measurement of ET -
Lysimeter, Field experiment
plot – soil moisture
depletion study, Water
balance method -
- 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 process by which water
vapour leaves the living plant
body and enters the atmosphere.
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.
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.
Under field conditions incoming
solar radiation supplies the energy
for the evapo-transpiration 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.
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.
(i) emergence and development of complete
vegetative cover, during which time
consumptive use rate increases rapidly from a
low value and approaches its maximum
(ii) the period of maximum vegetative cover
during which time the consumptive use rate
may be maximum if abundant soil moisture is
(iii) crop maturation stage, when for most crops,
the consumptive use rate begins to decrease.
Measurement of Evapotranspiratlon
2.Field experimental plots
3.Soil moisture depletion studies
4.Water balance method
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
Types of lysimeters:
(ii) weighing type.
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.
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
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
n = number of soil layers in the root zone
Water balance method.
The water balance method, also called
the inflow-outflow method, is suitable
for large areas (watersheds) over long
Precipitation = Evapotranspiration +
surface runoff +
sub-surface drainage +
change in soil water
Estimating Evapotranspiration from
A close relationship exists between the
rate of consumptive use by crops and the
rate of evaporation from a properly located
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
Estimating ET by
climatological data -
Blaney Criddle -
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,
Broadly these approaches fall in two
(1)purely empirical attempts to correlate
ET with one or more climatic factors
(2) the application of a more theoretical
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.
k t p
U = K.F = k. f = u =
U=seasonal consumptive use of water by the crop for a given period,
u=monthly consumptive use, inches
K=empirical seasonal consumptive use crop coefficient for the
F=sum of the monthly consumptive use factor(f) for the growing
K=empirical consumptive use crop coefficient for the month=u/f
t=mean monthly temperature, F
p=monthly daylight hours expressed as percentage of day light hours
of the year
Doorenbas and Pruitt (1975) have rejected the use of
crop coefficient(K)normally applied in the original
Blaney Criddle approach, because
(1)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
(2)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
(3)once PET has been determined by any standard
method, one set of crop factors (k c-) can be used
to determine crop ET.
the following relationship for ‘f’ factor
(expressed in mm/day) in Blaney-Criddle
f = p (0.46 t + 8.13), using t in C.
or f = 25.4 , using t in F.
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.
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
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
Qn = net radiation (mm of water )
= Qa (1- r)(0.18 + 0.55 n/N) - δTa4 (0.55 -0.092 √ed ) ( 0.10 +0.90 n/N )
r = reflection coefficient of evaporatiing surface,
0.0 6 for open water surface.
QA = Angot’s value of mean monthly extra
terrestrial radiation , mm of water /day .
n/N = ratio between actual and possible hours of
bright sunshine .
δ = 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.
= ﻻpsychrometric 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)
Ea=an aerodynamic component in which ,es is
considered equal t ea =0.35(ea-ed)(1+0.0098 u2)
u2=wind speed in miles/day at 2 miles per day at
any other height h in feet.
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
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 .
Rn = net radiation (same as Qn = Rns – Rnl )
Rns = the net incoming shortwave solar radiation
– Ra (1-α) (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 α 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.