Response of sensible, latent and CO2 fluxes
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L. Kergoat, F. Timouk, E. Mougin, E. Ceschia, P. Hiernaux, P. de Rosnay, V. Le Dantec PowerPoint PPT Presentation


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Response of sensible, latent and CO2 fluxes to lateral water redistribution and vegetation development in a Sahelian landscape at 15 °N. L. Kergoat, F. Timouk, E. Mougin, E. Ceschia, P. Hiernaux, P. de Rosnay, V. Le Dantec CESBIO, Toulouse, France C.R. Lloyd , C.M. Taylor,

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L. Kergoat, F. Timouk, E. Mougin, E. Ceschia, P. Hiernaux, P. de Rosnay, V. Le Dantec

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L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Response of sensible, latent and CO2 fluxes

to lateral water redistribution and vegetation development

in a Sahelian landscape at 15 °N

L. Kergoat, F. Timouk, E. Mougin, E. Ceschia,

P. Hiernaux, P. de Rosnay, V. Le Dantec

CESBIO, Toulouse, France

C.R. Lloyd , C.M. Taylor,

CEH, Wallingford UK.

17-04-2008, European Geophysical Union, Vienna


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

A lot has been said ...

from the Otterman, Charney, Jackson, Idso, Ripley debate in 1975

on albedo and (over) grazing and the Sahel drought

....

to Notaro et al. 2008 Global Change Biology

negative feedback : rainfall / land surface / rainfall

assuming bare soil evaporates more than grasses (!)

Almost nothing has been measured !

except SEBEX (near Niamey, 13.5 °N, 1990, 1991)

HAPEX-Sahel (near Niamey, 1992) bush fallow, millet, tiger bush

Land surface fluxes and energy budget in the Sahel

and nothing at all in central and northern Sahel.

Lat 15° N, Lon 0° : the most studied grid point in the Sahel

... by modelers only, sofar ....


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Land surface fluxes and energy budget in the Sahel

Objectives : to present SEB and turbulent fluxes at lat 15° N, lon 0°

and identify the factors controlling these fluxes.

Rnet = H + LE + G

If you install a flux tower at 15° N

You have 60 % chance to get this

24h averaged H


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Land surface fluxes and energy budget in the Sahel

You have 35 % chance to get this

24h averaged H


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Land surface fluxes and energy budget in the Sahel

You have 5 % chance to get this

24h averaged H

Large spatial variability !


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

The Hombori supersite : 3 main landscape units

60 km


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

soil type and water regime

rocky soil

high run-off

sandy soil

no run-off

loamy-clayed soil

lowland, ponds,

high run-on

60 km


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Agoufou, sandy soil, grassland, no run-off run-on

June 06

July 17

August 19


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Eguerit, rocky soil (gravel), no vegetation, high run-off


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Kelma, loamy-clayed soil, open forest, high run-on


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

H  Rnet 

high LE

H < 0

H = a Rnet

Timouk, Kergoat et al, subm.


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Daily evapotranspiration and Fco2

Agoufou ; High ETR, > 85% P rapidely returned to the atmosphere,

High Fco2 (reaches 18mmol m-2s-1 at 12h)

Kelma : Extremely high ETR (>> P),

Very high Fco2 (reaches 30mmol m-2s-1 at 12h)


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Simple scaling of sensible heat flux to supersite scale (60 km)

H = S wi. Hi

Averaged H close to Agoufou


L kergoat f timouk e mougin e ceschia p hiernaux p de rosnay v le dantec

Conclusions

High spatial variability of surface fluxes (including some very high ETR and Fco2 !)

 Lateral water redistribution and soil type controls :

- soil moisture in the root zone

- vegetation growth & vegetation type

- Rnet

- flux partitionning

 Landscape units large enough to maintain CBL gradients (mesoscale circulation?)

 Simple scaling shows that 60 km x 60 km average is close to Agoufou site

 Preliminary comparison implies ECMWF H flux is too high, more to come (ALMIP)

Thank you


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