWRI (ASCE-Environmental Water Resources Institute) version (from 2002) were discussed in detail. An example from Bushland, Texas for 10 days with highly advective evapotranspiration conditions was presented to demonstrate the agreement among the equations and the measured alfalfa evapotranspiration. The empirical Penman equation from Idaho matched the measured evapotranspiration (ET) data the closest, but its correlation through regression revealed a high bias and low correlation. The ASCE-EWRI hourly equation had the least bias, slope nearest 1.0, and highest correlation. The Penman-Monteith equation was demonstrated to accurately estimate alfalfa water use even in extreme, harsh conditions.

Last Modified: 4/17/2014
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