Land-surface processes in the globalenergy and water cycles.Part (a)

Schär, ETH Zürich

Sonia I. Seneviratne and Christoph Schär Land-Atmosphere-Climate Interactions Winter term 2006/07

Land-surface processes in the global energy and water cycles.

Part (a)

Christoph Schär

Institute for Atmospheric and Climate Science ETH Zürich

schaer@env.ethz.ch

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6

Outline

Global energy cycle

Global water cycle

Reservoirs of the water cycle Residence times in the water cycle

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Global energy balance

Sun

solar radiation

emitted infrared radiation reflected solar

radiation

Energy input = Energy output

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8

Incoming solar radiation

Earth’s surface 4 π r²

Earth’s shadow π r² extraterrestrial

solar constant S_o = 1367 W/m²

S =1 4S_o Mean incident solar radiation:

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Albedo

Albedo = Fraction of incoming radiation that is reflected

Surface properties and clouds are important for global energy balance:

• Cloud-albedo feedback

• Snow/ice-albedo feedback

• Vegetation-albedo feedback

Surface Albedo α

Clouds 100 m deep 0.4

500 m deep 0.7

Ocean zenith angle 30° 0.05

60° 0.10

85° 0.6

Ice 0.25-0.35

Snow old-new 0.45-0.85

Grassland 0.2-0.3

Forest 0.1-0.2

Global mean 0.3

Short Wave

–100% +33%

+67%

Long Wave +67%

–67%

S S·α

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10

Absorption by trace gases – greenhouse effect

(Peixoto and Oort, 1992)

Absorption [%]

Wave length [µ]

CH₄ N₂O O₂,O₃ CO₂ H₂O

H₂O vapor is the most important greenhouse gas.

CO₂ is the most important anthropogenic greenhouse gas.

shortwave longwave

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Global energy balance

Short Wave

global radiation

–100% +22% +8%

+42%

+28%

Long Wave +60%

–113% +101%

+10%

Latent Heat

–25%

Sensible Heat

–5%

–58% +5% +25%

CO2

H₂0

Space

Atmosphere

Land / Ocean

(based on data of Ohmura and Wild)

Transport of heat and water vapor

342 W/m²

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12

Outline

Global energy cycle Global water cycle

Reservoirs of the water cycle Residence times in the water cycle

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Water on planet Earth

Oceans 2620 96.5

Polar ice, sea ice, glaciers 47 1.7

Ground water 46 1.7

Permafrost 0.59 0.02

Lakes 0.35 0.013

Soil moisture 0.032 0.0012

Atmosphere 0.025 0.00093

Swamps 0.023 0.00083

Rivers 0.0042 0.00015

Biological water 0.0022 0.000081

Global mean Global percentage

depth [m] [%]

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Freshwater on planet Earth

Oceans 0

Polar ice, sea ice, glaciers 47 68.7

Ground water 21 30.1

Permafrost 0.59 0.86

Lakes 0.18 0.26

Soil moisture 0.032 0.047

Atmosphere 0.025 0.037

Swamps 0.023 0.033

Rivers 0.0042 0.0061

Biological water 0

Global mean Global percentage

depth [m] [%]

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Freshwater on planet Earth

Polar ice, glaciers, snow

d=358 km

Ground water, soil moisture, permafrost

d=275 km

Lakes d=56 km

Rivers, swamps d=30 km

Atmosphere d=29 km

0 100 200 km

Intercomparison using spheres with the respective volumes

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Global water cycle

Atmosphere 12.9

Oceans

1,338,000 Glaciers,

Polar ice 24,000 Soil moisture

16.5

Surface water

104

Ground water 10,800 116

(780 mm/y) 71

(470 mm/y) ~0 1 505

(1270 mm/y) 458

(1400 mm/y) 2.7 ~0

46 43.8 2.7

2.2 44.7

Numbers in Italic: Volumes [1000 km³] Numbers in normal: Fluxes [1000 km³/y]

(Numbers in brackets): Fluxes [mm/y], with respect to ocean / land-surface

All storage compartments have a zero balance in the longterm mean (in- and outgoing fluxes yield zero)!

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Outline

Global energy cycle Global water cycle

Reservoirs of the water cycle

• soil moisture

• ground water

• surface water

• snow / ice

Residence times in the water cycle

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Ground water Recharge

table

Storage and fluxes in the soil

Unsaturated (soil moisture):

Pores filled with water and air

Saturated (ground water):

Pores filled with water

Percolation

Ground water flow

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0%

100%

100%

0%

Soils can be described by grading curves

25%

50%

75%

75%

50%

25%

Mass fraction < diameter Mass fraction > diameter

Soil classification

USDA Classification Particle diameter

20% sand

73% silt

7% clay

Gravel {Kies}

Sand {Sand}

Silt {Schluff}

2 µm 0.002 mm

5 µm 20 µm 50 µm 0.05 mm

0.2 mm

2 mm

5 mm 20 mm

Clay {Lehm}

0.5 µm 0.5 mm

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Soil triangle

% Silt (2µm - 50 µm)

% Sand (50µm - 2 mm)

% Clay (< 2 µm)

(USDA Classification) Percentages are based on mass fraction

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Gravimetric measurement of soil moisture content

€

m

sample dried

sample (at 105°C for 2 days) Based on weight of sample:

€

m_sample

Volume of sample: V_sample Density of water: ρ_w

Volumetric soil moisture content, Volume fraction of soil moisture:

€

θ= V_water

V_sample =(m_sample−m_dried) ρ_w V_sample

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Measures of soil moisture content

Volumetric soil moisture content [1]

Gravimetric soil moisture content [1]

Relative saturation, saturation relative to pore volume [1]:

Water equivalent, water column [m]:

€

θ= V_water V_sample

€

θ_rel=V_water V_pores =θ

€ n

θ_g= m_water

m_dry = V_water ρ_water

V_sample ρ_dry = θ ρ_water ρ_dry

€

h_w=θ h_soil ^hsoil= depth of soil

n = volume fraction of pores

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Porosity and field capacity

Volume of sample: V_sample soil

sample

m

dried sample (at 105°C for 2 days)

m

saturated sample (confined)

m

saturated sample (unconfined,

no drainage after 2 days)

m

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26

Volumetric water content θ [1]: Volume fraction of soil moisture

Porosity n [1]: Volume fraction of pores

Field capacity FC [1]: Saturated water in balance with capillary forces

This definition of the field capacity is a bit casual (see later)

€

n= V_pores

V_sample =(m_sat −m_dried) ρ_w V_sample

Bodenwassergehalt, Porosität und Feldkapazität

€

θ= V_water

V_sample =(m_sample−m_dried) ρ_w V_sample

€

FC=V_{water at fc}

V_sample = (m_fc−m_dried) ρ_w V_sample

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Summary on soil moisture content

All pores saturated

Completely dry sample BodensubstratWasserLuft

0 1

Volumenanteil

n Porenvolumen FK Feldkapazität

θ Wassergehalt

soil substrateairwater

0 ≤ θ ≤ n

Volume fraction

Water content θ

1

0

Porosity n

Plants wilt Wilting point PWP

Maximum water content in balance with capillary forces Field capacity FK

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Typical values of n , FK, PWP

volume fraction

Porosity n 0.3 - 0.55

Field capacity FC 0.1 - 0.35

Permanent wilting point PWP 0.05 - 0.25

Dry 0

Coarse texture (z.B. Sand)

Fine texture (e.g. clay)

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Other methods to measure soil moisture

• Local measurements

• TDR (Time Domain Reflectometry):

measurement of dielectric constant

• Neutron probe

• Remote sensing

• microwaves

• GRACE (Gravity Recovery and Climate Experiment)

• Large-scale water balance

• Estimates of (P-E) and Q in a catchment provides information on terrestrial water storage changes (see later)

P E

Q

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Groundwater / aquifers

Unconfined Aquifer

Confined Aquifer Piezometric Surface

Groundwater Table Grundwasserspiegel Artesian Spring / Well

Impervious strata Impervious

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Töss-Aquifer

(Beyerle 1999)

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32

Vertical section across Töss-Aquifer

(Beyerle 1999)

Aquifer

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Measurement of the groundwater table

Location of groundwater wells in the Valais

Measurement of groundwater table using pressure sensors

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Surface water (rivers and lakes)

Example: central Switzerland

P Precipitation ET Evapotranspiration Q direct runoff G_out groundwater runoff G_in groundwater input

(Hydrologischer Atlas der Schweiz)

Concept of a catchment

“A water catchment area is a drainage basin or watershed, the region of land whose water drains past a specific point along a river or into a specified body of water such as a lake” (Wikipedia)

G_out P ET

G_in

Q

Water divide

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Major river basins in Europe

http://www.transboundarywaters.orst.edu/publications/atlas/atlas_html/graphics/imagemaps/europe_imagemaps.html

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Major river basins

Siehe auch: http://www.iucn.org/themes/wani/eatlas/

Amazonas

Kongo

Yangtzekiang Mississippi

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Major river basins

Area Mean runoff Runoff ratio¹⁾ [10³ km²] [m³/s] [%] R/P [1]

Amazonas 7,180 210,000 16.6 0.47

Kongo 3,822 42,000 3.3 0.25

Yangtzekiang 1,970 35,000 2.7 0.50

Orinoco 1,086 29,000 2.3 0.46

Brahmaputra 586 20,000 1.6 0.65

Parana 2,650 19,500 1.6 0.20

Donau 817 6,400 0.5

Rhein 190 2,200 0.2 ca 0.5

1)Runoff ratio: mean runoff / mean precipitation

(Baumgartner & Liebscher 1996, Dingman 1993)

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Polar ice and sea ice

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Snow and sea ice

Fläche

Global land surface: 149 · 10⁶ km² Global sea surface: 361 · 10⁶ km²

Sea ice cover Snow cover

Seasonal cycle

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Alpine snow cover

Climate change:

Warming of ΔT=3ºC Snow line climbes by about 500 m

Number of days with snow cover

Altitude

(Schüepp, Gensler and Bouet 1980)

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€

R_snow = ρ_snow ρ_w ⁼

h_w h_snow Water equivalent h_w [m]:

Water depth of the molten snow/ice cover (snow depth: h_snow^):

Relative snow density R_snow [1]:

Water equivalent of snow

€

h_w ρ_w =h_snow ρ_snow ⇒ h_w = ρ_snow ρ_w h_snow

(Dingman, modified from McKay 1970)

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44

Outline

Global energy cycle Global water cycle

Reservoirs of the water cycle Residence times in the water cycle

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Mean residence time

Fin S Fout

In the stationary case: F_in = F_out

The mean residence time τ of a water molecule in a particular storage S is determined by the size of the storage and the flux through it:

It corresponds to the time needed to fill the storage S with the flux F_in, or to the time needed to empty it with the flux F_out.

€

τ = S F_in = S

F_out

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Global water cycle

Atmosphere 12.9

Oceans

1,338,000 Glaciers,

Polar ice 24,000 Soil moisture

16.5

Surface water

104

Ground water 10,800 116

(780 mm/y) 71

(470 mm/y) ~0 1 505

(1270 mm/y) 458

(1400 mm/y) 2.7 ~0

46 43.8 2.7

2.2 44.7

Numbers in Italic: Volumes [1000 km³] Numbers in normal: Fluxes [1000 km³/y]

(Numbers in brackets): Fluxes [mm/y], with respect to ocean / land-surface

All storage compartments have a zero balance in the longterm mean (in- and outgoing fluxes yield zero)!

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Mean residence time

Example 1: Residence time in the atmosphere

F_out precipitation on land 116,000 km³/y

precipitation on sea 458,000 km³/y polar precipitation 2,700 km³/y total precipitation 576,700 km³/y

S storage 12’900 km³

τ = S / F_out = 0.022 y = 8.2 d

Example 2: Residence time in the ground water (global mean) F_in ground water formation 46'000 km³/y S storage 10'800'000 km³

τ = S / F_in = 235 y