Post on 14-Jan-2016
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Response of Sahelian vegetation to climatic variability
Consequences for the Surface-Atmosphere interactions
Mougin E.1, Hiernaux P.1, Kergoat L.1, Seghieri J. 1, Lavenu F.1, Tracol Y.1, Guichard F. 2, Jarlan L.2, Diarra L.3, Dembélé F.3, Karembé M.3, Mougenot B.1, Timouk F.1, de Rosnay P.1, Le Dantec V.1, Baup F.1, Mangiarotti S1.
1 CESBIO, 18 avenue Edouard Belin 31405 Toulouse / France2 CNRM/Météo-France, 42 avenue G. Coriolis 31057 Toulouse / France3 IER Institut d’Economie Rurale, Bamako, Mali
Presented by L. Kergoat
Eco-climatic zones of WA
Sahel usually delimited by 100 - 600 mm annual rainfall
Characteristics of the Sahelian vegetation
TreesShrubs
Wood layer}
Perennial grasses Annual grasses
Grass layer
crops (millet, sorghum)
variability of rainfall over Sahel (10°N – 20°N)
High spatial variability (courtesy Lebel et coll.)
Rainfall
add soil type and land use,you get vegetation variabilitywhich is very high.
Hiernaux (ILCA) and Diarra (IER), 1984, Cesbio+IER from 1996 onward
Long term sites network established in 1984 (extrem drought)Long term sites network established in 1984 (extrem drought)
Grass biomasstree densityspecies% bare soil
sample 1 km transectconsistent with satellite
09/84
09/86
09/89
09/93
09/85
09/87 09/88
08/90
09/00
09/92
09/99
Phenology and growth of Sahelian vegetation
• annuals : direct response to rainfall (temporal distribution of rainfall events)
Seasonal and interannual dynamics
Model STEP vs data
Mougin et al
Biomass variability explained by climate+vegetation model
Observation period
Interannual herbaceous productivity
Large variability + non linearity (rainfall distribution, dry spells)
STEP modelMougin et al
AVHRR trend
Anyamba and Tucker 2005
J Arid Envir. ‘greening’ of Sahel special issue
Consequences for Surface / Atmosphere Interactions
1) Characterize time/space variability of vegetation and fluxes response at site, landscape, and West African gradient scales
AMMA will do it
Sahel vegetation : Large temporal variability
Grass and crop phenology responds quickly
non linearity
Sahelian ecosystems show SOME resilience propertiesview of Sahel drought has changed since the 70’ 80’
Gourma mesoscale Site (~300 x 100 km²)
Hombori Supersite (15.4°N, 1.6°W)
Agoufou local Site (15.3°N, 1.5°W)
Bamba local Site (17.1°N, 1.3°W)
Niger River
Rainfall 50 mm
350 mm
450 mm
300 mm
100 mm
See postersSeghieri et alLe Dantec et aland TT3
Albedo of Agoufou grassland
2002
2003
albedo
rainfall
2005
2004
dry
dry
wet
wet
Day of year Guichard et al
2002
2003
2004
2005
Surface Net Radiation for Agoufou grassland
higher
higher
dry
dry
wet
wet
Guichard et al
Radiative balance, AgoufouAugust 2002 to 2005
-600
-500
-400
-300
-200
-100
0
100
200
300
400
500
2002
2003
2004
2005
IR IR SW SW Rnet (W.m-2)
-40
-30
-20
-10
0
10
20
IR IR SW SW Rnet (W.m-2)-400 +460 -250 +50 -140
2002
2003
2004
2005
Wet (2003 & 2005) =
low SW downbut low albedo
and low IR up
result : more energy for sensible and latent heat flux(and thetae)
Rn - G = LE +H
Models probably able to simulate this (ask ALMIP)Guichard et al
Tree leaf phenology
Fluxes ? Models ?
HiernauxSeghieri
Interannuel effects …for SOME species or SOME trees/shrubs
Consequences for Surface / Atmosphere Interactions
2) Sahelian paradoxes
‘le diable est dans les détails’. What would be the typical time scale of return to pre-drought state ? Water routing versus vegetation recovery ? What about models ?
Changes in vegetation -but different cases - and soil surface.
Increasing water table (Niger) : more runoff (crops, fallows), more infiltration from ponds. Favreau et al
Increased river runoff in the Sahelian zone, as opposed to decreased in Soudanian zone Mahé et al 05
Tiger bush shrinks, increase of runoff Valentin et al 01, Wu et al 05
Mare dAgoufou : temporary pond before 1984 permanent pond after the 1984 drought. Less rain, more run on. Why ? (crops not responsible) Hiernaux pers obs
3) The Quest for Memory : time lags effects ?
Sahelian droughts are better simulated when droughts impact ecosystems on time > 1 year e.g. Zeng et al 99
Lag effects are present in rainfall and/or remote sensing datae.g. Philippon et al
Current year -> current growth, Rn, ETR of annual grasses OKTree : leaf phenology depends on yr, yr-1 depending on tree species
yr-1, yr-2 seed bank effect ?Wind erosion ?Litter/nutrient effect ?Species dynamic effect ?
Population dynamics of grass species and links to rainfall (Gourma 84-93) Hiernaux, Diarra
Asymetry wet/dryhysteresis
Impact on fluxes ?
Similar shemes for trees, crops/fallow
LSM, SVAT ….
SUMMARY
Sahelian ecosystems : high time and space variability some resilience ?
Need for long term sites, network of sites. (really)
Intra-seasonal : some confidence in vegetation/SVAT models
Trends : some confidence in models (rain driven)
Multi year effect : tree phenology more to learn but we already know something (LE flux ?) Sahelian paradoxes : challenging !Series of interactions vegetation/surface/runoffno idea of associated fluxes changes
Surface / atmosphere interactions