Biophysical controls on evapotranspiration and water use efficiency in natural, semi-natural and managed African ecosystems

C. Brümmer, L. Merbold, S. Archibald, J. Ardö, A. Arneth, N. Brüggemann,

A. de Grandcourt, L. Kergoat, A. M. Moffat, E. Mougin, Y. Nouvellon,

L. Saint-Andre, M. Saunders, R. J. Scholes, E. Veenendaal, W. L. Kutsch

Biophysical controls on evapotranspiration and water

use efficiency in natural, semi-natural and managed

African ecosystems

Outline:

- Eddy tower locations

- E vs. P

- Lags between E and R

(E and D)

- Stomatal vs. available energy control

→ Priestley-Taylor

α

and g

- Water use efficiency

21 eddy covariance site years in 15 minutes

EC tower in Bontioli Nature Reserve, Burkina Faso, 2006

Site locations

11 sites:

- Burkina Faso

- Botswana

- Republic of Congo (3x)

- Mali (2x)

- South Africa

- Sudan

- Uganda

- Zambia

→ In total 21 site years of EC data

Tower site 2000 km 0 Savanna Grassland Cropland Shrubland Barren Wetland Water Land cover

Evergreen broadleaf forest

Grass-dominated Tree-dominated

0

500

1000

1500

2000

2500

0

500

1000

1500

Precipitation (mm)

E

v

apot

rans

pi

rat

ion (

m

m

)

1:1

Water deficit

Water excess

Annual - C4 dominated

Annual - C3 dominated

Wet season - C4 dominated

Wet season - C3 dominated

E vs. P – Annual and wet season

→ E plateaus with increasing P

→ E exceeds both annual and

wet season P at some sites with

low P

→ No significant differences

between C3 and C4 sites

E:P-ratio vs. P

Wet season fit: R2 _{= 0.69}

E = 35.50P-0.67

Annual - C4 dominated Annual - C3 dominated Wet season - C4 dominated Wet season - C3 dominated

→ Only dry sites exhibit

E:P-ratios >1

Climatic drivers:

Time lags between E and R

_n

(seasonal)

0 100 200 300 400 0 50 100 150 200

Day since start of hydrological year

R n (W m -2 ) Hin 01/02 Tch 06/07 Tch 07/08 Tch 08/09 0 100 200 300 400 0 20 40 60 80 100 R n (MJ m -2 month-1) E ( mm mo n th -1 )

↓

↓

Day since start of hydrological year

E ( m m da y -1 ) Hin 01/02 Tch 06/07 Tch 07/08 Tch 08/09 -100 -50 0 50 100 0.7 0.75 0.8 0.85 0.9 Lag (days) NCCC C3 C4

→ Both linear and hysteretic

relationships → R

lagging

behind E

→ C

sites seemed to be

more coupled

Only equatorial sites were

chosen

→ R

likely no significant

driving force on seasonal

basis

Climatic drivers:

Time lags between E and D (seasonal)

0 100 200 300 400 0 0.2 0.4 0.6 0.8 1

Day since start of hydrological year

D (k Pa) Hin 01/02 Kis 05/06 Tch 06/07 Tch 07/08 Tch 08/09 0 0.2 0.4 0.6 0 20 40 60 80 100 120 D (kPa) E ( mm mo n th -1 )

↓

↓

Day since start of hydrological year

E ( m m da y -1 ) Hin 01/02 Kis 05/06 Tch 06/07 Tch 07/08 Tch 08/09 -100 -50 0 50 100 0.7 0.75 0.8 0.85 0.9 0.95 1 Lag (days) NCCC C3 C4

→ Same pattern found for

the seasonal lag between E

and D

→ Declining E while D rises

suggests other controlling

mechanisms (e.g., stomata)

Climatic drivers:

Time lags between E and R

_n

; E and D (diurnal)

00:00 06:00 12:00 18:00 24:00 0 2 4 6 8 10 E ( mm) -200 0 200 400 600 800 Wet season 00:00 06:00 12:00 18:00 24:00 0 2 4 6 8 10 E R_n -200 0 200 400 600 800 R n (W m -2 ) Dry season 00:00 06:00 12:00 18:00 24:00 0 2 4 6 8 10 E ( mm) 0 1 2 3 4 5 Wet season 00:00 06:00 12:00 18:00 24:00 0 2 4 6 8 10 E D 0 1 2 3 4 5 D (k Pa) Dry season

Example site:

Zambia, Mongu (C

)

Climatic drivers:

Time lags between E and R

_n

; E and D (diurnal)

-4 -2 0 2 4 0.5 0.6 0.7 0.8 0.9 1 W et season, E-R n lag NCCC ma x C3 C4 -4 -2 0 2 4 0.5 0.6 0.7 0.8 0.9 1

Dry season, E-R

n lag C3 C4 -4 -2 0 2 4 0.5 0.6 0.7 0.8 0.9 1

W et season, E-D lag

Lag (hours) NCCC ma x C3 C4 -4 -2 0 2 4 0.5 0.6 0.7 0.8 0.9 1

Dry season, E-D lag

Lag (hours) C3 C4

→ Close coupling

bewteen E and R

→ Decoupling bewteen E

and D

→ Positive lag numbers

in WS ⇒ available

energy control on E

→ Negative lag numbers

⇒ at least to some extent

stomatal control on E

Priestley-Taylor

α

and canopy conductance (g

_c

)

DOY/Day since start of hydrological year

Pries tley -T a y lo r α BF-Bon 2006 ML-Ago 2007 ZA-Mon 2007-2008 0 100 200 300 0 5 10 15

DOY/Day since start of hydrological year g c ( mm s -1 ) 0 5 10 15 0 0.5 1 1.5 g c (mm s -1 ) Pries tley -T a y lo r α 0 2 4 6 8 10 0 2 4 6 D (kPa) g c ( mm s -1 )

→ Seasonal course of

α

was closely linked to

rainfall pattern

→ Stomatal control at

Zambian site (C

)

→ No significant stomatal

control at C

sites (seasonal);

water limitation during dry

season keeps

α

values <1.26

Priestley-Taylor

α

− Seasonal and system differences

0.4

0.6

0.8

1

1.2

a

ues

Pr

ie

s

tl

e

y

-T

ay

lor

al

pha

Annual WS

C3

C4

Annual

Annual

WS

WS

C3

C4

→ Higher evaporation ratio

at grass-dominated sites

AND/OR stomatal control

on E at tree-dominated sites

Water use efficiency

0

50

100

150

200

0

100

200

300

400

E

(mm month

or kg H

O m

_month

₎

G

PP (

g

C

m

m

ont

h

)

Burkina Faso - Bontioli

Botswana - Maun

Congo - Hinda

Congo - Kissoko

Congo - Tchizalamou

Mali - Agoufou

Mali - Kelma

South Africa - Kruger

Sudan - Demokeya

Zambia - Mongu

0

1

2

3

4

5

0

1

2

3

4

5

6

Mean monthly D (kPa)

Me

a

n

mo

n

th

ly

WUE

(

g

C

k

g

H

O)

Zambia - Mongu (C

)

Dependency of monthly WUE on D

0

1

2

3

4

5

0

1

2

3

4

5

6

Mean monthly D (kPa)

Me

a

n

mo

n

th

ly

WUE

(

g

C

k

g

H

O)

Zambia - Mongu (C

)

Botswana - Maun (C

)

Dependency of monthly WUE on D

0

1

2

3

4

5

0

1

2

3

4

5

6

Mean monthly D (kPa)

Me

a

n

mo

n

th

ly

WUE

(

g

C

k

g

H

O)

Zambia - Mongu (C

)

Botswana - Maun (C

)

Mali - Kelma (C

)

Dependency of monthly WUE on D

0

1

2

3

4

5

0

1

2

3

4

5

6

Mean monthly D (kPa)

Me

a

n

mo

n

th

ly

WUE

(

g

C

k

g

H

O)

Zambia - Mongu (C

)

Botswana - Maun (C

)

Mali - Kelma (C

)

Congo - Tchizalamou (C

)

Dependency of monthly WUE on D

0

1

2

3

4

5

0

1

2

3

4

5

6

Mean monthly D (kPa)

Me

a

n

mo

n

th

ly

WUE

(

g

C

k

g

H

O)

Zambia - Mongu (C

)

Botswana - Maun (C

)

Mali - Kelma (C

)

Congo - Tchizalamou (C

)

South Africa - Kruger (C

4

)

Mali - Agoufou (C

Water use efficiency in relation to MAP

0

500

1000

1500

0

2

4

6

8

10

Mean annual precipitation (mm)

W

UE

(

g

C k

g

H

O)

C3 dominated

C4 dominated

Conclusions

→ E plateaus with increasing P; seasonal course of E

mainly driven by water availability

→ E at C

sites were more

closely coupled (to R

and D)

than at C

sites

→ Non-equatorial C

sites

reach α values of 1.26 in WS

→ Variable WUE (1.5 to 4) among

sites; C

sites clearly dependent on D;

constant WUE during WS positively

correlated to MAP

Acknowledgements

Thank you!

Photo by Lutz Merbold

CarboAfrica Project

g

_c

vs. LAI

0

2

4

6

0

2

4

6

8

10

Peak LAI

M

ean annual

g

(mm s

)

C

C

0

2

4

6

0

2

4

6

8

10

Peak LAI

M

ean w

et

s

eas

on

g

(mm s

)

C

C