Abstract
Abstract. Changes in the flooded area of ponds in the Gourma region from 1950 to present are studied by remote sensing, in the general context of the current multi-decennial Sahel drought. The seasonal and interannual variations of the areas covered by surface water are assessed using multi-date and multi-sensor satellite images (SPOT, FORMOSAT, LANDSAT-MSS, –TM, and -ETM, CORONA, and MODIS) and aerial photographs (IGN). Water body classification is adapted to each type of spectral resolution, with or without a middle-infrared band, and each spatial resolution, using linear unmixing for mixed pixels of MODIS data. The high-frequency MODIS data document the seasonal cycle of flooded areas, with an abrupt rise early in wet season and a progressive decrease in the dry season. They also provide a base to study the inter-annual variability of the flooded areas, with sharp contrasts between dry years such as 2004 (low and early maximal area) and wetter years such as 2001 and 2002 (respectively high and late maximal area).The highest flooded area reached annually greatly depends on the volume, intensity and timing of rain events. However, the overall reduction by 20% of annual rains during the last 40 years is concomitant with an apparently paradoxical large increase in the area of surface water, starting from the 1970's and accelerating in the mid 1980's. Spectacular for the two study cases of Agoufou and Ebang Mallam, for which time series covering the 1954 to present period exist, this increase is also diagnosed at the regional scale from LANDSAT data spanning 1972–2007. It reaches 108% between September 1975 and 2002 for 91 ponds identified in central Gourma. Ponds with turbid waters and no aquatic vegetation are mostly responsible for this increase, more pronounced in the centre and north of the study zone. Possible causes of the differential changes in flooded areas are discussed in relation with the specifics in topography, soil texture and vegetation cover over the watersheds that feed each of the ponds. Changes in rain pattern and in ponds sedimentation are ruled out, and the impact of changes in land use, limited in the area, is found secondary, as opposed to what has often been advocated for in southern Sahel. Instead, major responsibility is attributed to increased runoff triggered by the lasting impact of the 1970–1980's droughts on the vegetation and on the runoff system over the shallow soils prevailing over a third of the landscape.
Highlights
The Sahel experienced an important decrease in precipitation during the second half of the 20th century, with severe droughts in 1972–1973 and again in 1983–1984 that have had a dramatic impact on the ecosystem and on the population living on the natural resources of this region (e.g. Dregne and Chou, 1992; Olsson, 1993; Hiernaux, 1996; Nicholson, 2001)
Changes in the flooded area of ponds in the Gourma region from 1950 to present are studied by remote sensing, in the general context of the current multi-decennial Sahel drought
The seasonal and interannual variations of the areas covered by surface water are assessed using multidate and multi-sensor satellite images (SPOT, FORMOSAT, LANDSAT-MSS, –TM, and -ETM, CORONA, and MODIS) and aerial photographs (IGN)
Summary
The Sahel experienced an important decrease in precipitation during the second half of the 20th century, with severe droughts in 1972–1973 and again in 1983–1984 that have had a dramatic impact on the ecosystem and on the population living on the natural resources of this region (e.g. Dregne and Chou, 1992; Olsson, 1993; Hiernaux, 1996; Nicholson, 2001). The Sahel experienced an important decrease in precipitation during the second half of the 20th century, with severe droughts in 1972–1973 and again in 1983–1984 that have had a dramatic impact on the ecosystem and on the population living on the natural resources of this region In some part of the Sahel, the rainfall deficit did not lead to a decrease in surface runoff or in water-table level, as it happened in the wetter Soudanian and Guinean zones further south in West Africa (Descroix et al, 2009). Evidence of an increase in water-table level has been reported in endorheic areas, such as in south-western Niger (Leduc et al, 2001). (2003, 2005a) outlined changes in hydrologic regime of rivers located in Burkina Faso, Mali and Niger, showing a discharge increase north of the 700-mm isohyets, and over northern.
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