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| Titel |
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) |
| VerfasserIn |
J. Gardelle, P. Hiernaux, L. Kergoat, M. Grippa |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 14, no. 2 ; Nr. 14, no. 2 (2010-02-16), S.309-324 |
| Datensatznummer |
250012185
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| Publikation (Nr.) |
 copernicus.org/hess-14-309-2010.pdf |
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| Zusammenfassung |
| 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. |
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