www.ec.gc.ca
OPEN WATER
EVAPORATION
NWRI
Raoul Granger
October/06
Newell Hedstrom
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).
Open Water Evaporation
• Hydro-Myth #1
The land and the sea are in harmony… so we can
transfer our knowledge of the land surface to the open
water...right?
• Hydro-Myth #2
For 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
Evaporation, W/m²
500
Land
400
Lake
300
200
100
0
-100
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Aug 24
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Aug 25
Lake Evaporation Observations:
Crean Lake 2005
800
Crean Lake - Aug 4/05
700
Flux, W/m²
600
LE
500
H
Rn
400
300
200
100
0
-100
0
600
1200
T ime
1800
2400
Weisman and Brutsaert (1973) showed that
lake evaporation involves advection, and that
one needs to have information on both the
land and water surfaces.
El = Ea + aρu* ⋅ (qs − qas ) ⋅ ( X f Z o )
Where the coefficients a and b are related to dimensionless advection
parameters
−b
Quill Lake, 1993
Evaporation, W/m²
500
Land
Lake
Lake (W-B)
400
300
200
100
0
-100
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Aug 24
Aug 25
Estimating Lake Evaporation
Will 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.
Crean Lake, 2006
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Crean Lake, 2006
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Crean Lake, 2006
Crean Lake 2006
400
LE (obs)
LE (W-B)
Flux, W/m²
300
200
100
0
-100
181.0
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181.5
182.0
182.5
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183.0
Julian Date
183.5
184.0
184.5
Ratio of transfer coefficients : stable conditions
1.2
1
Ke/Km = exp(-8.0Z/L)
Ke/Km
0.8
0.6
0.4
0.2
0
0.0001
0.001
0.01
0.1
z/L
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1
10
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)
100.0
Land
Ht, m
water
10.0
1.0
0.7
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0.8
0.9
1
Vapour Pressure, Kpa
1.1
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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Contents
• Text
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Contents
• Text
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