Lecture 4Evapotranspiration - measurement of ET -

Lysimeter, Field experiment plot – soil moisture

depletion study, Water balance method -

evaporation methods.

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.

TRANSPIRATION.

The process by which water vapour leaves the living plant body and enters the atmosphere.

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.

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 available

(iii) crop maturation stage, when for most crops, the consumptive use rate begins to decrease.

Measurement of Evapotranspiratlon

1.Lysimeter experiment 2.Field experimental plots3.Soil moisture depletion studies4.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 surrounding soil.

Types of lysimeters: (i)Non-weighing type (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.

xAixDi

MeiMbiERIRWR

n

i

1 100

)

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 soilAi = apparent specific gravity of the ith layer of the soil Di = depth of the ith layer of the soil within the root

zone, cmn = number of soil layers in the root zone D

Soil moisture depletion studies

xAixDiiMiM

un

i

1 100

)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 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

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

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

Lecture 5Estimating ET by

climatological data - Blaney Criddle -

modified Penman method

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.

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, U=seasonal consumptive use of water by the crop for a

given period, inchesu=monthly consumptive use, inchesK=empirical seasonal consumptive use crop coefficient for

the growing seasonF=sum of the monthly consumptive use factor(f) for the

growing seasonK=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

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 formula 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 time hours for a given month and latitude.

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 is considered equal to ea by assuming zero temperature gradient between surface(s) and air temperatures.]

Ta =mean air temperature in oK =273 +oC

EaQn

Eo

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.

psychrometric constant or the ratio of= ال 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 (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 .

Rn = net radiation (same as Qn = Rns – Rnl )

In which

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.