It is my (and FAO's) pleasure to announce the availability of a Spanish language translation
of FAO Irrigation and Drainage Paper no. 56 (Crop Evapotranspiration).

This translation was provided by Ricardo Trezza of the Univ. of the Andes, Venezuela, and
Ms. Magali Garcia, Univ. of San Andres, Bolivia, the text was edited by Juan Antonio
Sagardoy under the direction and review of Giovanni Muņoz, Water Development Officer
with FAO and with support provided by Pasquale Steduto, Service Chief of the Water
Resources, Management and Development Service of FAO.

The Spanish translation is available in printed form and has been added to the AGL On-line
Publications Database

(In this data base, you can select 'Irrigation and Drainage' as the Category and Spanish as
the language (with no keyword) and the translation of No. 56 will appear in the query list.)

A PDF version is available for on-line reading and/or downloading at (6.7MB), compliments of FAO.

A html version is under preparation.

The original authors of the No. 56 wish to thank Drs. Trezza and Garcia for their excellent
translation of the English version as well as the correction of several minor errors in some of
the examples. We wish to thank Messrs. Muņoz and Steduto of FAO for their support of this
translation exercise. Dr. Muņoz provided final compilation, formatting and adjustment to the

I wish to, at this time, call attention to a recent recommendation by Allen et al., (2006) in the
Journal of Agricultural Water Management, to change a coefficient in the FAO-Penman-
Monteith ETo equation when applied to hourly or shorter calculation time periods. This
recommendation is made to 'strengthen' the hourly or shorter ETo calculations by the FAO
ETo method so that calculations agree more closely with those made on a daily timestep
basis and with a number of research measurements.

The AWM paper describes this change and the reasons supporing it.  A pdf of the paper is
available at the following address (URL is case-sensitive):

The change in hourly surface resistance in the FAO-PM method is equivalent to the use of a
coefficient of 0.24 in the denominator of Eq. (53) on page 74 of the English and Spanish
translations for hourly calculation time steps during daytime (defined as when net radiation >
0) and a coefficient of 0.96 in the denominator for hourly time steps during nighttime in place
of the use of the original coefficient '0.34' that is currently shown in Eq. (53).

No changes are recommended for the FAO-PM ETo method for daily (24-h) time steps, so
that the coefficient in the denominator (in Eq. (6) on page 24 or 25) remains at 0.34 for 24-h
time steps.

Best regards,

Rick Allen
Univ. Idaho