Evaporation and Transpiration

Evaporation

Evaporation is the process by which water isconverted from its liquid form to its vapor form andthus transferred from land and water masses to theatmosphere.

Evaporation from the oceans accounts for 80% of thewater delivered as precipitation, with the balanceoccurring on land, inland waters and plant surfaces.

Rate of evaporation

Wind speed:

The higher the wind speed, the more evaporation

Temperature:

The higher the temperature, the more evaporation

Humidity:

The lower the humidity, the more evaporation

Factors effecting evaporation

Strength of intermolecular forces

Surface area

Atmospheric Pressure

Humidity

Radiation

Wind Velocity

Temperature

Vapor pressure

Strength of intermolecular forces

The ease of evaporation of a liquid is related to thestrength of the attractive forces between themolecules in the liquid. In polar liquids cohesiveforces are strong while in non-polar liquids thecohesive forces are very weak and the moleculesescape easily

Surface area

The larger the exposed surface area of the liquid the greater is the number of molecules escaping from its surface. Evaporation is directly proportional to the area exposed.

Some important factors

If humidity is more, the water holding capacity of air is less, so lessevaporation. If water content is less in the air, more evaporation, will takeplace.

If atmospheric pressure is more, the evaporation is less and vice versa

Evaporation rate varies directly with the difference of vapor pressurebetween air and water.

Evaporation is directly proportional to radiation. Solar energy near theequator is more, therefore evaporation is much more.

The increase in wind velocity increases evaporation. Wind removes theevaporated water and thereby creates space for new evaporated water.

The rate of evaporation increases as the temperature of a liquid isincreased, as it is an endothermic process. For example, a glass of hot waterevaporates more rapidly than a glass of cold water.

Dalton’s Law of Evaporation

Rate of evaporation is proportional to the differencebetween saturation vapor pressure (SVP) at watertemperature(ew) and actual vapor pressure in the air (ea )

In the initial stages, the rate of evaporation is morethan the rate of condensation because only smallnumbers of molecules are present in the gaseous state.

The state where the rate of evaporation becomes equalto the rate of condensation is called a state of dynamicequilibrium.

Vapor pressure

Magnitude of vapor pressure

The magnitude of vapor pressure depends upon the

following three factors

Nature of liquid

Temperature of the liquid

Presence of impurities

Measurement Of Evaporation

This is done by the following methods

Using evaporimeters

Using empirical equations

By analytical methods

Evaporimeters

These are pans containingwater which are exposed tothe atmosphere. Loss ofwater by evaporation fromthese pans is measured atregular intervals (daily).

Meteorological data such ashumidity, wind velocity, airand water temperatures,and precipitation are alsomeasured and noted alongwith evaporation

USWB Class A Evaporation Pan

A pan of diameter 1210mm and depth255mm

Depth of water is maintained between18 and 20cm

The pan is made of unpainted GI sheet

The pan is placed on a woodenplatform of height 15cm above groundlevel to allow free air circulation belowthe pan

Evaporation is measured by measuringthe depth of water in a stilling well witha hook gauge

ISI Standard Pan

Specified by IS: 5973 and known as the modified Class APan

A pan of diameter 1220mm and depth 255mm, Coppersheet 0.9mm thick, tinned inside and painted white outside

Placed on a square wooden platform of width 1225mm andheight 100mm above ground level to allow free aircirculation below the pan

A fixed point gauge indicates the level of water

Water is added to or removed from the pan to maintain the water level at afixed mark using a calibrated cylindrical measure. The top of the pan iscovered with a hexagonal wire net of GI to protect water in the pan from birds.Presence of the wire mesh makes the temperature of water more uniformduring the day and night. Evaporation from this pan is about 14% lower ascompared to that from an unscreened pan

ISI Standard Pan

Colorado Sunken Pan

920mm square pan made ofunpainted GI sheet, 460mmdeep, and buried into theground within 100mm ofthe top

Main advantage of this pan– its aerodynamic andradiation characteristics aresimilar to that of a lake

Disadvantages – difficult todetect leaks, expensive toinstall, extra care is neededto keep the surroundingarea free from tall grass,dust etc

Principle of Pan evaporation

The principle of the evaporation pan is the following:

The pan is installed in the field, the pan is filled with a known quantity of water (thesurface area of the pan is known and the water depth is measured)

The water is allowed to evaporate during a certain period of time (usually 24 hours).For example, each morning at 7 o'clock a measurement is taken. The rainfall, if any,is measured simultaneously

After 24 hours, the remaining quantity of water (i.e. water depth) is measured A the amount of evaporation per time unit (the difference between the two

measured water depths) is calculated; this is the pan evaporation: E pan (in mm/24hours)

The E pan is multiplied by a pan coefficient, K pan, to obtain the ETo

ETo = K pan × E panwith:

ETo: reference crop evapotranspirationK pan: pan coefficientE pan: pan evaporation

USGS Floating Pan

A square pan of 900mm sides and450mm deep

Supported by drum floats in themiddle of a raft of size 4.25m x4.87m, it is set afloat in a lake witha view to simulate thecharacteristics of a large body ofwater

Water level in the pan ismaintained at the same level asthat in the lake, leaving a rim of75mm

Diagonal baffles are provided in thepan to reduce surging in the pandue to wave action

Disadvantages – High cost ofinstallation and maintenance,difficulty in making measurements

Drawbacks of Evaporation pans

Evaporation pans are not exact models of large reservoirs. Their major drawbacks are the following:

They differ from reservoirs in the heat storage capacity and heattransfer characteristics from the sides and the bottom (sunken andfloating pans aim to minimize this problem). Hence evaporation froma pan depends to some extent on its size (Evaporation from a pan ofabout 3m dia is almost the same as that from a large lake whereas thatfrom a pan of about 1m dia is about 20% in excess of this).

The height of the rim in an evaporation pan affects wind action overthe water surface in the pan. Also it casts a shadow of varying size onthe water surface.

The heat transfer characteristics of the pan material are different fromthat of a reservoir.

Hence evaporation measured from a pan has to be corrected to get theevaporation from a large lake under identical climatic and exposureconditions.

Pan coeffecient

Lake Evaporation = Pan Coefficient x Pan Evaporation

Sl. No. Types of Pan Average Value Range

1 Class A Land Pan 0.70 0.60 – 0.80

2 ISI Pan (Modified

Class A)

0.80 0.65 – 1.10

3 Sunken Pan 0.78 0.75 – 0.86

4 USGS Floating Pan 0.80 0.70 – 0.82

Evaporation Stations

WMO recommends the following values of minimum density of evaporimeters .

Arid Zones – 1 station for every 30,000 sq.km

Humid Temperate Zones – 1 station for every 50,000 sq.km

Cold regions – 1 station for every 1,00,000 sq.km

Typical hydro-meteorological station

Recording rain gauge and non-recording raingauge

Stevenson box with maximum, minimum, wet, and dry bulb thermometers

Wind anemometer and wind vane

Pan evaporimeters

Sunshine Recorder etc

Empirical Equations

Most of the available empirical equations for estimating lake evaporation are a Dalton type equation of the general form.

Meyer’s Formula

Rohwer’s Formula

Wind Velocity

In the lower part of the atmosphere, up to a height of about500m above the ground level, wind velocity follows theone-seventh power law as

Analytical Methods Of Evaporation Estimation

Water Budget Method

Energy Budget Method

Mass Transfer Method

Water Budget Method

If the unit oftime is kept verylarge, estimatesof evaporationwill be moreaccurate. It isthe simplest ofall the methods,but the leastreliable

Energy Budget Method

It involves application of thelaw of conservation ofenergy

Energy available forevaporation is determinedby considering the incomingenergy, outgoing energy,and the energy stored in thewater body over a knowntime interval

Estimation of evaporationfrom a lake by this methodhas been found to givesatisfactory results, witherrors of the order of 5%,when applied to periods lessthan a week

Energy Balance in a water body

This is the energy balancein a period of 1 day. Allenergy terms are incalories/ sq.mm/day.

If time periods are shortHs , Hi can be neglected asthey are negligibly small .

All terms except Ha, caneither be measured orevaluated indirectly .

Ha is estimated usingBowen’s ratio

Comparison Of Methods

Analytical methods can provide good results.However, they involve parameters that are difficultto assess.

Empirical equations can at best give approximatevalues of the correct order of magnitude.

In view of the above, pan measurements find wideacceptance in practice.

Methods to Reduce Evaporation

The annual evaporation from water bodies, in Pakistan,can range from 1- 2 meters .The bigger the surfacemore evaporation. It can be reduced by one or more ofthe following :

Reduction of surface area of reservoir.

Wind breakers. Trees are planted on the windward side of the reservoir. This is useful & effective for small reservoirs

Mechanical covers. The reservoirs are totally covered with cover. This is effective but very expensive.

Monomolecular Films. A thin film of chemical is spread, which reduces the evaporation.

Transpiration

Transpiration is the process of water being takeninto and evaporating from plants

Evapotranspiration

Is a term used to describe the sum of evaporation and planttranspiration from the Earth's land surface to atmosphere

Evaporation accounts for the movement of water to the airfrom sources such as the soil, canopy interception, and waterbodies

Transpiration accounts for the movement of water within aplant and the subsequent loss of water as vapor throughstomata in its leaves

Evapotranspiration is an important part of the water cycle

Evaporation and transpiration occur simultaneously and thereis no easy way of distinguishing between the two processes

Potential Evapotranspiration (PET)

Potential Evapotranspiration (PET)

ASSIGNMENT 4

Estimating evapotranspiration

Catchment water balance

Hydro meteorological equations

Energy balance

Catchment water balance

Evapotranspiration may be estimated by creating anequation of the water balance of a drainage basin. Theequation balances the change in water stored within thebasin (S) with inputs and exports:

The input is precipitation (P), and the exports areevapotranspiration (which is to be estimated), streamflow (Q), and groundwater recharge(D). If the change instorage, precipitation, stream flow, and groundwaterrecharge are all estimated, the missing flux, ET, can beestimated by rearranging the above equation as follows;

ET = P- ∆S – Q - D

Hydro meteorological equations

Blaney- Criddle equation

A purely empirical formuladeveloped based on datafrom arid Western US

Assumes that PET is relatedto the hours of sunshine andtemperature (these aremeasures of solar radiationin an area)

PET (in cm) in a cropgrowing season

Penman equation

where:

m = Slope of the saturation vapor pressure curve (Pa K-1)Rn = Net irradiance (W m-2)ρa = density of air (kg m-3)cp = heat capacity of air (J kg-1 K-1)ga = momentum surface aerodynamic conductance (m s-1)δe = vapor pressure deficit (Pa)λv = latent heat of vaporization (J kg-1)γ = psychrometric constant (Pa K-1)

Penman-Monteith variation

λv = Latent heat of vaporization. Energy required per unit mass of water vaporized. (J/g)Lv = Volumetric latent heat of vaporization. Energy required per water volume vaporized.

(Lv = 2453 MJ m-3)E = Mass water evapotranspiration rate (g s-1 m-2)ETo = Water volume evapotranspired (m3 s-1 m-2)Δ = Rate of change of saturation specific humidity with air temperature. (Pa K-1)Rn = Net irradiance (W m-2), the external source of energy fluxcp = Specific heat capacity of air (J kg-1 K-1)ρa = dry air density (kg m-3)δe = vapor pressure deficit, or specific humidity (Pa)ga = Conductivity of air, atmospheric conductance (m s-1)gs = Conductivity of stoma, surface conductance (m s-1)γ = Psychrometric constant (γ ≈ 66

Pa K-1)

Energy balance

A third methodology to estimate the actual evapotranspiration isthe use of the energy balance.

Where λE is the energy needed to change the phase of waterfrom liquid to gas, Rn is the net radiation, G is the soil heat fluxand H is the sensible heat flux. Using instruments like ascintillometer, soil heat flux plates or radiation meters, thecomponents of the energy balance can be calculated and theenergy available for actual evapotranspiration can be solved.

λE = Rn + G – H

The SEBAL algorithm solves the energy balance at the earthsurface using satellite imagery. This allows for both actual andpotential evapotranspiration to be calculated on a pixel-by-pixelbasis. Evapotranspiration is a key indicator for watermanagement and irrigation performance. SEBAL can map thesekey indicators in time and space, for days, weeks or years

Experimental Method for measuring ET

weighing lysimeter

Potential evaporation in Huwaii

References

^ http://www.oslpr.org/download/en/2000/0031.pdf ^ Swank, W., and Douglass, J. 1974, Science.

185(4154):857-859 ^ Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. (1998).

Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations. ISBN 92-5-104219-4. http://www.fao.org/docrep/X0490P/x0490p00.HTM. Retrieved 2007-10-08

^ http://www.waterwatch.nl/tools0/sebal.htWater Evaluation And Planning system (WEAP)