Soil Water Management
Evapotranspiration and Irrigation Water Requirments
Mid Snake Water Quality





FAO Irrigation and Drainage Paper No. 56

Crop evapotranspiration
(guidelines for computing crop water requirements)

Richard G. Allen, Luis S. Pereira, Dirk Raes,
Martin Smith
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 where information on the ordering of the publication is provided or by sending an e-mail to 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 Requirements" (1998).

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.

The (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).

The (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.