Surface energy (and water) budget in the Sahel during the multi-decadal drought: combining remote sensing

and in situ data.

Laurent Kergoat (CNRS/LMTG, Toulouse), Pierre Hiernaux, Françoise Guichard, Johanna Ramarohetra, Manuela Grippa, Franck Timouk, Eric Mougin and AMMA land surface and remote sensing groups(CEH, LTHE, HSM, LA)

Monitoring the Sahel droughtLargest multi decadal precipitation change, huge impact on population (water, food)

mean 350 mm

wet 50’ : pre drought

Data from DNM Mail

19842004

15°N, 1W

drought still important, maybe a recent trend towards recovery

3 debated questions

-1) Debate on positive feedback between surface and rainfall able (or not) to significantly strengthen monsoon anomaly

Vegetation decay increases surface albedo (and Rnet ?)Extremely few data

- 2) Debate greening or desertification ? Satellite series show a greening over 1982-present, Many studies point towards land degradation

-3) Debate on increased runoff in the Sahel: man-driven (crop area, woof cut) ?

Collection of results from the AMMA project (EU).Anyamba and Tucker 3005

Eddy fluxes and radiation stations networkvegetation, moisture...

9°40

13°4015°3017°

AMMA set up (starting 2005)

data from CEH, LMTG, HSM, LTHE, LA

60 km

The Hombori supersite : 3 main landscape units

June 06August 19

Seasonal cycle :sensible heat flux. Importance of surface runoff

Timouk et al. J. Hydrol. 2009

Evaluation of LSM: ALMIP exercisefair agreement for H flux but ....

Boones et al. BAMS 2009

Evaluation of satellite LandSAF and gewex-SRB products, ECMWF ISF and AMMA re-analysis

surface incoming radiationlarge overestimation of SWinc mainly due to dus (reverse for LW !)

Ramier et al. 2009

and ...ALMIP / GRACE significant differences in annual amplitude of soil moisture, partly due to dry season ETR.

Grippa et al 2010, WRR, in revisionALMIP 2 : mesoscale LSM model/data comparison, starting 2011 !

Surface net radiation for climate modelsAMMAMIP intercomparison (forced SST)

Hourdin et al BAMS 2010

OBS

too flat, large biases, even or mainly before the rain season !

Interannual variabilty : In situ and MODIS albedo

Satellite / in situ agreement on seasonal and interannual variability

Dramatic effect of the 2004 drought

Consistent with 1984-present difference in Meteosat albedo

(Latanziani and Govaerts, Eumetsat)Variability of the LW budget even more important ! (not shown)

Provides solid bases for feedback modeling (positive likely)

2004 drought

fire

Samain et al. JGR 2008

However, at decadal time scales, other processes become important« less rain, more water in ponds »

1966

1996

2007Gardelle et al., HESS, 2010

+ 100 increase in pond surface (n=91)

increase tree cover in valleys

On shallow soils, increased runoff, vegetation decay, soil erosion

2008

1954

shallow soils: Changes in Tin Adjar catchment, 16° N, 1954 - 2008

Ramarohetra 2010

CORONA. 15/10/1965 SPOT 28/12/03

2,25 6,52

11,65

37,26

41,44

0,8818,66

12,20

38,71

15,02

4,64

10,77

1954 2008

sand

rock trees

vegetated clayed valley

Decadal-scale effects of the Sahel drought:

sandy soil (65%): strong resilience of ecosystems (Hiernaux et al. 2009)complete recovery of trees and grassesconsistent with the ‘greening’ Sahel and rainfall

‘trend’

shallow soils (30 %)vegetation decay, triggered by the 1980’ droughtsincreased and concentrated runoff (gullies)important erosion. consistent with ‘degradation’. still decaying, completely climate driven in the Gourma !

lowlands, valleys, run-on areas (5%)spectacular increase of pond surface, large increase of trees coverconsistent with the greening Sahelcompletely cimate driven in the Gourma !

Decadal-scale effects of the Sahel drought:

Long-term effects on surface surface fluxes and water cycle

Delayed ETR cycle Rnet changes depending on both SW and LWdepending on plant cover changesdepending on soil erosion (and soil colour)more than an albedo story !

Involves threshold response of eco-hydrosystems to climatemay not be reversible (erosion)

Combines with land use changes (Southern Sahel)

Combines with dust emission and surface radiation budget

Thank you