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EVAPORATION, TRANSPIRATION &

EVAPOTRANSPIRATION

ALEX.K.GEORGEBSF-10-002

Evaporation is the process during which a liquid changes into gas.

Water changes to vapour through the absorption of heat

One of the fundamental component of hydrological cycle

Essential requirements in the process are1. The source of energy to vaporize the liquid water

(solar or wind) 2. The presence of gradient of concentration between

the evaporating surface and the surrounding air.

Evaporation is defined as a function of the differences in the vapour pressure of the water and the vapour pressure of the air.

-Dalton (1882)

E = (es - ed) f(u)

E = evaporation

es = saturation vapour pressure at the temperature of evaporating surface

ed = saturation vapour pressure at the dew point temperature of the atmosphere

f(u) = a function of the wind velocity.

1. Degree of saturation of surface2. Temperature of surface and air3. Humidity 4. Wind velocity5. Vegetation cover

PAN EVAPORIMETERPICHE EVAPORIMETER

Transpiration is the process by which water vapour leaves the living plant body and enters the atmoshere.

Michel (1978)It involves continuous flow of water from soil in to

plant and out through stomata (leaves) to the atmosphere.

Basically an evaporation process.Transpiration Ratio: The amount of water transpired

by a crop in its growth to produce unit weight of dry matter.

1. Climate1. Light intensity2. Atmospheric vapour pressure3. Temperature 4. Wind

2. Soil1. Availability of water

3. Plant factors1. Extent and efficiency of root system2. Leaf area3. Leaf arrangement & structure4. Stomatal behaviour

A potometer sometimes known transpirometer is a device used for measuring the rate  transpiration.

Types:1. Ganong's Potometer2. Darwin's Potometer3. Gaurrea's Potometer4. farmer potometer

POTOMETER

Evapotranspiration (ET) is the quantity  of  water  transpired by  the  plants  during  their growth  or  retained  in  plant tissue,  plus  the  moisture evaporated  from  the  surface of the soil and the vegetation.

                     Michel (1978)

It accounts for the movement of water to the air from sources such as the soil, canopy interception, and water bodies.

POTENTIAL EVAPOTRANSPIRATION

(PET)

Theoretical amount of moisture that could be lost from the surface to the atmosphere if it were available.

The amount of moisture which, if available, would be removed from a given land area by evapotranspiration.

Expressed in units of water depth.

EFFECTIVE EVAPOTRANSPIRATION

(EET)

Actual amount of water lost due to evapotranspiration from the soil along with actively growing plant or crop.

depends upon plant and soil characteristics, and upon the amount of available water in the soil.

1. Lysimeter experiment2. Field experimental plots3. Soil moisture depletion studies4.Water balance/budget method5. Eddy covariance

6.By using US-open pan evaporimeter7. Energy balance

Lysimeter  is  adevice  in  which  a volume  of  soil  planted  with vegetation is located in a container to isolate it hydrologically from the surronding soil.

Having  a  weighing  device  and  a drainage  system,  which  permit continuous measurement of  excess water  and draining  below  the  root zone  and  plant  water  use,  and hence evapotranspiration.  

The  amount  of  water  lost  by evapotranspiration  can  be  worked out  by  calculating  the  difference between  the  weight  before  and after the precipitation input.

ET = P + (I – D) + S WHERE, ET = EVAPOTRANSPIRATIONP = PRECIPITATIONI = IRRIGATION WATERD = EXCESS WATER DRAINED FROM BOTTOMS = INCREASE OR DECREASE IN STORAGE OF

SOIL MOISTURE

Direct method of measuring evapotranspiration

fast fluctuations of vertical wind speed are correlated with fast fluctuations in atmospheric water vapour density.

Directly estimates the transfer of water vapour (evapotranspiration) from the land (or canopy) surface to the atmosphere.

ETo = KC × E pan where, ETo : reference crop

evapotranspiration KC : crop coefficient E pan : pan evaporation

Formula –Formula – ETET = P – Q – = P – Q – ΔΔS - S - ΔΔDD where, ΔS= watershed storage variation (mm): Send–Sbeginning

P = Precipitation (mm)Q = Stream flow (mm)ΔD = Seepage out – seepage in (mm)ET = evaporation and transpiration (mm)

FORMULA- Rn - G - H = λETWhere,Rn : Net surface radiation flux density (Wm-2)G : Ground heat flux density (Wm-2)H : Sensible heat flux density (Wm-2)λET : Latent heat flux density (Wm-2)λ : Latent heat of vaporization of water (Jkg-1)

1. Thornthwaite equation

2. Hargreaves equation

3. Net radiation (Rn) based method

e = 1.6(10t/I)a

WHERE:e = un adjusted potential ET (cm/month)t = mean air temperature (celcius)I = annual or seasonal heat indexa = an emperical exponent computed

ET = 0.0023 (Tm + 17.8)(T max – Tmin )1/2.Ra where, Tm – daily mean temperature Ra – extraterrestrial radiation [MJ m-2 day-1].

ET = 0.489 + 0.289 Rn + 0.023 T mean

where Rn [MJ m-2 day-1] is net radiation.

When a surface evaporates, it looses energy and cools itself.

It is that cooling that can be observed from space.

Satellites can map the infrared heat radiated from Earth, thus enabling to distinguish the cool surfaces from the warm surfaces.

Satellite map of evapotranspiration of whole world in the season of winter and summer

winter summer

Energy availability - The more energy available, the greater the rate of Evapotranspiration. It takes about 600 calories of heat energy to change 1 gram of liquid water into a gas.

Humidity gradient - The rate and quantity of water vapour entering into the atmosphere both become higher in drier air.

Water availability - Evapotranspiration cannot occur if water is not available.

Wind speed – higher the wind speed, greater will the rate of evapotranspiration.

Physical attributes of the vegetation - factors as vegetative cover, plant height, leaf area index and leaf shape and the reflectivity of plant surfaces can affect rates of evapotranspiration.

Soil characteristics - Soil characteristics that can affect evapotranspiration include its heat capacity, and soil chemistry and albedo.

http://www.eoearth.org/article/Evapotranspiration http://www.oslpr.org/download/en/2000/0031.pdf http://www.waterwatch.nl/tools0/sebal.html Gray, D.M. (ed.) (1970) Handbook on the Principles of

Hydrology, Can. Nat. Com. Int. Hydrol.Decade, Nat. Res. Council Can., Ottawa.

Tim Davie, Fundamentals of hydrology (Routledge, 2003) Rana, G. and N. Katerji. 2000. Measurement and estimation

of actual evapotranspiration in the field under Mediterranean climate.

Michael A.M, 1978. Irrigation Theory and Practice. Vikas Publishing House Pvt Ltd, New Delhi.

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