When ever a rainy day makes you feel sad, remember that water is the molecule of life' J' lvarez

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When ever a rainy day makes you feel sad,
remember that water is the molecule of life. J.
Álvarez
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• Chapter III EVAPOTRANSPIRATION

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INTRODUCTION
Chapter 3 Evapotranspiration

• Evapotranspiration (ET) is a combination of two
  processes evaporation y transpiration.
• Evaporation is a physical process that involves
  conversion of liquid water into vapors in the
  atmosphere.

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INTRODUCTION
Chapter 3 Evapotranspiration

• Transpiration is a physical process that involves
  flow of liquid water from the soil to the surface
  of leaves/ branches and trunk and conversion of
  liquid water from the plant tissue into water
  vapors in the atmosphere.
• Evaporation, transpiration and evapotranspiration
  processes are important for estimating crop
  irrigation requirements and for the irrigation
  scheduling.

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INTRODUCTION
Chapter 3 Evapotranspiration

• For crop irrigation requirements, it is necessary
  to estimate ET by on site measurements or by
  using meteorological data.
• On site measurements are very costly and are
  mostly employed to calibrate ET methods utilizing
  climatological data.

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INTRODUCTION
Chapter 3 Evapotranspiration

• There are number of proposed equations that
  require meteorological data and some are used to
  estimate the ET for periods of one day or more.
• Not all methods are equally precise and reliable
  for different regions of the world.

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POTENTIAL EVAPOTRANSPIRATION (ET)
Chapter 3 Evapotranspiration

• Potential evapotranspiration is a water loss from
  the soil surface completely covered by
  vegetation. (ET).
• Meteorological processes determine the
  evapotranspiration of a crop.

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POTENTIAL EVAPOTRANSPIRATION (ET)
Chapter 3 Evapotranspiration

• The evapotranspiration depends on three factors
• Vegetation
• Water availability in the soil
• Behavior of stomates.

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POTENTIAL EVAPOTRANSPIRATION (ET)
Chapter 3 Evapotranspiration

• Vegetation affects the ET in various forms. It
  affects ability of soil surface to reflect light.
  The vegetation changes amount of absorbed energy
  by the soil surface.
• As the plants are water stressed, stomates close
  resulting in the reduction of a water loss and
  carbon dioxide absorption.

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POTENTIAL EVAPOTRANSPIRATION (ET)
Chapter 3 Evapotranspiration

• Changes in the soil moisture affects direct
  evaporation from the soil surface and available
  water to the plants.
• This is a factor that is not considered in the
  potential evapotranspiration equation.

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Chapter 3 Evapotranspiration

• WEATHER STATION

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Chapter 3 Evapotranspiration
WEATHER STATION
View of climatological station
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MATHEMATICAL MODELS FOR (PET)
Chapter 3 Evapotranspiration

• There are different methods to estimate or
  measure the ET and the potential
  evapotranspiration (PET). The precision and
  reliability vary from one method to another, some
  provide only an approximation.

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MATHEMATICAL MODELS FOR (PET)
Chapter 3 Evapotranspiration

• The most frequently used techniques are
  Hydrological method or water balance method,
  climatological methods and micrometeorological
  methods.
• Many investigators have modified the equations
  that are already established.

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MATHEMATICAL MODELS FOR (PET)
Chapter 3 Evapotranspiration

• Every researcher has its preferred formula, that
  may give good results.
• Every researcher has preference. However each
  formulae, depending where it was evaluated, may
  or may not result in the first or the last place.
  

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WATER BALANCE METHOD
Chapter 3 Evapotranspiration

• This technique employs periodic determination of
  rainfall, irrigation, drainage and soil moisture
  data. The hydrological method uses water balance
  equation
• PI SW - RO D ET 0
• In this equation, every variable can be measured
  with precision with the lysimeters. The ET can be
  calculated as a residual, knowing values of all
  other parameters.

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CLIMATIC METHODS
Chapter 3 Evapotranspiration

• Using meteorological data, numerous equations
  have been proposed. Also, numerous modifications
  have been made to the available formulae for
  application to a particular region.

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PENMAN METHOD
Chapter 3 Evapotranspiration

• The Penman formula was presented in 1948. It is
  based on Net radiation, air temperature, wind
  velocity and deficit in the vapor pressure. He
  gave the following equation
• PET Rn /(a b) Ea
• c b

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LYSIMETER WITH COMPONENTS
Chapter 3 Evapotranspiration

• lthttp//www.regional.org.au/au/asssi/supersoil2004
  /s15/oral/1083_meissnerr.htmgt

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LYSIMETER WITH COMPONENTS
Chapter 3 Evapotranspiration

• lthttp//www.regional.org.au/au/asssi/supersoil2004
  /s15/oral/1083_meissnerr.htmgt

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PENMAN METHOD MODIFIEDD BY MONTEITH
Chapter 3 Evapotranspiration

• LE - s (Rn S) Pa Cp (es ea) / ra
• (s b) ( ra rc) / ra

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PENMAN METHOD MODIFIEDD BY MONTEITH
Chapter 3 Evapotranspiration

• This method has been successfully used to
  estimate the ET of a crop. The Penman-Monteith
  equation is limited to research work
  (experimentation) since the ra and rc data are
  not always available.

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PENMAN METHOD MODIFIEDD BY DOORENBOS AND PRUITT
Chapter 3 Evapotranspiration

• PET c W Rn (1 W) F(u) (ea ed)
  

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PENMAN METHOD MODIFIEDD BY DOORENBOS AND PRUITT
Chapter 3 Evapotranspiration

• The Penman formula is not popular, because it
  needs obtained data only available at the
  majority of the meteorological weather stations.
• Estimations of PET using Penman formula can be
  complex. The equation contains too many
  components, which should be measured or
  estimated, when data is not available.

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THORNWAITE METHOD
Chapter 3 Evapotranspiration

• This method uses monthly average temperature and
  the length of the day.
• PET 16 Ld 10 T / I a

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THORNWAITE METHOD
Chapter 3 Evapotranspiration

• The application of the equation to short periods
  of time can lead to an error.
• During short periods, the average temperature is
  not an adequate measure of the received
  radiation.
• During long terms, the temperature and the ET are
  similar functions of the net radiation.

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BLANEY- CRIDDLE METHOD
Chapter 3 Evapotranspiration

• The original Blaney- Criddle equation was
  developed to predict the consumptive use of PET
  in arid climates.
• This formula uses percentage of monthly sunshine
  hours and monthly average temperature.
• PET Km F
• This method is easy to use and the necessary data
  are available.

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BLANEY AND CRIDDLE METHOD MODIFIED BY FAO
Chapter 3 Evapotranspiration

• PET C P (0.46 T 8)
• where
• PET Potential evapotranspiration, mm/ day.

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BLANEY AND CRIDDLE METHOD MODIFIED BY FAO
Chapter 3 Evapotranspiration

• Doorenbos and Pruitt recommended individual
  calculation for each month. They indicated that
  it may be necessary to increase its value for
  high elevations.

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BLANEY AND CRIDDLE METHOD MODIFIED BY SHIH
Chapter 3 Evapotranspiration

• PET 25.4 K MRs (1.8 T 32) / TMRs
• where
• PET Monthly potential
  evapotranspiration, mm.
• K Coefficient for this modified
  method.
• MRs Monthly solar radiation, cal/ cm2.
• T Monthly average temperature,
  C.
• TMRs Sum monthly solar radiation during
  the year, cal/cm2.

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JENSEN- HAISE METHOD
Chapter 3 Evapotranspiration

• The Jensen-Haise equation 9 resulted from about
  3,000 measurements of the ET taken in the Western
  Regions of the United States for a 35-years
  period.
• PET Rs (0.025 T 0.08)

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JENSEN- HAISE METHOD
Chapter 3 Evapotranspiration

• This method seriously underestimates ET under
  conditions of high movements of atmospheric air
  masses. However it gives reliable results for
  calm atmospheres.

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STEPHENS-STEWART METHOD
Chapter 3 Evapotranspiration

• Stephens-Stewart utilized solar radiation data.
  It is similar to the original Jensen-Haise
  method.
• PET 0.01476 (T 4.905) MRs/ b

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PAN EVAPORATION METHOD
Chapter 3 Evapotranspiration

• Class A pan is commonly used instrument to
  measure evaporation. The relationship between PET
  and pan evaporation can be expressed as
• PET Kp PE

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PAN EVAPORATION METHOD
Chapter 3 Evapotranspiration

• The evaporation pan integrates the climate
  factors and has proven to give accurate
  estimations of PET.

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HARGREAVES METHOD
Chapter 3 Evapotranspiration

• Hargreaves method uses a minimum of
  climatological data. The formula is as follow
• PET MF (1.8 T 32) CH

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HARGREAVES METHOD
Chapter 3 Evapotranspiration

• The Hargreaves original formula for the PET was
  based on radiation and temperature as shown
  below
• PET 0.0135 x RS (T 17.8)

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CLASS A EVAPORIMETER PAN
Chapter 3 Evapotranspiration

• lthttp//www.sce.ait.ac.th/facilities/irrlab/irr_st
  ation.htmgt

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CLASS A EVAPORIMETER PAN
Chapter 3 Evapotranspiration

• lthttp//www.agrometeorology.org/index.php?id38gt

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MODIFIED HARGREAVES AND SAMANI METHOD
Chapter 3 Evapotranspiration

• Finally after several years of calibration,
  equation /13/ was presented as follows
• PET 0.0023 Ra x (T 17.8) x (TD)0.50

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MODIFIED HARGREAVES AND SAMANI METHOD
Chapter 3 Evapotranspiration

• This equation requires only maximum and minimum
  temperature data. This data is normally
  available. This formula is precise and reliable.

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LINACRE METHOD
Chapter 3 Evapotranspiration

• PET 700 Tm / 100 1 15 T- Td
• ( 80 T)

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LINACRE METHOD
Chapter 3 Evapotranspiration

• The variations in PET values by this formula are
  0.3 mm/ day based annual data and 1.7 mm/ day
  based on daily data.

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MAKKINK METHOD
Chapter 3 Evapotranspiration

• Makkink developed a regression equation to
  estimate the PET using radiation measurements
• PET Rs s/(a b) 0.12

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MAKKINK METHOD
Chapter 3 Evapotranspiration

• This formula provides good results in humid and
  cold climates, and in arid regions.

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RADIATION METHOD
Chapter 3 Evapotranspiration

• Doorenbos and Pruitt 4 presented following
  radiation equation, which is a modified Makkink
  formula 16
• PET c (W Rs)

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RADIATION METHOD
Chapter 3 Evapotranspiration

• This method was employed in the Equator zone, in
  small islands and in high latitudes. Solar
  radiation maps provide the necessary data for the
  formula.

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REGRESSION METHOD
Chapter 3 Evapotranspiration

• The simple lineal regression equation is given as
  follow
• PET a Rs b

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REGRESSION METHOD
Chapter 3 Evapotranspiration

• This regression method is simple and easy to use.
  However, it is not frequently used because of
  highly empirical nature.

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PRIESTLY-TAYLOR METHOD
Chapter 3 Evapotranspiration

• In the absence of atmospheric air mass movement,
  Priestly and Taylor showed that the PET is
  directly related to evaporation equilibrium
• PET A s/( S B) (Rn S)

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PRIESTLY-TAYLOR METHOD
Chapter 3 Evapotranspiration

• This method is of semi-empirical in nature. It
  is reliable in humid zones, and is not adequate
  in arid regions.

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LOCAL CALIBRATION
Chapter 3 Evapotranspiration

• Local calibration is always necessary to obtain
  most precise and good estimates of the crop water
  requirements.
• For the Blaney- Criddle method, ET can be
  estimated using measurements of the soil
  moisture, in lysimeters, and that can measure
  water entering and going out.

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LOCAL CALIBRATION
Chapter 3 Evapotranspiration

• Only ambient temperatures and rainfall data are
  necessary for complete calibration when
  determining appropriate monthly crop coefficient.
  
• The Jensen- Haise method 9 is recommended for
  periods of 5 to 30 days.

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LOCAL CALIBRATION
Chapter 3 Evapotranspiration

• For a monthly calibration, the ET can be
  estimated by soil moisture measurements, inlet
  and outlet flows, in lysimeters, etc.
• The Penman equation can provide precise
  estimations from a month to an hour depending on
  the calibration method.

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CROP EVAPOTRANSPIRATION (ETc)
Chapter 3 Evapotranspiration

• To obtain the ETc (consumptive use), is necessary
  to know crop and ambient conditions.
• The ETc indicates amount of water consumed for a
  given crop stage and the irrigation requirements
  can be determined.

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CROP EVAPOTRANSPIRATION (ETc)
Chapter 3 Evapotranspiration

• The Blaney- Criddle method does not need a crop
  coefficient.
• Doorenbos and Pruitt 4, provided an appropriate
  crop coefficient to estimate the ET for specific
  crops.

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CROP EVAPOTRANSPIRATION (ETc)
Chapter 3 Evapotranspiration
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CROP COEFFICIENTS
Chapter 3 Evapotranspiration

• The crop coefficients (Kc), are related to the
  type of crop, the physiology of a crop, crop
  stage, days after planting, the degree of
  coverage and the PET.
• It is important to know, how these were obtained.
  There are empirical relations between ETc and
  PET
• Kc ETc / PET

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CROP COEFFICIENTS
Chapter 3 Evapotranspiration

• The combined Kc includes evaporation from the
  soil surface and the plant surface. The
  evaporation depends on the soil moisture and soil
  characteristics. The transpiration depends on
  the amount and nature of leaf area index of a
  plant and the available soil moisture to the root
  zone. The Kc can be adjusted to the available
  soil moisture and evaporation on the surface.

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Chapter 3 Evapotranspiration

• Example of a crop coefficient curve.

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REFERENCE CROP
Chapter 3 Evapotranspiration

• The alfalfa is frequently selected as a reference
  crop because, it has high ET rates in arid
  regions.
• Under these conditions, the PET is equal to the
  daily ET.
• The daily ET rates can be measured with the
  sensitive lysimeters.

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CROP COEFFICIENT
Chapter 3 Evapotranspiration

• Crop coefficients are given in table 5. It is
  possible to estimate the consumptive water use
  (ETc) using the crop coefficient and the
  calculated PET relation
• ETc Kc PET

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CROP COEFFICIENT
Chapter 3 Evapotranspiration

• Here are examples of crop coefficients for
  different crops

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BIBLIOGRAPHY
Chapter 3 Evapotranspiration

• 1. Allen, R. G., 1986. A Penman for all seasons.
  J. Irrigation and drainage Division of ASCE,
  112 (4) 348 - 368.
• 2. Doorenbos, J. and W. O. Pruitt, 1977. Crop
  Water Requirements. FAO Irrigation and Drainage
  Division of ASCE Paper 24, Food and Agriculture
  Organization of the United Nations, Rome, pages
  156.
• 3. Goyal, M. R., 1988. Potential
  evapotranspiration for the South Coast Puerto
  Rico with the Hargreaves Samani Technique. J.
  Agric. Univ. P.R., 72 (1) 41 - 50