by
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
FAO - Food and Agriculture Organization of the United Nations
Rome, 1998
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M-56
ISBN 92-5-104219-5
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© FAO 1998
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List of principal symbols and acronyms
Chapter 1 - Introduction to evapotranspiration
Units
Factors affecting evapotranspirationWeather parameters
Crop factors
Management and environmental conditionsReference 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 equationPenman-Monteith equation
Aerodynamic resistance (ra)
(Bulk) surface resistance (rs)Reference surface
FAO Penman-Monteith equationChapter 3 - Meteorological data
Meteorological factors determining ET
Atmospheric pressure (P)
Latent heat of vaporization (l)
Psychrometric constant (g)Estimating missing climatic data
Estimating missing humidity data
Estimating missing radiation data
Missing wind speed dataAn alternative equation for ETo when weather data are missing
Chapter 4 - Determination of ETo
Calculation procedure
ETo calculated with different time stepsCalculation procedures with missing data
Pan evaporation method
Part B - Crop evapotranspiration under standard conditions
Chapter 5 - Introduction to crop evapotranspiration (ETc)
Factors determining the crop coefficient
Single and dual crop coefficient approaches
Crop coefficient curveChapter 6 - ETc - Single crop coefficient (Kc)
Length of growth stages
Crop coefficientsTabulated 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)Alfalfa-based crop coefficients
Transferability of previous Kc valuesChapter 7 - ETc - Dual crop coefficient (Kc = Kcb + Ke)
Transpiration component (Kcb ETo)
Basal crop coefficient (Kcb)
Determination of daily Kcb valuesEvaporation component (Ke ETo)
Calculation procedure
Upper limit Kc max
Soil evaporation reduction coefficient (Kr)
Exposed and wetted soil fraction (few)
Daily calculation of Ke
Part C - Crop evapotranspiration under non-standard conditions
Chapter 8 - ETc under soil water stress conditions
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 relationshipChapter 9 - ETc for natural, non-typical and non-pristine vegetation
Initial growth stage
Mid and late season stages
Water stress conditionsMid-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
ConclusionMid-season stage - Adjustment for stomatal control
Late season stage
Estimating ETc adj using crop yieldsChapter 10 - ETc under various management practices
Areas surrounded by vegetation having similar roughness and moisture conditions
Clothesline and oasis effectsManagement induced environmental stress
Chapter 11 - ETc during non-growing periods
Bare soil
Surface covered with dead vegetation
Surface covered with live vegetation
Frozen or snow covered surfaces
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