open water evaporation - usask.ca€¦ · open water evaporation •hydro-myth #1 the land and the...
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OPEN WATER EVAPORATIONNWRI
Raoul GrangerOctober/06
Newell Hedstrom
www.ec.gc.ca
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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
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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
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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
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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)
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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