Plants and Environment: The hydrological cycle

Peter Bauer-Gottweinpbg@env.dtu.dk

Outline

•

The global hydrological cycle and the plants’

role in it

•

Plants and water quantity–

Plant evapotranspiration

at the local scale

–

Plant evapotranspiration

at the regional scale•

Plants and salinization

The global hydrological cycle and the plants’

role in it

Global Water Balance (Fluxes in 103 km3/a)

ET Oceans

430

Natural ET 50

ET agriculture

20

ET irrigation agriculture

2

Land

Acess. runoff

13

Precipitation over land

110Total ET over land

70

Total discharge

40

Inaccess. runoff

29 ET rainfed

agr. 18

Human withdrawals

4Discharge withdrawals

2

Rainfall over the Oceans

390

TransportAtmosphere

40

Oceans

Residual access. Runoff 9

Compare with discharge River Rhine at its mouth 2300 m3/s ≅

0.073 103

km3/a

Modified after Postel et al., 1996

Global Aspect of Water

•

70% of freshwater demand is by irrigated agriculture•

Irrigated agriculture is three times as productive as rainfed

agriculture

•

1 t grain needs 1000-2000 t water•

There is a world market for grain•

New trend: biofuel

production

The global aspect of water is food production

Net irrigation

requirements (mm/yr)

Döll, 2002

Ü0 240 480 720 960120Kilometers

water

evergreen needleleaf forest

evergreen broadleaf forest

deciduous needleleaf forest

deciduous broadleaf forest

mixed forest

closed shrublands

open shrublands

woody savannahs

savannahs

grasslands

permanent wetlands

croplands

urban and built-up

cropland natural vegetation mosaic

permanent snow and ice

barren and sparsely vegetated

unclassified

Agricultural water use in the Aral Basin

Global Trends (1960-2005)

Grain production was able to balance population growthProductivity was increased twofold

Twofold increase of populationWorld population, 1000 peopleSource WRI, http://earthtrends.wri.org/

0

400000

800000

1200000

1600000

2000000

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Cropped area almost constantCultivated area, 1000 hectaresSource WRI, http://earthtrends.wri.org/

0

500000

1000000

1500000

2000000

2500000

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005Twofold increase in grain productionCereal production, 1000 tonsSource WRI, http://earthtrends.wri.org/

0

2000000

4000000

6000000

8000000

10000000

1960 1970 1980 1990 2000

Global Trends (1960-2005)

Decreasing food prices (until recently…)Grain prices in year 1990 dollars per tonSource: International Rice Research Institutehttp://www.irri.org/science/ricestat/index.a

sp

Six fold increase in fertilizer useWorld fertilizer use, 1000 tons per yearSource WRI, http://earthtrends.wri.org/

Increased productivity is partly due to irrigation agriculture

0

20000

40000

60000

80000

100000

120000

140000

160000

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Increase of irrigated areaGlobal irrigated area, 1000 hectaresSource WRI, http://earthtrends.wri.org/

0

50000

100000

150000

200000

250000

300000

1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

200

400

600

800

1000

1200

1400

1960 1970 1980 1990 2000

RiceWheatMaize

From Birur et al., 2008http://www.fapri.iastate.edu/outlook2008/

The Biofuel

Boom

Impact on Food Prices

IMF, http://www.imf.org/external/np/res/commod/index.asp

World Food Price Index, 2005=100

50

70

90

110

130

150

170

190

Jun-80 Jun-85 Jun-90 Jun-95 Jun-00 Jun-05

Crop evapotranspiration requirements

0C CET K ET= ⋅ETC

crop evapotranspiration (mmd-1)KC

crop coefficient (-)ET0

Reference crop evapotranspiration (mmd-1)

Reference ET can be calculated using Penman-Monteith, Priestley-Taylor or Hargreaves equations

FAO-56, http://www.fao.org/docrep/X0490E/X0490E00.htm

FAO-56 Reference ETReference surface:“A hypothetical well-watered reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 sm-1 and an albedo

of 0.23.”

( )

( )

2 ,2 ,22

02

9000.408 ( )

2731 0.34

s aRn G u e eT

ETu

γ

γ

Δ − + −+=

Δ + +FAO reference ET

FAO-56, http://www.fao.org/docrep/X0490E/X0490E00.htm

Idea: Separate climate and land surface factors

ET0

:

mm/dayRn:

MJ/m2/dayG:

MJ/m2/dayT2

: °Cu2

: m/s

es,2

: kPaea,2

: kPaΔ: kPa/°Cγ: kPa/°C

Units:

Crop Coefficient

•

Almost independent of climate and location –

universally applicable

•

Takes into account crop height, albedo, canopy resistance and soil evaporation

•

World standard for calculation of agricultural water demand (FAO-56)

Time

KC

1 year

Plants and water quantity

Plant Evapotranspiration

at the local scale

How do trees uptake water?

Eagleson, 1970

PhotosynthesisPlants transpire enormous volumes of water but only need a few percent of that water for their physiological needs. During the day plants use radiant energy to photosynthesise energy rich glucose molecules from water and CO2

2 2 6 12 6 26CO 6H O C H O 6O→+ +

radiant energy

Respiration is the process of obtaining energy from energy rich complex molecules like glucose. It is essential to living organisms.

Stomata

•

10-30μm in length, 50-500/mm2

of leaf, 0.3-1% of leaf area•

Primary purpose: CO2

uptake. Transpiration is an unavoidable water loss

Sap Flow Method

Sap Flow: Results

MSc

Sara Lerer

Modeling: Flux-Gradient Law

rC

, representing the plant (s/m)

rah

, representing the atmospheric surface layer (s/m)

ea

, actual water vapour

pressure in the atmosphere (kPa)

es

, saturation water vapour

pressure in the leaf (kPa)

LeafStomata

s a

c ah

e eET C

r r−

=+

The water flux is proportional to the water vapour

pressure difference and inversely proportional to the total resistance

Factors affecting rc

•

Leaf Area Index (LAI)•

Water availability

•

Temperature•

Illumination

•

Air humidity

Various physically based and (semi-) empirical parameterizations are available for all factors

Factors affecting rah

•

Surface roughness•

Wind speed

•

Stability of the atmosphere

The aerodynamic resistance is a property of the atmospheric surface layer and expresses the efficiency of vertical turbulent transport through the surface layer

Plant Evapotranspiration

at the Regional scale

The Surface Energy Balance

0 Rn H ET Gλ= + + −Rn: Net radiation (W/m2)H: Sensible Heat flux (W/m2)λE: Latent heat flux (W/m2)G: Soil heat flux (W/m2)All fluxes are counted positive upwards

Rn

G

H λET

Eqivalence

of volumetric

and energy

units

A volumetric water flux (Evapotranspiration) can

always

be

expressed

as an energy flux (latent heat), since

a fixed

amount

of energy

per unit of water

is used

to convert

the water

from the liquid

phase

to the gas phase

wET= v ETλ ρ λ⋅ ⋅

latent heat fluxW/m2

Evapotranspirationm3/m2/s=m/s

ρw

Density of liquid water (=1000 kgm-3)ET

Evapotranspiration

(ET) rate (mmday-1)λv

Latent heat of vaporization ( ≅

2.256 MJkg-1)

Penman-Monteith

Equation( )

1

s an p a

ah

c

ah

e eR G crET

rr

ρλ

γ

−Δ − +

=⎛ ⎞

Δ + +⎜ ⎟⎝ ⎠

The PM Equation can be derived by combining• The surface energy balance• The flux-gradient law• Linearization of the water vapor pressure saturation curve (Clausius-Clapeyron)• Bowen ratio: Same rah

for water and heat

ρa

Mean air density (=1.2 kgm-3)ea

water vapour pressure (Pa; 1millibar=100Pa)es

saturation water vapour pressure (Pa)G

Soil heat flux (W/m2)Rn

Net radiation (W/m2)λET

latent heat flux (W/m2). λET = λv

ρw

ETcp

Specific heat of air (cp ≅

0.00101 MJkg-1K-1)Δ

Slope of saturation vapor pressure curve (PaK-1) γ

Psychrometric

constant (kPaK-1). γ=1.608pcp

/λv

Measuring Evapotranspiration

at

the regional scale

Remote Sensing: Triangle Method

MSc

Thesis Sara Lerer

The Triangle Methodmax

,min max

i

i

ETEF

ET Hλ ϕ ϕ

λ γ ϕ ϕΔ −

= = ⋅+ + Δ −

Evaporative Fraction

Jiang & Islam, 2001

NDVI (Normalized Difference Vegetation Index)

Normalized Difference Vegetation Index (NDVI)

Green vegetation has a characteristic spectral reflectance.

NIR vis

NIR vis

r rNDVI

r r−

=+

NDVI is a measure of density and state of the vegetation cover.Note that NDVI can also be used to detect water surfaces (NDVI<0).

ΔLST (Land Surface Temperature)

The TriangleMay 22, 2007 April 23, 2007

Mean AET (mm/day)

Plants and salinization

Shallow sand aquifer

Swamp Qdry

ET

Qin

Infiltration is roughly proportional to coastline length

Shashe River Valley: Setting

Tham

alaka

ne Fa

ult

Kunyere

Fault

Boronyana

Shashe

Nxotega

Maun

P. Bauer et al., Journal of Hydrology 316 (1-4):163-183, 2006

Distance from River Valley (m)

Rel

ativ

e E

leva

tion

(m)

BH 5746

BH 8258

BH 8256

BH 8253

BH Z10587

500 1000 1500 2000 2500-15

-10

-5

0

5

Water Level Oct. 1991Water Level Feb. 2000Water Level Dec. 2002

Shashe Field Data

Shashe River

Boronyane River

Nxotega River

12

6

Con

duct

ance

(mS)

N

8 km

MAUN

Distance from River Valley (m)

Rel

ativ

e

E

leva

tion

(m)

BH 5746

BH 8258

BH 8256

BH 8253BH Z10587

500 1000 1500 2000 2500-15

-10

-5

0

5

EC Apr. 1997EC Dec. 2002

0

10

20 G

roun

dwat

er E

Cm

S/c

m

The SystemTranspiration

Water is moving out of the domain, salts are left behind →Concentrations rise

concentration

transpiration

accumulation

toxicity

Standard implementation of transpiration in GW models

s iji j

h hS K qt x x

⎛ ⎞∂ ∂ ∂ ⎟⎜= +⎟⎜ ⎟⎟⎜⎜∂ ∂ ∂⎝ ⎠

,max( , , ) ( , ) ( )ET ET ETq x y h q x y f h= ⋅

( )( )ij i

i j i

c cD v c m

t x x xθ

θ θ⎛ ⎞∂ ∂ ∂ ∂⎟⎜= − +⎟⎜ ⎟⎟⎜⎜∂ ∂ ∂ ∂⎝ ⎠

( , , , )ET ET ETm q x y h c c= − ⋅

for c<

for c

ET

ET

c c

c

μ

μ μ

=

= ≥

( ) 1 for h>ESETf h =

( ) 1

for ES-d<h<ES

ETES h

f hd−

= −

( ) 0 h<ES-dETf h =

Flow Transport

The willow tree toxicity test

e.g. Larsen et al., ES&T, 2005

Results from willow tree experiments:

Phytotoxicity

of NaCl

11

1 /NRT

cτ= −

+

NRT: Normalized relative transpiration

ln(c), c in kg m-3

NR

T 240 (%

)

-3 -2 -1 0 1 2 3 40

10

20

30

40

50

60

70

80

90

100normalized NRT240 dataexcluded 24 hour databest fit95% confidence interval

In agreement with published literature, e.g. Maas & Hoffman (1977), v. Genuchten & Hoffman (1984)

-3 = 0.39 kg mτ

Results from willow tree experiments:

Uptake of NaCl0 5 10 15 20

-10

0

10

20

30

c in kg m-3

c T in k

g m

-3

full concentration range

24h48h72h96h168h192h240h

0 2 4 6 8-2

0

2

4

6

8

c in kg m-3

c T in k

g m

-3

zoomed inset

Break-through point

c: salt concentration in watercT

: uptake concentration

Model of uptake by plant roots

Root compartment

passive uptake of salt and waterenzymatic removal of saltuptake of water and salt

Model of plant roots

2

,max

,max ,maxmax

,ma

max

x

0

1 4( )

2

R T

TRW

T

R RW

T TM

R

RW

R RW

TM

M RWT R

R

Tdc

V q cdt

c cKqM Kq qc K vM M

qv K c

MK K c

K

c

Kq

cM

β β αγα

α

β

γ

⋅ ⋅⋅

+= = ⋅ − −

− − −=

= −

= − −

⋅⋅

=

Michaelis-Menten

kinetics for removal

Steady-state in the root compartment

Model Results: Original Concept

2700 years

5400 years

8100 years

Model Results: New Concept

2700 years

5400 years

8100 years

Conclusions•

Plants are key players in the hydrological cycle

•

Plants (irrigated agriculture) is the major water user on the global scale

•

Plant evapotranspiration

is still one of the most challenging processes –

both

in terms of modeling and monitoring•

Plants have a key role in soil and groundwater salinization