Less rain, more water in ponds: a remote sensing study of the dynamics of surface waters from 1950 to present in pastoral Sahel (Gourma region, Mali)

摘要

Changes in the flooded area of ponds in the Gourma region from 1950 topresent are studied by remote sensing, in the general context of the currentmulti-decennial Sahel drought. The seasonal and interannual variations of theareas covered by surface water are assessed using multi-date and multi-sensorsatellite images (SPOT, FORMOSAT, LANDSAT-MSS, –TM, and -ETM, CORONA, andMODIS) and aerial photographs (IGN). Water body classification is adapted toeach type of spectral resolution, with or without a middle-infrared band, andeach spatial resolution, using linear unmixing for mixed pixels of MODISdata. The high-frequency MODIS data document the seasonal cycle of floodedareas, with an abrupt rise early in wet season and a progressive decrease inthe dry season. They also provide a base to study the inter-annualvariability of the flooded areas, with sharp contrasts between dry years suchas 2004 (low and early maximal area) and wetter years such as 2001 and 2002(respectively high and late maximal area).The highest flooded area reachedannually greatly depends on the volume, intensity and timing of rain events.However, the overall reduction by 20% of annual rains during the last40 years is concomitant with an apparently paradoxical large increase in thearea of surface water, starting from the 1970's and accelerating in the mid1980's. Spectacular for the two study cases of Agoufou and Ebang Mallam, forwhich time series covering the 1954 to present period exist, this increase isalso diagnosed at the regional scale from LANDSAT data spanning 1972–2007.It reaches 108% between September 1975 and 2002 for 91 ponds identified incentral Gourma. Ponds with turbid waters and no aquatic vegetation are mostlyresponsible for this increase, more pronounced in the centre and north of thestudy zone. Possible causes of the differential changes in flooded areas arediscussed in relation with the specifics in topography, soil texture andvegetation cover over the watersheds that feed each of the ponds. Changes inrain pattern and in ponds sedimentation are ruled out, and the impact ofchanges in land use, limited in the area, is found secondary, as opposed towhat has often been advocated for in southern Sahel. Instead, majorresponsibility is attributed to increased runoff triggered by the lastingimpact of the 1970–1980's droughts on the vegetation and on the runoffsystem over the shallow soils prevailing over a third of the landscape.