Evaporation Principles

8/20/2019 Evaporation Principles

1/37

ESTIMATING EVAPORATION

FROM WATER SURFACES

(With emphasis on shallow water

bodies)

1ET Workshop12‐Mar‐2010


2/37

•

• Estimating methods

include:

– pan coe c ent x measure pan evaporat on

– water balance

– energy balance

– mass transfer

– combination techniques

• Emphasis will be practical methods



3/37

• – th

• Dalton (1802),

E = f( ū ) (eo – ea )

• Bowen 1926 , t e Bowen ratio, t e ratio o

sensible heat

to

latent

heat

gradients

( Δt/ Δe)

• Applications were made to lake evaporation

by Cummings and Richardson 1927; McEwen

1930



4/37

•

• Incoming solar

radiation

is

the

main

source

• In contrast to an , not a net so ar ra iation

is absorbed

on

the

surface

• In pure water, about 70% is adsorbed in the

top 5 m (16 ft)

• Solar radiation

adsorbed

below

the

surface

is

“stored ener ”



5/37

•

be more

difficult

than

estimating

soil

heat

flux (G)

• Part of solar radiation may penetrate to great

depths depending

on

the

clarity

of

the

water

• Stored energy affects the evaporation rate

• Exam l t m rat r r fil in wat r:

– profiles during increasing solar cycle

– rofiles durin decreasin solar c cle



6/37

Solar Radiation Penetrates Deep in Water

Evaporation pans are two Evaporation pans are two

shallow and hold too shallow and hold too

much “warmth” at themuch “warmth” at the

surface.

surface.

Therefore, they can Therefore, they can

overestimate the overestimate the

evaporation from large evaporation from large

reservoirs and lakes.reservoirs and lakes.


7/37

• , , , .

m (190

ft)

deep,

average

40

m

• Thermal profiles

during

increasing

Rs

• Thermal profiles during decreasing Rs

• Exam le tem eratures b de th and time

• Reason for studying evaporation—improve



8/37

LakeBerryessa

LakeBerryessa

California, USACalifornia, USA

LB

05LB10

LB04

LB12

LB03

LB 06

(New-USBRWS)

LB07A-E

LB 01

-

LB02

Portable WS

LB 08

(Dam)LB 09

(USBR)

LB 11

-WS



9/37

Temperature Profile Data - 7/10/03

0

10 12 14 16 18 20 22 24 26 28

-10

-5

-20

-15

D

e p t h , m

-30

-25

Temperature, C

LB 01 LB 02 LB 03 LB 04 LB 05 Avg



10/37

-

0

10 12 14 16 18 20 22 24 26 28

-5

-15

-

D e p t h ,

m

-25

-

-30Temperature, C



11/37

30.0

2005

20.0

25.0

15.0

W a t

e r t e m p ,

10.0

5.0

1 / 1

2 / 1

3 / 1

4 / 1

5 / 1

6 / 1

7 / 1

8 / 1

9 / 1

1 0 / 1

1 1 / 1

1 2 / 1

0.15 m 5.2 m 7.6 m 12.8 22.6 m



12/37

• The pan site was moved from the original site

• Measured

pan

evaporation

x original

coefficients

underestimated reservoir evaporation and inflow

inflows late in the summer

• The obvious

solution

– move

the

pan

site,

and

rec ec t e pan coe c ents

• Data were needed to justify to the USBR the need to chan e the an site

• View of

the

evaporation

pan

site

• Estimated rate of energy storage



13/37



14/37

15.0

5.0

10.0

- 1

-5.0

0.0

Q t ,

M J m - 2 d a

-10.0

-1 .

M - 0

3

J - 0

3

S - 0

3

N - 0

3

J - 0

4

M - 0

4

M - 0

4

J - 0

4

S - 0

4

N - 0

4

J - 0

5

M - 0

5

M - 0

5

J - 0

5

S - 0

5

N - 0

5

J - 0

6

- - - - - - -



15/37

• Maximum ener stora e rates of 5 to 10 MJ m‐2

d‐1

to a depth

of

25

m

measured

in

Lake

Berryessa and about 10 MJ m‐2 d‐1 to a depth of 45 m measured in Lake Mead

• Water surface

temperatures

reached

a max mum n u y n an n ugus n a e

Mead (lag is related to depth of water)

• Advected energy can be large in reservoirs on



16/37

8.0

9.0

d

- 1

5.0

6.0

7.0

a t i o n , m

m

3.0

4.0

t e d

e v a p o

0.0

1.0

2.0

E s t i m

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Davis-Parker Parker-Imperial Imperial-Morelos



17/37

‐

30.0

35.0

25.0

15.0

20.0

° C

5.0

10.0

0.0

J‐05 F‐05 M‐05 A‐05 M‐05 J‐05 J‐05 A‐05 S‐05 O‐05 N‐05 D‐05

12‐Mar‐2010 ET Workshop 17

Surface Temp


18/37

•

• Aerodynamic methods

– use ma n y on arge a es an reservo rs

• Example – American

Falls

reservoir

in

Idaho

by

Allen et al.

– estimates using water and air temperature

– estimated evaporation

relative

to

ET r

• Why the low summer rate? Cold inflow water?



19/37

Temperature of water from American Falls outfall follows Temperature of

Air

Year 2000

American Falls Tem eratures

15

20

25

30

u r e ,

C

2004

-5

0

510

T e m p e r

a

Year 2004-

24-Feb09-Mar

23-Mar

06-Apr 26-Apr

11-May

25-May09-Jun

22-Jun

07-Jul20-Jul

03-Aug

17-Aug01-Sep

14-Sep

28-Sep12-Oct

26-Oct

09-Nov22-Nov

07-Dec

20-Dec

Date

Water Temp 10 day mean Air Temp


20/37

Evaporation ratio for Alfalfa

American Falls, Evaporation/ETr ratios

2004

0.70.80.9

/ E T r

0.4

0.50.6

p o

r a t i o n

00.10.2.

E v a

0 1 2 3 4 5 6 7 8 9 10 11 12Month

ETrF from Am.Falls study 2004 Aerodynamic ETrF from Tmean


21/37

•

– Equilibrium temperature

(Edlinger et

al.

1968)

–

(1974), Fraederich et al. (1977), de Bruin (1982), and

Finch (2001)

• Finite difference model (Finch and Gash, 2002)

• Pan eva oration x an coefficient

• Energy balance

and

combination

methods

• o w



22/37

• Pan coefficient studies

• Rohwer (1931)

– a

classic

detailed

study

conducted on the CSU campus

• Rohwer compared evaporation from a Class A

pan and

an

85

‐diameter

(26

m)

reservoir

• Young (1947) also did a classic study in CA

• Others: Kohler (1954); Kohler et al. (1959);

Farnsworth et

al.

(1982)

• Fetch effects and obstructions (fixed & variable)



23/37

Ratio of Lake Evaporation to Class Pan Evaporation

1.20

0.80

1.00

p a n

- 1

0.40

0.60

k e

e v a p

E

0.00

0.20 L


Lake Elsinore Lake Okeechobee



24/37

1

0.8

0.9

0.6

0.7

y = ‐0.000x3 + 0.016x2 ‐ 0.056x + 0.623

0.4

0.5

0.2

0.3

Ratio ‐ Reservoir to Class A pan, Apr‐Nov ‐ Rohwer 1931

0

0.1

3 4 5 6 7 8 9 10 11 12


. .


25/37

30.0

25.0

‐

20.0

C

15.0

T e m p ,

5.0

.

0.0

Sep Oct Nov Apr May Jun Jul Aug Sep Oct Nov Apr May Jun Jul Aug Sep Oct Nov


Air temp‐1‐inch Water temp


26/37

12.0

14.0

8.0

10.0

, m m d

1

Jun-Jul

6.0

E v a p o r a t i o n

Sep

Oct

2.0

.

Nov

Dec

0.0

0 1 10 100 1000

Upwind fetch of irr igated grass, m



27/37

•

– buildings

–

– other (shown in previous example)

• ar a e

– adjacent corn field (most common), can have a

ma or e ec

– weeds and other adjacent crops



28/37

• Aerod namic used mainl on lar e water bodies

• Energy balance

(requires

detailed

measurements)

• Combination methods: Penman (1948, 1956, 1963); Penman‐Monteith (1965); Priestley‐Taylor (1972)

• All require estimating net radiation using standard

difficult for deep water bodies)

• Reference ET x coefficient (E = ET x K )

– for shallow

water

bodies

– for ice‐free water bodies



29/37

•

• Example rates

of

storage

– pea rates can range rom to m‐ ‐

– equivalent to 2 to 4 mm d‐1 evaporation

• Example rates calculated from lake studies

– Pretty Lake in Indiana

– Williams Lake

in

Minnesota

12‐Mar

‐2010 ET

Workshop 29


30/37

Reported Energy Storage Rates - Pretty Lake (63-65) & Wil li ams Lake (82-86)

10.0

15.0

- 1

0.0

5.0

t , M

J

m

- 2

-10.0

-5.0 Q

Day of Year

Prett Lake Williams Lake Pol . Prett Lake Pol . Williams Lake

30ET Workshop12

‐Mar

‐2010


31/37

‐

• ‐ .

• ET ref x coefficient for

open

ice

‐free

water

(K w )

• w ere ET ref is or s ort grass ET os , mm ‐

• First check

input

weather

data

for

quality

0.408 Δ(Rn ‐G) + γ [900/(T + 273)] u2 (es ‐ea)

‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐re

Δ + γ (1 + 0.34 u2)

31ET Workshop12

‐Mar

‐2010


32/37

–

•

– Elevation 2297

m

(7536

ft)

above

sea

level

– , .

• Estimated energy storage

• Estimated evaporation

– May‐October

– ET ref x K w also

agrees

with

PM

in

November

• Results Ma ‐October

32ET Workshop12

‐Mar

‐2010


33/37

0.5

1.0

0.0 m

2 d

- 1

-0.5 Q t ,

M

J

-1.5

-1.0


Est-Qt

33ET Workshop12

‐Mar

‐2010


34/37

8.0

Home Lake, Alamosa, CO

7.0

.

5.0

.

, m m d

- 1

3.0

4.0

i m a t e d e v a

1.0

2.0 E s t

0.0

Apr May Jun Jul Aug Sep Oct

E - PM E = ETo x 1.10

34ET Workshop12

‐Mar

‐2010


35/37

ROHWER’S CLASS A PAN

COEFFICIENTS

• p

• Based on

mean

ratios

(polynomial)

– pr . ugust .

– May 0.63 September 0.78

– June 0.67 October 0.77

– July 0.71

– Average, April

‐October 0.70

35ET Workshop12

‐Mar

‐2010


36/37

ESTIMATED EVAPORATION

HOME LAKE

– MAY

‐OCT.

• ‐ re

• mm

• 894 906 890 892

• (35.2) (35.7) (35.0) (35.1)

• Percent of PM

• 100 101 100 99.8

• All ive similar values for Ma throu h October

36ET Workshop12

‐Mar

‐2010


37/37

ESTIMATED EVAPORATION

LAKE BERRYESSA

3 YR

AVG

• PM Penman P‐T USBR ori inal

• mm• (inches)

• 1,325 1,425 1,277 955

• (52.2) (56.1) (50.3) (37.7)• 100 108 96 72

• The same Rn and Qt used for first three methods

• a ues

con rm

e ects

o

poor

pan

s te• P‐T equation does not have wind speed

37ET Workshop12

‐Mar

‐2010

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Evaporation Principles

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