Crop evapotranspiration
iii
Preface
This publication presents an updated procedure for calculating reference and crop
evapotranspiration from meteorological data and crop coefficients. The procedure, first presented
in the FAO Irrigation and Drainage Paper No. 24 'Crop Water Requirements', is termed the 'K
c
ET
o
' approach, whereby the effect of the climate on crop water requirements is given by the
reference evapotranspiration ET
o
and the effect of the crop by the crop coefficient K
c
. Other
procedures developed in FAO Irrigation and Drainage Paper No. 24 such as the estimation of
dependable and effective rainfall, the calculation of irrigation requirements and the design of
irrigation schedules are not presented in this publication but will be the subject of later papers in
the series.
Since the publication of FAO Irrigation and Drainage Paper No. 24 in 1977, advances in research
and more accurate assessment of crop water use have revealed the need to update the FAO
methodologies for calculating ET
o
. The FAO Penman method was found to frequently
overestimate ET
o
while the other FAO recommended equations, namely the radiation, the Blaney-
Criddle, and the pan evaporation methods, showed variable adherence to the grass reference crop
evapotranspiration.
In May 1990, FAO organized a consultation of experts and researchers in collaboration with the
International Commission for Irrigation and Drainage and with the World Meteorological
Organization, to review the FAO methodologies on crop water requirements and to advise on the
revision and update of procedures.
The panel of experts recommended the adoption of the Penman-Monteith combination method as a
new standard for reference evapotranspiration and advised on procedures for calculating the
various parameters. The FAO Penman-Monteith method was developed by defining the reference
crop as a hypothetical crop with an assumed height of 0.12 m, with a surface resistance of 70 s m
-1
and an albedo of 0.23, closely resembling the evaporation from an extensive surface of green grass
of uniform height, actively growing and adequately watered. The method overcomes the
shortcomings of the previous FAO Penman method and provides values that are more consistent
with actual crop water use data worldwide. Furthermore, recommendations have been developed
using the FAO Penman-Monteith method with limited climatic data, thereby largely eliminating
the need for any other reference evapotranspiration methods and creating a consistent and
transparent basis for a globally valid standard for crop water requirement calculations.
The FAO Penman-Monteith method uses standard climatic data that can be easily measured or
derived from commonly measured data. All calculation procedures have been standardized
according to the available weather data and the time scale of computation. The calculation methods,
as well as the procedures for estimating missing climatic data, are presented in this publication.
In the 'K
c
-ET
o
' approach, differences in the crop canopy and aerodynamic resistance relative to the
reference crop are accounted for within the crop coefficient. The K
c
coefficient serves as an
aggregation of the physical and physiological differences between crops. Two calculation methods
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