Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56

Table of Contents


Richard G. Allen
Utah State University
Logan, Utah, USA

Luis S. Pereira
Instituto Superior de Agronomia
Lisbon, Portugal

Dirk Raes
Katholieke Universiteit Leuven
Leuven, Belgium

Martin Smith
Water Resources, Development and Management Service

FAO - Food and Agriculture Organization of the United Nations
Rome, 1998

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ISBN 92-5-104219-5

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© FAO 1998

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Table of Contents



List of principal symbols and acronyms

Chapter 1 - Introduction to evapotranspiration

Evapotranspiration process

Evapotranspiration (ET)

Factors affecting evapotranspiration

Weather parameters
Crop factors
Management and environmental conditions

Evapotranspiration concepts

Reference crop evapotranspiration (ETo)
Crop evapotranspiration under standard conditions (ETc)
Crop evapotranspiration under non-standard conditions (ETc adj)

Determining evapotranspiration

ET measurement
ET computed from meteorological data
ET estimated from pan evaporation

Part A - Reference evapotranspiration (ETo)

Chapter 2 - FAO Penman-Monteith equation

Need for a standard ETo method
Formulation of the Penman-Monteith equation

Penman-Monteith equation
Aerodynamic resistance (ra)
(Bulk) surface resistance (rs)

Reference surface
FAO Penman-Monteith equation

Missing climatic data

Chapter 3 - Meteorological data

Meteorological factors determining ET

Solar radiation
Air temperature
Air humidity
Wind speed

Atmospheric parameters

Atmospheric pressure (P)
Latent heat of vaporization (l)
Psychrometric constant (g)

Air temperature
Air humidity

Calculation procedures


Calculation procedures

Wind speed

Wind profile relationship

Climatic data acquisition

Weather stations
Agroclimatic monthly databases

Estimating missing climatic data

Estimating missing humidity data
Estimating missing radiation data
Missing wind speed data

Minimum data requirements

An alternative equation for ETo when weather data are missing

Chapter 4 - Determination of ETo

Penman-Monteith equation

Calculation procedure
ETo calculated with different time steps

Calculation procedures with missing data
Pan evaporation method

Pan evaporation
Pan coefficient (Kp)

Part B - Crop evapotranspiration under standard conditions

Chapter 5 - Introduction to crop evapotranspiration (ETc)

Calculation procedures

Direct calculation
Crop coefficient approach

Factors determining the crop coefficient

Crop type
Soil evaporation
Crop growth stages

Crop evapotranspiration (ETc)

Single and dual crop coefficient approaches
Crop coefficient curve

Flow chart of the calculations

Chapter 6 - ETc - Single crop coefficient (Kc)

Length of growth stages
Crop coefficients

Tabulated Kc values
Crop coefficient for the initial stage (Kc ini)
Crop coefficient for the mid-season stage (Kc mid)
Crop coefficient for the end of the late season stage (Kc end)

Construction of the Kc curve

Annual crops
Kc curves for forage crops
Fruit trees

Calculating ETc

Graphical determination of Kc
Numerical determination of Kc

Alfalfa-based crop coefficients
Transferability of previous Kc values

Chapter 7 - ETc - Dual crop coefficient (Kc = Kcb + Ke)

Transpiration component (Kcb ETo)

Basal crop coefficient (Kcb)
Determination of daily Kcb values

Evaporation component (Ke ETo)

Calculation procedure
Upper limit Kc max
Soil evaporation reduction coefficient (Kr)
Exposed and wetted soil fraction (few)
Daily calculation of Ke

Calculating ETc

Part C - Crop evapotranspiration under non-standard conditions

Chapter 8 - ETc under soil water stress conditions

Soil water availability

Total available water (TAW)
Readily available water (RAW)

Water stress coefficient (Ks)
Soil water balance
Forecasting or allocating irrigations
Effects of soil salinity
Yield-salinity relationship
Yield-moisture stress relationship
Combined salinity-ET reduction relationship

No water stress (Dr < RAW)
With water stress (Dr > RAW)


Chapter 9 - ETc for natural, non-typical and non-pristine vegetation

Calculation approach

Initial growth stage
Mid and late season stages
Water stress conditions

Mid-season stage - Adjustment for sparse vegetation

Adjustment from simple field observations
Estimation of Kcb mid from Leaf Area Index (LAI)
Estimation of Kcb mid from effective ground cover (fc eff)
Estimation of Kcb full

Mid-season stage - Adjustment for stomatal control
Late season stage
Estimating ETc adj using crop yields

Chapter 10 - ETc under various management practices

Effects of surface mulches

Plastic mulches
Organic mulches


Contiguous vegetation
Overlapping vegetation
Border crops

Small areas of vegetation

Areas surrounded by vegetation having similar roughness and moisture conditions
Clothesline and oasis effects

Management induced environmental stress

Alfalfa seed
Sugar beets

Chapter 11 - ETc during non-growing periods

Types of surface conditions

Bare soil
Surface covered with dead vegetation
Surface covered with live vegetation
Frozen or snow covered surfaces

Annex 1. Units and symbols

Annex 2. Meteorological tables

Annex 3. Background on physical parameters used in evapotranspiration computations

Annex 4. Statistical analysis of weather data sets

Annex 5. Measuring and assessing integrity of weather data

Annex 6. Correction of weather data observed in non-reference weather sites to compute ETo

Annex 7. Background and computations for Kc for the initial stage for annual crops

Annex 8. Calculation example for applying the dual Kc procedure in irrigation scheduling


A. Basic concepts and definitions
B. ET equations
C. ET and weather measurement
D. Parameters in ET equations
E. Crop parameters in PM equation
F. Analysis of weather and ET data
G. Crop evapotranspiration
H. Crop coefficients
I. Lengths of crop growth stages
J. Effects of soil mulches
K. Non-growing season evapotranspiration
L. Soil water holding characteristics
M. Rooting depths
N. Salinity impacts on evapotranspiration
O. Soil evaporation
P. Factors affecting ETc
Q. Soil water balance and irrigation scheduling
R. General

FAO technical papers