8/2/2019 56349-27238-SWC 302 -Lec 4&5

1/24

Lecture 4Evapotranspiration -measurement of ET -

Lysimeter, Field experimentplot soil moisture

depletion study, Water

balance method -evaporation methods.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

2/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

3/24

TRANSPIRATION.

The process by which water

vapour leaves the living plant

body and enters the atmosphere.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

4/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

5/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

6/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

7/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

8/24

Measurement of Evapotranspiratlon

1.Lysimeter experiment2.Field experimental plots

3.Soil moisture depletion studies

4.Water balance method

8/2/2019 56349-27238-SWC 302 -Lec 4&5

9/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

10/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

11/24

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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

12/24

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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

13/24

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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

14/24

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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

15/24

Lecture 5Estimating ET by

climatological data -Blaney Criddle -

modified Penmanmethod

8/2/2019 56349-27238-SWC 302 -Lec 4&5

16/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

17/24

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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

18/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

19/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

20/24

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 )

8/2/2019 56349-27238-SWC 302 -Lec 4&5

21/24

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.

8/2/2019 56349-27238-SWC 302 -Lec 4&5

22/24

=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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

23/24

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

8/2/2019 56349-27238-SWC 302 -Lec 4&5

24/24

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.