OPEN WATER EVAPORATIONNWRI

Raoul GrangerOctober/06

Newell Hedstrom

www.ec.gc.ca

11/22/2006 Page 2

Objectives of Evaporation Studies

• Provide a correct description of the open water evaporation for short-term (hourly, daily) calculations.– The advection process.– Application to remote sensing

Evaporation Models are parameterizations of one or more of the conditions required for evaporation to occur:

For evaporation to occur there must be:- a supply of water at the surface, - a supply of energy to satisfy the requirement for the phase

change, and - a transport mechanism to carry the vapour away from the

surface (wind, vapour gradient).

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Open Water Evaporation

• Hydro-Myth #1The land and the sea are in harmony… so we can transfer our knowledge of the land surface to the open water...right?

• Hydro-Myth #2For a lake, availability of water is not an issue, so evaporation must be related only to the energy supply…right?

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Lake Evaporation Observations:Quill Lake, 1993 - open water and land surface

-100

0

100

200

300

400

500

Aug 23 Aug 24 Aug 25

Evap

orat

ion,

W/m

² Land Lake

Lake Evaporation Observations:Crean Lake 2005

Crean Lake - Aug 4/05

-100

0

100

200

300

400

500

600

700

800

0 600 1200 1800 2400Time

Flux

, W/m

²

LEHRn

Weisman and Brutsaert (1973) showed that lake evaporation involves advection, and that one needs to have information on both the land and water surfaces.

Where the coefficients a and b are related to dimensionless advection parameters

bofassal ZXqquaEE −⋅−⋅+= )()(*ρ

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Quill Lake, 1993

-100

0

100

200

300

400

500

Aug 23 Aug 24 Aug 25

Evap

orat

ion,

W/m

²

Land Lake Lake (W-B)

Estimating Lake EvaporationWill require a knowledge of the water surface

temperature, combined with a boundary layer model capable of representing the advection of energy.Need to redo Weisman-Brutsaert development

with better parameterizations for stable conditions.

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Crean Lake, 2006

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Crean Lake, 2006

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Crean Lake, 2006

Crean Lake 2006

-100

0

100

200

300

400

181.0 181.5 182.0 182.5 183.0 183.5 184.0 184.5

Julian Date

Flux

, W/m

²

LE (obs)LE (W-B)

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Ratio of transfer coefficients : stable conditions

Ke/Km = exp(-8.0Z/L)

0

0.2

0.4

0.6

0.8

1

1.2

0.0001 0.001 0.01 0.1 1 10

z/L

Ke/

Km

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Boundary Layer Investigation

• Upwind and Downwind tethersonde profiles were obtained on Sept. 1/06– Validation of boundary layer development– Estimation of Evaporation from Boundary Layer

Integration

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Humidity Profiles Crean Lake (13:30)

1.0

10.0

100.0

0.7 0.8 0.9 1 1.1

Vapour Pressure, Kpa

Ht,

m

Landwater

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Things to do

• Redo Weisman-Brutsaert advection analysis with better parameterizations for stable conditions.

• Complete the boundary layer integration work.• Begin collection of MODIS images; test for

applicability with remote sensing.

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www.ec.gc.ca

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Contents

• Text

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Contents

• Text