Evapotranspiration - measurement of ET - Lysimeter, Field experiment plot

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Lecture 4 Evapotranspiration - measurement of ET
- Lysimeter, Field experiment plot soil
moisture depletion study, Water balance method -
evaporation methods.
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• EVAPORATION.
• The process during which a liquid changes into a
  gas.
• One of the fundamental components of the
  hydrological cycle by which water changes to
  vapour through the absorption of heat energy.
• This is the only form of moisture transfer from
  land and oceans into the atmosphere.

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TRANSPIRATION. The process by which water
vapour leaves the living plant body and enters
the atmosphere.
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Evapo-transpiration (ET). or consumptive use
(Cu) The quantity of water transpired by plants
during their growth or retained in the plant
tissue, plus the moisture evaporated from the
surface of the soil and the vegetation.
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POTENTIAL EVAPO-TRANSPIRATION (PET) Thornthwaite
(1948) defined it as the evapo-transpiration from
a large vegetation covered land surface with
adequate moisture at all times. He felt that
since the moisture supply was not restricted the
PET depended solely on available energy. Penman
(1947) defined PET as the ET from an actively
growing short green vegetation completely shading
the ground and never short of moisture
availability. Jensen (1968) assumed PET as the
upper limit of ET that would occur with a well
watered agricultural crop having an
aerodynamically rough surface such as Lucerne
with 30 to 50 cm of top growth.
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• Under field conditions incoming solar radiation
  supplies the energy for the evapotranspiration
  process.
• Wind is important in removing water vapour from
  the cropped area and the prevailing temperature
  and humidity conditions result from the
  interaction of the two processes.
• Usually a close relationship exists between net
  incoming solar radiation and evapotranspiration.

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• The stage of growth of the crop has a
  considerable influence on its consumptive use
  rate, especially for annual crops which generally
  have three distinct stages of growth.
• emergence and development of complete vegetative
  cover, during which time consumptive use rate
  increases rapidly from a low value and approaches
  its maximum
• the period of maximum vegetative cover during
  which time the consumptive use rate may be
  maximum if abundant soil moisture is available
• crop maturation stage, when for most crops, the
  consumptive use rate begins to decrease.

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• Measurement of Evapotranspiratlon
• Lysimeter experiment
• Field experimental plots
• Soil moisture depletion studies
• Water balance method

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• Lysimeter studies involve the growing of crops in
  large containers (lysimeters) and measuring their
  water loss and gains.
• A lysimeter can be defined as a device in which a
  volume of soil planted with vegetation is located
  in a container to isolate it hydrologically from
  the surrounding soil.
• Types of lysimeters
• Non-weighing type
• weighing type.

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The major limitations are the reproduction of
physical conditions such as temperature, water
table, soil texture and density etc., within the
lysimeter comparable to those outside in the
field.
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Field experimental plots.
WR seasonal water requirement, cm IR total
irrigation water applied, cm ER - seasonal
effective rainfall, cm Mbi moisture percentage
at the beginning of the season in the ith layer
of the soil Mei moisture
percentage at the end of the season in the ith
layer of the soil Ai apparent specific gravity
of the ith layer of the soil Di depth of the
ith layer of the soil within the root zone, cm n
number of soil layers in the root zone D
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Soil moisture depletion studies
u Water used from root zone between sampling,
cm M1i moisture percentage at first sampling in
the ith layer of the soil M2i moisture
percentage at second sampling in the ith layer of
the soil Ai apparent specific gravity of the
ith layer of the soil Di depth of the ith
layer of the soil within the root zone, cm n
number of soil layers in the root zone
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Water balance method. The water balance method,
also called the inflow-outflow method, is
suitable for large areas (watersheds) over long
periods. Precipitation Evapotranspiration
surface runoff sub-surface drainage
change in soil water contents
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Estimating Evapotranspiration from Evaporation
Data A close relationship exists between the
rate of consumptive use by crops and the rate of
evaporation from a properly located evaporation
pan. The standard US Weather Bureau Class A
open pan evaporimeter described earlier or the
sunken screen open pan evapori-meter may be used
for the measurement. Evapotranspiration pan
evaporation x crop factor
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Lecture 5 Estimating ET by climatological data -
Blaney Criddle - modified Penman method
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Evapotranspiration is often predicted on the
basis of Climatological data. Relate the
magnitude and variation of ET to one or more
climatic factors such as temperature, day length,
humidity, wind, sunshine, etc. Broadly these
approaches fall in two classes, (1)purely
empirical attempts to correlate ET with one or
more climatic factors (2) the application of a
more theoretical approach.
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Blaney and Criddle (1950) observed that the
amount of water consumptively used by crops
during their growing seasons was closely related
with mean monthly temperature and daylight hours.
U K.F ? k. f ?u
In which, Useasonal consumptive use of water by
the crop for a given period, inches umonthly
consumptive use, inches Kempirical seasonal
consumptive use crop coefficient for the growing
season Fsum of the monthly consumptive use
factor(f) for the growing season Kempirical
consumptive use crop coefficient for the
monthu/f tmean monthly temperature,
?F pmonthly daylight hours expressed as
percentage of day light hours of the year
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• Doorenbas and Pruitt (1975) have rejected the use
  of crop coefficient(K)normally applied in the
  original Blaney Criddle approach, because
• the original crop coefficient(K) are heavily
  depend on local conditions ,and wide varieties of
  K values reported in literature make the
  selection of this value rather difficult
• the relationship between Blaney-Criddle f-values
  and can be adequately described for a wide range
  of temperatures for areas having minor variations
  in relative humidity, sunshine and wind velocity
• once PET has been determined by any standard
  method, one set of crop factors (k c) can be
  used to determine crop ET.

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the following relationship for f factor
(expressed in mm/day) in Blaney-Criddle formula
f p (0.46 t 8.13), using t in ?C.
or f , using t in ?F.
in which, t the mean of daily maximum and
minimum temperature in ?C or ?F over the month
considered p the mean daily percentage of
annual day time hours for a given month and
latitude.
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Penman Formula

• Eo Evaporation from open water surface
  ,mm/day
• ? slope of saturation vapour pressure vs
  temperature
• curve (dEa /dT) at the mean air
  temperature Ta, mm Hg
• per oC
• Ea saturation vapour pressure of the
  evaporating surface
• (es) in mm Hg at mean air temperature Ta.
  here es is
• considered equal to ea by assuming zero
  temperature
• gradient between surface(s) and air
  temperatures.
• Ta mean air temperature in oK 273 oC

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• Qn net radiation (mm of water )
• Qa (1- r)(0.18 0.55 n/N) - dTa4 (0.55
  -0.092 ved ) ( 0.10 0.90 n/N )
• r reflection coefficient of evaporatiing
  surface, 0.0 6 for open water surface.
• QA Angots value of mean monthly extra
  terrestrial radiation , mm of water /day .
• n/N ratio between actual and possible hours of
  bright sunshine .
• d Stefan Boltzman constant .
• ed saturation vapour pressure of the
  atmosphere , in mm Hg , at dew point
  temperature (RHmean /100) ea, in which RH
  is the mean relative humidity.

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• ? psychrometric constant or the ratio of
  specific heat of air to the latent heat of
  evaporation of water (0.49) for 0 celcius and mm
  Hg)
• Eaan aerodynamic component in which ,es is
  considered equal t ea 0.35(ea-ed)(10.0098 u2)
• u2wind speed in miles/day at 2 miles per day at
  any other height h in feet.

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• MODIFIED PENMAN FORMULA
• ETo W . R n (1- w) .f(u) .(ea ed )
• radiation term aerrodynamic term.
• ETo the refernce crop evapotranspiration
  in mm / day (not adjusted)
• ea saturation vapour pressure in mbar at
  the mean air
• temperature in 0C
• ed mean actual vapour pressure of the air
  in mbar
• ea (RH mean /100 ) in which ,RH
  relative humidity. This can also be
  determined from dry and wet bulb temp. or dew
  point temp.
• F(u) a wind related function .
• (1- W ) a temperature and elevation related
  weighting factor for the effect of wind
  and humudity on ETc.
• W a temperature and elevation related
  weighting factor for the effect of wind and
  humudity on ETc .

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• Rn net radiation (same as Qn Rns Rnl )
• In which
• Rns the net incoming shortwave solar
  radiation Ra (1-a) (0.25 0.50 n/N ) in which
  Ra is same as QA or extra terrestrial
  radiation expressed in equivalent evaporation
  inn mm/day , n/N is the same as explained in
  Penman , and a is same as r or reflection
  coefficient the value of which is taken as 0.25
  for most crops gives conversion factors for RA
  to Rns for a given reflection of 25 per cent and
  ratios for n/N, and
• Rnl the net long wave radiation f(t)
  .f(ed).f(n/N), the values of which are given in
  Appendix F ,Tables F11,F12,F13 respectively.