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Laurent Kergoat (CNRS/LMTG, Toulouse),

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),

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Laurent Kergoat (CNRS/LMTG, Toulouse),

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  1. 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)

  2. Monitoring the Sahel drought Largest multi decadal precipitation change, huge impact on population (water, food) 15°N, 1W Data from DNM Mail mean 350 mm 2004 1984 wet 50’ : pre drought drought still important, maybe a recent trend towards recovery

  3. 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

  4. AMMA set up (starting 2005) Eddy fluxes and radiation stations network vegetation, moisture... 17° 15°30 13°40 9°40 data from CEH, LMTG, HSM, LTHE, LA

  5. June 06 August 19 The Hombori supersite : 3 main landscape units 60 km

  6. Seasonal cycle : sensible heat flux. Importance of surface runoff Timouk et al. J. Hydrol. 2009

  7. Evaluation of LSM: ALMIP exercise fair agreement for H flux but .... Boones et al. BAMS 2009

  8. Evaluation of satellite LandSAF and gewex-SRB products, ECMWF ISF and AMMA re-analysis surface incoming radiation large overestimation of SWinc mainly due to dus (reverse for LW !) Ramier et al. 2009

  9. and ... ALMIP / GRACE significant differences in annual amplitude of soil moisture, partly due to dry season ETR. Grippa et al 2010, WRR, in revision ALMIP 2 : mesoscale LSM model/data comparison, starting 2011 !

  10. Surface net radiation for climate models AMMAMIP intercomparison (forced SST) OBS too flat, large biases, even or mainly before the rain season ! Hourdin et al BAMS 2010

  11. Interannual variabilty : In situ and MODIS albedo 2004 drought fire Samain et al. JGR 2008 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)

  12. However, at decadal time scales, other processes become important« less rain, more water in ponds » + 100 increase in pond surface (n=91) increase tree cover in valleys 1966 1996 Gardelle et al., HESS, 2010 2007

  13. shallow soils: Changes in Tin Adjar catchment, 16° N, 1954 - 2008 1954 2008 Ramarohetra 2010 On shallow soils, increased runoff, vegetation decay, soil erosion

  14. SPOT 28/12/03 CORONA. 15/10/1965 trees rock vegetated clayed valley sand 2008 1954

  15. Decadal-scale effects of the Sahel drought: sandy soil (65%): strong resilience of ecosystems (Hiernaux et al. 2009) complete recovery of trees and grasses consistent with the ‘greening’ Sahel and rainfall ‘trend’ shallow soils (30 %) vegetation decay, triggered by the 1980’ droughts increased 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 cover consistent with the greening Sahel completely cimate driven in the Gourma !

  16. 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 LW depending on plant cover changes depending on soil erosion (and soil colour) more than an albedo story ! Involves threshold response of eco-hydrosystems to climate may not be reversible (erosion) Combines with land use changes (Southern Sahel) Combines with dust emission and surface radiation budget Thank you

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