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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 #175125
Title: A RECOMMENDATION ON STANDARDIZED SURFACE RESISTANCE FOR HOURLY CALCULATION OF REFERENCE ETO BY FAO56 PENMAN-MONTEITH METHOD

Author
item ALLEN, RICHARD - UNIV. OF IDAHO
item PRUITT, WILLIAM - UNIV. OF CALIF.-RETIRED
item WRIGHT, JAMES - ARS KIMBERLY-RETIRED
item Howell, Terry
item VENTURA, FRANCESCA - UNIV. OF BOLOGNA-ITALY
item SNYDER, RICHARD - UNIV. OF CALIF
item ITENFISU, DANIEL - ALBERTA AGRIC.-CANADA
item STEDUTO, PASQUALE - FAO-ROME
item BERENGENA, JOAQUIN - INST. DE INVESTIGACION-SP
item YRISARRY, JAVIER - SPAIN
item SMITH, MARTIN - FAO-MADAGASCAR
item PEREIRA, LUIS - INST SUPERIOR DE AGRONOMI
item RAES, DIRK - KATHOLIEKE UNIV LEUVEN
item PERRIER, ALAIN - NATL. AGRONOMIQUE-PARIS
item ALVES, ISABEL - SPAIN
item WALTER, IVAN - WALTER ENGR-DENVER
item ELLIOTT, RONALD - OKLA STATE UNIV.

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/2005
Publication Date: 1/20/2006
Citation: Allen, R.G., Pruitt, W.O., Wright, J.L., Howell, T.A., Ventura, F., Snyder, R., Itenfisu, D., Steduto, P., Berengena, J., Yrisarry, J.B., Smith, M., Pereira, L.S., Raes, D., Perrier, A., Alves, I., Walter, I., Elliott, R. 2006. A recommendation on standardized surface resistance for hourly calculation of reference ETO by FAO56 Penman-Monteith method. Agricultural Water Management. 81:1-22.

Interpretive Summary: The current availability of hourly weather data at more sites has increased the demand for equations to predict hourly crop water use. The standardized reference crop water developed by the American Society of Engineers in cooperation with the Irrigation Association proposed a revised surface resistance be used for a short reference crop like clipped grass that used differing daytime and night me values. This paper evaluated this revised parameter definition in comparison with measured grass water use using weighing lysimeters at many sites around the world. This new American standardized equation fit the data well. It is recommended as a new world-wide accepted procedure for estimating crop water use when hourly weather data are used.

Technical Abstract: The increased development and installation of networks of electronic weather stations worldwide has increased the availability of weather data for calculating ETo on an hourly basis. There has been question and debate as well as studies on the appropriate expression and parameterization for the surface resistance (rs) parameter of the Penman-Monteith (PM) equation and the associated coefficient for the reduced form FAO-PM equation when applied hourly. This paper reviews the performance of the FAO-PM method using rs = 70 s m-1 for hourly periods and using a lower rs = 50 s m-1 value during daytime and rs = 200 s m-1 during nighttime. Variability in hour to hour trends in rs among locations and dates makes it difficult, if not impossible, to establish a consistent algorithm for rs. However, the relatively good and consistent accuracy in ETo when using a constant rs = 50 s m-during daytime gives good as a standardized parameter and coefficient for calculating study in the U.S., and studies by European and American recommend that the FAO-PM ETo method from FAO-56, shorter basis, use rs = 50 s m-1 for daytime and rs = 200 use will provide, on average, good agreement with computations step basis. No changes are suggested for the FAO-PM method steps, where rs = 70 s m-1 should continue to be used.