L. Kergoat, F. Timouk, E. Mougin, E. Ceschia,P. Hiernaux, P. de Rosnay, V. Le DantecCESBIO, Toulouse, France

C.R. Lloyd , C.M. Taylor,CEH, Wallingford UK.

Response of sensible, latent and CO2 fluxes to lateral water redistribution and vegetation development

in a Sahelian landscape at 15 °N

17-04-2008, European Geophysical Union, Vienna

Land surface fluxes and energy budget in the Sahel

A lot has been said ...

from the Otterman, Charney, Jackson, Idso, Ripley debate in 1975on albedo and (over) grazing and the Sahel drought

....

to Notaro et al. 2008 Global Change Biology negative feedback : rainfall / land surface / rainfallassuming 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

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 ....

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° NYou have 60 % chance to get this

24h averaged H

Land surface fluxes and energy budget in the Sahel

You have 35 % chance to get this

24h averaged H

Land surface fluxes and energy budget in the Sahel

You have 5 % chance to get this

24h averaged H

Large spatial variability !

Land surface fluxes and energy budget in the Sahel

60 km

The Hombori supersite : 3 main landscape units

rocky soilhigh run-off

loamy-clayed soil lowland, ponds,high run-on

sandy soilno run-off

60 km

soil type and water regime

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

June 06

July 17

August 19

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

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

H = Rnet

H < 0

high LE

H Rnet

Timouk, Kergoat et al, subm.

Daily evapotranspiration and Fco2

Agoufou ; High ETR, > 85% P rapidely returned to the atmosphere, High Fco2 (reaches 18mol m-2s-1 at 12h)

Kelma : Extremely high ETR (>> P), Very high Fco2 (reaches 30mol m-2s-1 at 12h)

Simple scaling of sensible heat flux to supersite scale (60 km)H = wi. Hi

Averaged H close to Agoufou

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