FAO Irrigation and Drainage Paper
(guidelines for computing crop water requirements)
Richard G. Allen, Luis S. Pereira, Dirk Raes,
1998, 290 pp., ISBN 92-5-104219-5
This publication presents an updated procedure for calculating
reference and crop evapotranspiration from meteorological data
and crop coefficients. The procedure, first presented in FAO Irrigation
and Drainage Paper No. 24, Crop water requirements, in 1977, allows
estimation of the amount of water used by a crop, taking into
account the effect of the climate and the crop characteristics.
The publication incorporates advances in research and more accurate
procedures for determining crop water use as recommended by a
panel of high-level experts organized by FAO in May 1990. The
first part of the guidelines includes procedures for determining
reference crop evapotranspiration according to the FAO Penman-Monteith
method. These are followed by updated procedures for estimating
the evapotranspiration of different crops for different growth
stages and ecological conditions.
If outside the US, to order the publication directly from FAO,
visit the following site http://www.fao.org/icatalog/inter-e.htm
where information on the ordering of the publication is provided
or by sending an e-mail to Publications-Sales@fao.org.
The current price from FAO is $35.00. A
list of sales agents for FAO technical papers is maintained at
The document can be reviewed online in HTML
or in PDF (6.3M) formats. An errata
sheet (MS Word) (PDF) for the document exists last updated February 2006 and corrected examples 35 and 36 exist.
Spreadsheet programs that support
calculation examples in FAO-56:
"Crop Evapotranspiration: Guidelines for Computing Crop Water
The following zip files contain Excel and Quattro-Pro spreadsheets
that reproduce many of the calculation examples given in FAO-56.
The files maybe downloaded by clicking on the file name.
(20Kb) contains FAO56EToExam.xls and FAO56EToExam.wb1. These spreadsheets contain step by step calculations for the FAO-PM equation for examples 8 - 20 of FAO-56. In particular, examples 17, 18, and 19 are presented for monthly, daily, and hourly computation time steps. These spreadsheet computations exactly reproduce the examples in the text and may be useful for chasing down problems in other spreadsheets or programs. This version was updated on October 16, 2003.
(75Kb) file contains FAO56Ax7.xls and FAO56Ax7.wb1 spreadsheets. These spreadsheets provide examples (24 and 25) for computing the Kc during the initial period (Kc ini) and for reproducing the figures 29 and 30. (Annex 7).
(228Kb) file contains FAO56Ax8.xls, FAO56Ax8.wb1and FAO56Ax8.wk3 spreadsheets. These spreadsheets reproduce the example in Annex 8 for applying the Kc "dual" method of FAO-56. This version was updated on October 16, 2003.
Spreadsheets were created by Dr. Richard G. Allen, University
of Idaho, 1999.