Articles
PROPOSED REVISION TO THE FAO PROCEDURE FOR ESTIMATING CROP WATER REQUIREMENTS
Article number
449_2
Pages
17 – 34
Language
Abstract
The forthcoming revision of the FAO ID-24 publication (Crop Water Requirements) utilizes the FAO form of the Penman-Monteith equation to predict grass reference evapotranspiration (ET) and includes modified procedures for estimating crop coefficients (Kc). The Kc of the initial growing period has been reexpressed using an integrated, volumetric-based decay equation to describe the effect of the surface soil water balance on evaporation.
The Kc table of the original FAO ID-24 publication, which had multiple entries for four wind-relative humidity classes, has been condensed into one column representing Kc‘s for conditions of low wind and moderate relative humidity.
Kc‘s for other climatic conditions are derived using an equation containing wind speed and relative humidity.
Procedures are introduced for applying FAO Kc‘s as "basal" coefficients, where increases in Kc during evaporation from wet soil are computed using decay functions based on a moisture balance of the soil surface.
The Kc table of the original FAO ID-24 publication, which had multiple entries for four wind-relative humidity classes, has been condensed into one column representing Kc‘s for conditions of low wind and moderate relative humidity.
Kc‘s for other climatic conditions are derived using an equation containing wind speed and relative humidity.
Procedures are introduced for applying FAO Kc‘s as "basal" coefficients, where increases in Kc during evaporation from wet soil are computed using decay functions based on a moisture balance of the soil surface.
Authors
R.G. Allen, M. Smith, L.S. Pereira, W.O. Pruitt
Keywords
Evapotranspiration, evaporation, crop-water use, guidelines
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