03a evaporation

7/23/2019 03a Evaporation

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Evaporation,Evaporation,

Transpiration,Transpiration,

EvapoTranspirationEvapoTranspiration

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Losses of Precipitation

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EvaporationEvaporation

Evaporation happensEvaporation happens

in several waysin several ways Open water evaporationOpen water evaporation

Transpiration fromTranspiration fromleavesleaves

Evaporation from soilEvaporation from soil

and land surfaceand land surface

Evapotranspiration

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Terminology Terminology EvaporationE vaporation – process by which liquid– process by which liquid

water passes directly to the vapor phasewater passes directly to the vapor phase

The rate of evaporation depends upon theThe rate of evaporation depends upon the

water temperature and the temperaturewater temperature and the temperatureand humidity of the air above the water.and humidity of the air above the water.

Humidity refers to the amount of moistureHumidity refers to the amount of moisture

in the air; more specically:in the air; more specically:

bsolute humidity ! mass of water per unit volume of air "usually #rams bsolute humidity ! mass of water per unit volume of air "usually #ramswater per cubic meter of air$water per cubic meter of air$

%aturation humidity ! ma&imum amount of moisture the air can hold at a%aturation humidity ! ma&imum amount of moisture the air can hold at a

#iven temperature#iven temperature

'elative humidity ! the absolute humidity over the saturation humidity'elative humidity ! the absolute humidity over the saturation humidity

"i.e.( the percent ratio of the amount of moisture in the air to the total"i.e.( the percent ratio of the amount of moisture in the air to the total

amount it could possibly hold$amount it could possibly hold$

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TranspirationTranspiration

TranspirationTr anspiration ! process by which! process by which

liquid water passes from liquid toliquid water passes from liquid to

vapor throu#h plant metabolism vapor throu#h plant metabolism

)lants ta*e up water for their own)lants ta*e up water for their own

use "i.e.( for buildin# plant tissue$(use "i.e.( for buildin# plant tissue$(

Only about +, of what they suc* upOnly about +, of what they suc* up

#ets used; the rest is released to the#ets used; the rest is released to the

atmosphere throu#h leaves.atmosphere throu#h leaves.

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EvapotranspirationEvapotranspiration

-hen studyin# water in the eld( one-hen studyin# water in the eld( one

cannot separate water lost to evaporationcannot separate water lost to evaporation

from transpiration lossesfrom transpiration losses

t is typical to lump them to#ether ast is typical to lump them to#ether asevapotranspiration "E!T$.evapotranspiration "E!T$. )otential evapotranspiration)otential evapotranspiration is the water loss that would occur if thereis the water loss that would occur if there

is an unlimited supply of water available for transpiration andis an unlimited supply of water available for transpiration and

evaporation.evaporation.

n reality( the amount of water that transpires or evaporates is limitedn reality( the amount of water that transpires or evaporates is limited

by the amount of water that is available. f the amount of waterby the amount of water that is available. f the amount of water

available is less than the potential( then theavailable is less than the potential( then the actual evapotranspirationactual evapotranspiration will be lower than the potential.will be lower than the potential.




Factors InuencingFactors Inuencing

EvaporationEvaporation Ener#y supply for vapori/ationEner#y supply for vapori/ation

"latent heat$"latent heat$ %olar radiation%olar radiation

TemperatureTemperature Ener#y input for evaporation to proceedEner#y input for evaporation to proceed

The hi#her the temperature the hi#herThe hi#her the temperature the hi#her

the capacity of air to absorb more waterthe capacity of air to absorb more water




Factors inuencingFactors inuencing

evaporationevaporation Transport of vapor away fromTransport of vapor away from

evaporative surfaceevaporative surface -ind velocity over surface-ind velocity over surface

%pecic humidity #radient above%pecic humidity #radient abovesurfacesurface




-ind has a ma0or e1ect on-ind has a ma0or e1ect on

evaporation( Eevaporation( E 2y convection wind removes vapor!laden2y convection wind removes vapor!laden

airair 3eepin# the boundary layer thin and3eepin# the boundary layer thin and

maintainin# a hi#h transfer rate of watermaintainin# a hi#h transfer rate of water

from liquid to vapor phasefrom liquid to vapor phase

-ind is also turbulent( increasin# the-ind is also turbulent( increasin# the

di1usion several orders of ma#nitude overdi1usion several orders of ma#nitude over

that of molecular di1usionthat of molecular di1usion




Factors inuencingFactors inuencing

evaporationevaporation 4e#etated surfaces56rop resistance 4e#etated surfaces56rop resistance

The transpiration of cropped surface isThe transpiration of cropped surface is

usually less than the evaporation of anusually less than the evaporation of an

open water surfaceopen water surface %upply of moisture to the surface%upply of moisture to the surface

Evapotranspiration "ET$Evapotranspiration "ET$

)otential Evapotranspiration ")ET$ –)otential Evapotranspiration ")ET$ –moisture supply is not limitedmoisture supply is not limited




Measurement ofMeasurement of

evaporationevaporation "a$ Evaporimeters"a$ Evaporimeters

6lass !Evaporation pan6lass !Evaporation pan

% standard pan% standard pan

6olorado %un*en pan6olorado %un*en pan

7% #eolo#ical survey 8oatin# pan7% #eolo#ical survey 8oatin# pan




Evaporation from aEvaporation from a

Water Surface Water Surface %implest form of evaporation%implest form of evaporation

9rom free liquid of permanently9rom free liquid of permanently

saturated surfacesaturated surface




Evaporation from a PanEvaporation from a Pan

ational -eather %ervice 6lass ational -eather %ervice 6lass typetype

nstalled on a wooden platform in anstalled on a wooden platform in a#rassy location#rassy location

9illed with water to within .<9illed with water to within .<inches of the topinches of the top

Evaporation rate is measured byEvaporation rate is measured bymanual readin#s or with an analo#manual readin#s or with an analo#output evaporation #au#eoutput evaporation #au#e

h

rea( A

6%

w ρ

a ρ

AE m wv ρ =

dt

dh E −=

n R s H

%ensibleheat to air

et radiation 4apor 8ow rate

Heat conductedto #round

G




The most commonly use evaporimeter is the class A pan

This is a pan !ith "#"m $iameter an$ $epth of ##%mmthe pan is set "%& mm a'ove the groun$ to allo! air to movefreely aroun$Evaporation is measure$ $aily as the amount of !aterevaporate$ from the pan At the 'eginning of the $ay, the pan is (lle$ to the %&mm

from the top an$ is left to evaporate for #) hrsTo $etermine the amount of !ater evaporate$ $uring theDecember 28, 2015 19



Pan coe*icientPan coe*icient

Evaporation pans are not e&act modelsEvaporation pans are not e&act models

of lar#e reservoirs and have theof lar#e reservoirs and have the

followin# drawbac*s:followin# drawbac*s:

They di1er in heat storin# capacity andThey di1er in heat storin# capacity andheat transfer from the sides and bottomheat transfer from the sides and bottom

The hei#ht of the rim in an evaporation panThe hei#ht of the rim in an evaporation pan

a1ects the wind action over the surfacea1ects the wind action over the surface

The heat transfer characteristics of the panThe heat transfer characteristics of the pan

are di1erent from that of the reservoirare di1erent from that of the reservoir




+alues for Pan +alues for Pan

oe*icient poe*icient p

Types of Pan Average value

-ange

6lass =and )an >.?> >.@>!>.A>

% pan >.A> >.@<!+.+>

6olorado %un*en)an

>.?A >.?<!>.A@

7%B% 8oatin#pan

>.A> >.?>!>.A




.'/ Empirical Metho$s.'/ Empirical Metho$s

=ar#e empirical equations are available to =ar#e empirical equations are available toestimate 9ree water evaporation usin# commonlyestimate 9ree water evaporation usin# commonly

available meteorolo#ical data.available meteorolo#ical data.

Cost formulae are based on the Dalton!typeCost formulae are based on the Dalton!type

equation and can be e&pressed as:equation and can be e&pressed as:

-here;-here;

EE== free water evaporation in mm5day free water evaporation in mm5day

eess saturation vapour pressure at the water!surface saturation vapour pressure at the water!surface

temperature in mm of mercurytemperature in mm of mercury

eeaa actual vapour pressure of overlyin# air at a specied actual vapour pressure of overlyin# air at a specied

hei#hthei#ht

f"u$ wind speed correctionf"u$ wind speed correction

3 is a coe1icient3 is a coe1icient

))(( a s L eeu Kf E −=




Meyer0s

e1uation2

( ) ( )V eeC E a s 06215.01+−=

E 3 evaporation in mm4monthes 3 saturation vapour pressure

ea 3 actual vapour pressure in mm

+ 3 monthly mean !in$ velocity in 5m4hr 3 "% for small shallo! pon$s




.c/ Analytical Metho$s of.c/ Analytical Metho$s of

Estimating EvaporationEstimating Evaporation -ater!bud#et method-ater!bud#et method

Ener#y 2alance CethodEner#y 2alance Cethod

erodynamic method "Cass!transfer erodynamic method "Cass!transfermethod$method$

)enman Cethod "6ombined method$)enman Cethod "6ombined method$




.i/ Water67u$get Metho$.i/ Water67u$get Metho$

The water bud#et method is the simplest ofThe water bud#et method is the simplest of

the three analytical methods.the three analytical methods.

t involves writin# the hydrolo#icalt involves writin# the hydrolo#ical

continuity equation for the free watercontinuity equation for the free waterstora#e systemstora#e system

The ideal way of estimatin# evaporationThe ideal way of estimatin# evaporation

from any la*e( reservoir( or catchmentfrom any la*e( reservoir( or catchment

would be to measure the variouswould be to measure the variouscomponents of in8ow( out8ow and stora#ecomponents of in8ow( out8ow and stora#e

and apply the mass balance equation.and apply the mass balance equation.




The evaporation in any time interval may

'e compute$ from

( ) Q I S S E

−+−= 21

Where I an$ 8 are the volumes of ino!an$ outo!, S

" an$ S

# are the initial an$

(nal storage respectively




.ii/ Energy Metho$.ii/ Energy Metho$ t is an application of the law of conservation oft is an application of the law of conservation of

ener#yener#y 6ontinuity6ontinuity

64 contains liquid and vapor phase water64 contains liquid and vapor phase water

=iquid phase=iquid phase

∫∫ ⋅+∫∫∫ ∀=−CS

w

CV

wv d dt d m dAV ρ ρ

0=

dt

dh Aw ρ =

o 8ow of liquido 8ow of liquid

water throu#h 6%water throu#h 6%

AE m wv ρ =

dt

dh E −=

hw ρ

a ρ

vm

dt

dh E −=

n R s H

G

The air in contact !ith the groun$ or !ater surface is !arme$ an$ thenThe associate$ o! of energy thru the air is terme$ sensi'le heat u9 :sDecember 28, 2015 27



Energy Metho$Energy Metho$

Ener#y Eq. for -ater inEner#y Eq. for -ater in64 64

ssume: ssume:

+. 6onstant temp of water in 64 +. 6onstant temp of water in 64 . 6han#e of heat is chan#e in internal ener#y of water. 6han#e of heat is chan#e in internal ener#y of water

evaporatedevaporated

hw ρ

a ρ

vm

dt

dh E −=

n R s H

G

vvml dt

dH =

G H Rdt

dH sn −−=

G H Rml snvv −−= AE m w ρ =

( )G H R Al

E snwv

−−= ρ

1

'ecall'ecall::

wv

nr

l

R E

ρ =

e#lectin# sensible ande#lectin# sensible and

#round heat 8u&es#round heat 8u&es

-ate of heat input to the system




( )β ρ +−

=1 L

G R E

w

n L

a s

a s

ee

T T

−

−= γ β

- n 3 net ra$iation, ; 3 :eat u9 into the

groun$, L 3 latent heat of vapori<ation, p isthe 'arometric pressure in m'

3= Sign of changes in morning an$evenin since :s is >ve u !ar$ $urin the

=−

−=

a s

aw

ee

T T γ β

β

a s s T T H −∝

a svv

eeml −∝




.iii/ Aero$ynamic.iii/ Aero$ynamic

Metho$Metho$ nclude transportnclude transport

of vapor away fromof vapor away from

water surface aswater surface as

function of:function of: Humidity #radientHumidity #radient

above surfaceabove surface

-ind speed across-ind speed across

surfacesurface

n R

E

et radiation

Evaporation

ir 9low




Aero$ynamic Metho$ Aero$ynamic Metho$

z z sa uee M E )( −=

Where Ea 3evaporation 'y the aero$ynamicmetho$,

M 3 mass6transfer coe*icient, es 3

saturation vapour pressure at !atertemperature, e< 3 vapour pressure of the air

at level ?, u< 3 !in$ velocity at level ? P

C

M w

E a

ρ

ρ

622.0=

Where @ ! 3 $ensity of !ater, @a 3

$ensity of air, P 3 atmosphericpressure at level ?, E 3 'ul5evaporation coe*icientDecember 28, 2015 32



.iv/ om'ine$ Metho$.iv/ om'ine$ Metho$

Evaporation is calculated byEvaporation is calculated by erodynamic method erodynamic method

Ener#y supply is not limitin#Ener#y supply is not limitin#

Ener#y methodEner#y method

4apor transport is not limitin# 4apor transport is not limitin#

ormally( both are limitin#( so use aormally( both are limitin#( so use a

combination methodcombination method

%ensible heat 8u& is di1icult to estimate%ensible heat 8u& is di1icult to estimate

ssume it is proportional to the vapor heat 8u& ssume it is proportional to the vapor heat 8u& β β 2owen ratio 2owen ratio

Ener#y balance equation "B>$Ener#y balance equation "B>$

( )vv s ml H β =

( )

( )β

ρ

+=

−−=

1

1

ml R

G H R Al

E

vn

snwv




-ecall +apor Pressure-ecall +apor Pressure

+=

T

T e s

3.237

27.17exp611

2)3.237(

4098

T

e

dT

de s s

+==∆




om'ine$ Metho$om'ine$ Metho$

.ont/.ont/ 6ombinin#6ombinin#

Ener#y balanceEner#y balance

erodynamic Cethods erodynamic Cethods

6ombined Cethod6ombined Cethod

-ell suited to small areas-ell suited to small areas

with detailed datawith detailed data et 'adiationet 'adiation

ir Temperature ir Temperature

HumidityHumidity

-ind %peed-ind %peed

ir )ressure ir )ressure

wv

nr

l

R E

ρ =

ar E E E γ

γ

γ +∆+

+∆∆

=

2)3.237(

4098

T

e

dT

de s s

+

==∆

r E E γ +∆

∆= 3.1 Priestly Taylor

z z sa uee M E )( −=




E9ampleE9ample

Elev m(Elev m(

)ress +>+.F *)a()ress +>+.F *)a(

-ind speed F m5s(-ind speed F m5s(

et 'adiation >> -5m(et 'adiation >> -5m(

ir Temp < de#6( ir Temp < de#6(

'el. Humidity G>,('el. Humidity G>,(

7se 6ombo Cethod to nd7se 6ombo Cethod to nd

EvaporationEvaporationkJ/kg244110)25*36.22500(

237010501.2

3

6

=−=

−=

x

T xl v

mm/day10.7997*102441

2003

=== xl

R E

wv

nr

ρ




E9ample .ont/E9ample .ont/

Elev m(Elev m(

)ress +>+.F *)a()ress +>+.F *)a(

-ind speed F m5s(-ind speed F m5s(

et 'adiation >> -5m(et 'adiation >> -5m(





( )

mm/day45.7

)day1/s86400(*)m1/mm1000(*126731671054.4 11

=−= −

x E a

Pa3167=ase

Pa12673167*4.0* === asha e Re




E9ample .ont/E9ample .ont/

Elev m(Elev m(

)ress +>+.F *)a()ress +>+.F *)a(

-ind speed F m5s(-ind speed F m5s( et 'adiation >> -5m(et 'adiation >> -5m(





Pa/degC7.188)253.237(

3167*40982 =

+=∆

738.0=+∆∆ γ

mm/day2.745.7*262.010.7*738.0 =+=+∆

++∆∆

= ar E E E γ

γ

γ

262.0=+∆ γ

γ




E9ampleE9ample

et 'adiation >> -5m(et 'adiation >> -5m( ir Temp < de#6( ir Temp < de#6(

7se )riestly!Taylor Cethod to nd7se )riestly!Taylor Cethod to nd

Evaporation rate for a water bodyEvaporation rate for a water body

r E E γ +∆

∆= 3.1 Priestly Taylor

mm/day10.7=r E 738.0=+∆∆γ

mm/day80.610.7*738.0*3.1 == E

03a evaporation

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