Combined uses of supervised classification and Normalized Difference Vegetation Index techniques to monitor land degradation in the Saloum saline estuary system

Saltwater contamination constitutes a serious problem in Saloumestuary, due to the intermittent
and reverse tide flows of the Saloum River. This phenomenon is caused by the runoff deficit,
which forces the advance of saltwater 60 km upstream, contaminating surface water and thus
causing the degradation of biodiversity and large areas of agricultural soils in this region. The
present study aims to evaluate the consequences of saltwater contamination in the last three
decades in this estuary by assessing the land-cover dynamics. Thus, latter consists of tracking
the landscape-changing process over time to identify land-cover transitions. These transitions
are closely related to the ecosystem-setting condition and can be used to assess the combined
impacts of both natural and human-induced phenomena over a given period of time. In this
study, special attention was given to mangrove degradation and to temporal progression of the
salty barren soils locally called ‘‘tan’’. The loss of mangrove areas to tan and the general increase
in salty barren soil areas can reflect the increase in the level of salinization in the study area over
the time period under consideration. To fulfill this objective, four Landsat satellite images from
the same season in the years 1984, 1992, 1999, and 2010 were used to infer time series land-use
and land-cover maps of the Saloumestuary area. In addition to satellite imagery, rainfall records
were used to evaluate climatic variation in terms of high-to-low precipitation during the time
span considered. Spectral analysis indicated that from 1984 to 2010, mangroves and savanna/
rain-fed agriculture are converted to ‘‘tan’’ (denuded and salty soils). In addition, these results
showed that significant changes in land use/land cover occur within the whole estuary system
and reflecting therefore environmental degradation, such as land desertification and salinization,
and vegetation degradation which reflect the advanced of salinity