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Transcript of 56349-27238-SWC 302 -Lec 4&5
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Lecture 4Evapotranspiration -measurement of ET -
Lysimeter, Field experimentplot soil moisture
depletion study, Water
balance method -evaporation methods.
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EVAPORATION.
- The process during which a liquidchanges into a gas.
- One of the fundamental components
of the hydrological cycle by whichwater changes to vapour through the
absorption of heat energy.
- This is the only form of moisturetransfer 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 consumptiveuse (Cu)
The quantity of water transpired byplants during their growth or retainedin the plant tissue, plus the moisture
evaporated from the surface of thesoil and the vegetation.
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POTENTIAL EVAPO-TRANSPIRATION (PET)
Thornthwaite (1948) defined it as the evapo-transpiration from a large vegetation covered landsurface with adequate moisture at all times. He felt thatsince the moisture supply was not restricted the PETdepended solely on available energy.
Penman (1947) defined PET as the ET from an activelygrowing short green vegetation completely shading theground and never short of moisture availability.
Jensen (1968) assumed PET as the upper limit of ET thatwould occur with a well watered agricultural crop havingan aerodynamically rough surface such as Lucerne with30 to 50 cm of top growth.
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Under field conditions incoming
solar radiation supplies the energyfor the evapotranspiration process.
Wind is important in removing water
vapour from the cropped area andthe prevailing temperature andhumidity 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 considerableinfluence on its consumptive use rate,
especially for annual crops which generallyhave three distinct stages of growth.
(i) emergence and development of completevegetative cover, during which time
consumptive use rate increases rapidly from alow value and approaches its maximum
(ii) the period of maximum vegetative coverduring which time the consumptive use rate
may be maximum if abundant soil moisture isavailable
(iii) crop maturation stage, when for most crops,the consumptive use rate begins to decrease.
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Measurement of Evapotranspiratlon
1.Lysimeter experiment2.Field experimental plots
3.Soil moisture depletion studies
4.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 inwhich a volume of soil planted withvegetation is located in a container to
isolate it hydrologically from thesurrounding soil.
Types of lysimeters:
(i)Non-weighing type
(ii) 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.xMMb
ERIRWR
n
i
110
)
WR = seasonal water requirement, cmIR = total irrigation water applied, cmER =- seasonal effective rainfall, cmMbi = 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 ithlayer of the soil
Ai = apparent specific gravity of the ith layer of the soilDi = depth of the ith layer of the soil within the root zone, cmn = number of soil layers in the root zone D
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Soil moisture depletion studies
xiMiMun
i
1 10
)21
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 soilAi = apparent specific gravity of the ith layer of the soil
Di = depth of the ith layer of the soil within the rootzone, 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 suitablefor large areas (watersheds) over longperiods.
Precipitation = Evapotranspiration +surface runoff +
sub-surface drainage +
change in soil water
contents
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Estimating Evapotranspiration fromEvaporation Data
A close relationship exists between therate of consumptive use by crops and therate of evaporation from a properly locatedevaporation pan.
The standard US Weather Bureau Class Aopen pan evaporimeter described earlier or
the sunken screen open pan evapori-metermay be used for the measurement.
Evapotranspiration = pan evaporation x
crop factor
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Lecture 5Estimating ET by
climatological data -Blaney Criddle -
modified Penmanmethod
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Evapotranspiration is often predicted onthe basis of Climatological data.
Relate the magnitude and variation of ETto one or more climatic factors such astemperature, day length, humidity, wind,
sunshine, etc.
Broadly these approaches fall in two
classes,(1)purely empirical attempts to correlateET with one or more climatic factors(2) the application of a more theoreticalapproach.
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Blaney and Criddle (1950) observed that the amount ofwater consumptively used by crops during their growingseasons was closely related with mean monthly
temperature and daylight hours.
U = K.F = k. f = u =In which,
U=seasonal consumptive use of water by the crop for a given period,inches
u=monthly consumptive use, inchesK=empirical seasonal consumptive use crop coefficient for the
growing season
F=sum of the monthly consumptive use factor(f) for the growingseason
K=empirical consumptive use crop coefficient for the month=u/ft=mean monthly temperature, Fp=monthly daylight hours expressed as percentage of day light hours
of the year
100
ptk
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Doorenbas and Pruitt (1975) have rejected the use ofcrop coefficient(K)normally applied in the originalBlaney Criddle approach, because
(1)the original crop coefficient(K) are heavily dependon local conditions ,and wide varieties of K valuesreported in literature make the selection of thisvalue rather difficult
(2)the relationship between Blaney-Criddle f-valuesand can be adequately described for a wide rangeof 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 usedto determine crop ET.
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the following relationship for f factor
(expressed in mm/day) in Blaney-Criddleformula
f = p (0.46 t + 8.13), using t in C.or f = , using t in F.
1004.25
tp
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 timehours 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 Hgper oC
Ea = saturation vapour pressure of the evaporating surface
(es) in mm Hg at mean air temperature Ta. [here es isconsidered equal to ea by assuming zero temperaturegradient between surface(s) and air temperatures.]
Ta =mean air temperature in oK =273 +oC
EaQnEo
Q t di ti ( f t )
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Qn = net radiation (mm of water )= Qa (1- r)(0.18 + 0.55 n/N) - Ta4 (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= Angots value of mean monthly extra
terrestrial radiation , mm of water /day .
n/N = ratio between actual and possible hours ofbright sunshine .
= Stefan Boltzman constant .
ed= saturation vapour pressure of theatmosphere , in mm Hg , at dew pointtemperature =(RHmean /100) * ea, in whichRH is the mean relative humidity.
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=psychrometric constant or the ratio of specificheat of air to the latent heat of evaporation ofwater (0.49) for 0 celcius and mm Hg)
Ea=an aerodynamic component in which ,es isconsidered equal t ea =0.35(ea-ed)(1+0.0098 u2)
u2=wind speed in miles/day at 2 miles per day atany other height h in feet.
MODIFIED PENMAN FORMULA
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MODIFIED PENMAN FORMULA
ETo * =W . R n + (1- w) .f(u) .(eaed )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 in0
Ced = mean actual vapour pressure of the air in mbar= ea *(RH mean /100 ) in which ,RH == relative
humidity. This can also be determined from dryand 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 .
R
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Rn = net radiation (same as Qn = Rns Rnl )In which
Rns = the net incoming shortwave solar radiation
Ra (1-) (0.25 +0.50 n/N ) in which Ra is same asQA or extraterrestrial radiation expressed inequivalent evaporation inn mm/day , n/N is the
same as explained in Penman , and is same as ror reflection coefficient ; the value of which istaken as 0.25 for most crops gives conversionfactors for RA to Rns for a given reflection of 25 per
cent and ratios for n/N, andRnl = 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.