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Þröstur Þorsteinsson and Einar Sveinbjörnsson

©2008 bild-werkstatt.de

Location In SW-Iceland

Lake diemensions

Area 83 km2

Mean depth 34 m

Max depth 114 m

Volume 2873 106 m3

Measurement of Þingvallavatn

Sigurjón Rist

Pioneer in water research in Iceland

Decades of research

Guðmann Ólafsson and Hörður (his son)

monitored lake ice formation and break-up

Sigurjón Rist at work Influx of water Influx nearly all via groundwater flow

Flux ~100 m3 s-1

91 m3 s-1 Groundwater

4 m3 s-1 Precipitation

2 m3 s-1 Öxará

3 m3 s-1 Surface water

Renewal time thus ~330 days

Mean water velocity 43 m per day

Groundwater91%

Surface water

5%

Precipitation4%

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Ground-water flow

Typical temperature profiles Few measurements

Mainly by Sigurjón Rist and co-workers

Only one where water is deepest

So – fairlyschematicprofiles shownhere

Data

Radiation (short- and longwave) from Reykjavík

Weather (wind speed, wind direction, temparature) from Þingvellir

Lake ice – Guðmann Ólafsson and Hörður (his son)

Some data lost in fire 2000

View towards Botnsúlur on March 15, 2008. Photo Einar Sveinbjörnsson

Lake ice observations The lake ice usually forms in early January

No lake ice formed

1922/23

1928/29

1956/57 – possibly

2002/03

2003/04 – very little

2005/06

Days of lake ice

0

20

40

60

80

100

120

140

1952 1957 1960 1974 1977 1980 1983 1986 1989 1992 1995 1999 2002 2005 2008

Da

ys

Ice

Some ice

No ice

Air temperature At

Þingvellir

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Win

d s

pe

ed

(m

s-1

)

Wind direction (°)

Radiation budget Incoming solar

radiation, Qs

Incoming long-wave radiation (clouds and atmosphere), Qh

Outgoing radiation (I)

Albedo (a)aQs

Short wave radiation Incoming solar radiation in Reykjavík

Data Incoming longwave radiation from

the atmosphere and clouds

Sun angle Most 49°

Albedo is a function of the sun angle

Fresnel equation for reflection off a plane surface of pure water

Diffuse radiaton, lower limit of a usually 5 – 6 %

Waves also have an effect – water surface not a flat plane usually

Water turbity

But, still mostly a function of solar elevation

AlbedoFresnelGrishchenko

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Modeling of the lake temperature Lake modelled with N layers

Want to examine when lake ice can form

Explore the sensitivity to climate change

Temperature measurements Three locations

4 times over the year 2008

Made by Náttúrufræðistofa Kópavogs

Basic equation to solve Change in T with

time and depth:

Radiation at the surface

Discrete equation Model as N layers

1<i<N-1

i=1

i=N

Bowen ratio Calculation of latent heat require knowledge of air

moisture – which is rearly known

Therefore, use the ratio of sensible to latent heat, Bowen ratio

But, what should the value be?

Is ~10 over tropical ocean

Is 0.1 in deserts

Studies over N-Atlantic indicate B ~ 0.65

L

HB

Sensible heat H )(259.1 zsz TTUH

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Net radiation

IQQR hs

Results Pretty color

picture – but not only that ...

Comparison with measured T Temperature profiles at 3 locations

4 times: 8 May, 2 Jul, 1 Sep, and 15 Oct, 2008

De

pth

(m

)

Temperature (°C) Temperature (°C) Temperature (°C)

Measured and calculated TD

ep

th (

m)

De

pth

(m

)

Temperature (°C) Temperature (°C)

Without eddy diffusion

ke = 0

ke ≠ 0

Teddy - T

Conclusions With a model that “accuratly” simulates lake

temperature as a function of climate we can

Predict when lake ice can form

Examine the sensitivity to temperature change, and other variables, for lake ice formation

FIN