Evidence of an increase in pond surface during the multidecennial drought in pastoral Sahel J. Gardelle, P. Hiernaux, L. Kergoat, M. Grippa, and E. Mougin CNRS, CESBIO, Toulouse, France 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 in the sahelian Gourma (Mali) period and is based on long time series of remotely sensed data. 2 Site, data and methods SatelliteSensor Spatial resolution Spectral resolutionYear of acquisitionGround coverage Number of images SPOT 1HRV20 mG, R, NIR199060km*60km2 SPOT 4HRVIR20 mG, R, NIR, MIR2005 to km*60km14+5 FORMOSAT-28 mB,G,R,NIR200724km*24km30 LANDSAT MSS57 mG, R, NIR km*180km 3 TM28,5 mB, G, R, NIR MIR1986 and ETM28,5 m / 30 mB, G, R, NIR, MIR1999 to TerraMODIS250 mR, NIR2000 to km*1200km366 CORONAKH-4A2,79 mPAN1965 and km*230km8 Aerial photographs1.06 mPAN1954 and km*10km2 List of satellite and aerial data. Rainfall anomaly in Hombori (Gourma). 3 Spectral characterisics of the ponds : ‘blue’ ponds and ‘red’ ponds « blue »: no vegetation, high sediment loading. « Turbid ». « red »: vegetation in the pond less 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 Results: a dramatic increase in ponds’ area 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 together 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 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