evidence of an increase in pond surface during the multidecennial drought in pastoral sahel j....
TRANSCRIPT
0
100
200
300
400
1975 2002 1975 2002 1975 2002
Turbid water
Clear water
North Centre South
Flooded pond area (ha)
+ 130%
+ 270%
+ 27%
+ 517%
- 15%
+ 635%
- 5%
+ 99%
+ 16%
Evidence of an increase in pond surface during the multidecennial drought in pastoral Sahel
J. Gardelle, P. Hiernaux, L. Kergoat, M. Grippa, and E. MouginCNRS, CESBIO, Toulouse, France ([email protected])
1 The question : What is the response of ponds to the drought in central Sahel ?
The Sahel experienced the most important decrease in precipitation during the second half of the 20th century, with severe droughts having a dramatic impact on the population. However, in apparent contradiction with the negative precipitation trend, evidences of an increase in watertables level and in river discharge have been reported for some areas of southern Sahel, for example in South Western Niger).No information is available for the northern pastoral areas. This study is focused on the evolution of ponds over the 1954 - 2007 in the sahelian Gourma (Mali) period and is based on long time series of remotely sensed data.
2 Site, data and methods
Satellite SensorSpatial
resolutionSpectral resolution
Year of acquisition
Ground coverage
Number of
images
SPOT 1 HRV 20 m G, R, NIR 1990 60km*60km 2
SPOT 4 HRVIR 20 m G, R, NIR, MIR 2005 to 2006 60km*60km 14+5
FORMOSAT-2 8 m B,G,R,NIR 2007 24km*24km 30
LANDSAT
MSS 57 m G, R, NIR 1975
170km*180km
3
TM 28,5 m B, G, R, NIR MIR 1986 and 1987 2
ETM28,5 m / 30
mB, G, R, NIR, MIR 1999 to 2002 5
Terra MODIS 250 m R, NIR 2000 to 20081200km*1200k
m366
CORONA KH-4A 2,79 m PAN 1965 and 1966 17km*230km 8
Aerial photographs 1.06 m PAN 1954 and 1996 10km*10km 2
List of satellite and aerial data.
Rainfall anomalyin Hombori (Gourma).
3 Spectral characterisics of the ponds : ‘blue’ ponds and ‘red’ ponds
« blue »: no vegetation, high sediment loading. « Turbid ».
« red »: vegetation in the pondless sediment, « clear ».
Methods:
Classification is easy when MIR available. Differenciation of several water bodies (« red », « blue » ponds) is necessary whitout MIR band.
Visual interpretation for panchromatic images (CORONA, arerial photographs). Details in Gardelle et al..
Agoufou : a ‘blue’ pond.
Toundourou: a ‘red’ pond
Comparison MODIS / SPOT and MODIS/FORMOSAT- Ponds larger than 40 ha can be monitored with MODIS, giving acces to the seasonal cycle and inter-annual variability over 2000-2007
5 Results: a dramatic increase in ponds’ area 1954 -2007
Evolution of the Agoufou pond at the same spatial scale and same season ! 1966, 1996, 1999
Same for the Bangui Mallam pond
Agoufou pond area: seasonal cycle for 2000-2007together with historical data.
Agoufou pond area as a function of annual rainfall showing a clear change in pond regime starting in the early 90 6 A regional phenomenon :
survey of 122 ponds in 1975 and 2002 shows dramatic increase of pond areas, mostly turbid ponds.
7 Conclusions- Large increase ponds over the 1954-2007 period- Increased runoff not caused by crop extension in this area (almost NO crops).- Change in natural vegetation is the main ‘suspect’.- Future of this phenomenon is unknown What if plants recover ?
typical runoff generating surface, covering 35% of the region.
not a bias due to rainfall variability or seasona lsampling.
Cumulated change in pond surface in october (2002 compared to 1975) for the northern, central and sourthenr zone of the Landsat scene.
References : Gardelle et al. submitted to HESS
4 High and low resolution data
location of the ponds in the 3 zones of the study area
maps with contours of the different satellite and aerial images spanning 1954-2007