Remote Sensing Mapping of the Rice Field and Vegetal Cover and the Impacts of Soil Factors on Herbaceous in the Sub-Watershed of Boutolate (Lower Casamance, Senegal)

Abstract

Rice cultivation in lowlands plays an important role for population livelihood in lower Casamance. This economic activity is known to be affected by many contraints which are climate change and soil salinity. soil salinity is observed in rice fields and in the vegetation cover of Boutolate sub-watershed which covers an area of 6594,21 ha. Due to this situation, anti-salt micro-dams were built to retain water and also changes in land use around the area. This work aims to study rice field area and vegetation cover dynamics in the sub-bassin area located in the south-western of Lower Casamance. An overall time period of 26 years was considered in our study. The satellite imagery Landsat TM of 1984 and 2003 and ETM+ of 2000 and 2010 combined with field ground data was used to analyze the rice field area, vegetation cover dynamics and to understand the impacts of climatic and anthropogenic factors on its evolution. Floristic inventory was also done using plot size 3x3m along the upstream and downstream of micro-dams. In each plot, soil samples were taken for laboratory analysis to determine the impacts of geological factors on herbaceous strata in cultivable paddy area. The results showed a decrease of area paddy during these periods. The decrease in cultivated area was due to rainfall deficit which increased soil salinity and the worse management and monitoring plan after micro-dams were established in 2004. These caused floods which affected 0,93% of upstream rice-growing units in 2010 . The increase of shrub-tree savannah between 1984 and 2010 was due to the natural regeneration. The natural regeneration was favored by the decrease of fraudulent logging related to insecurity in the area and the expansion of fruit trees, particularly Anacardium occidentale. Anacardium occidentale area increased between 2000 and 2010. The soil physical parameters influenced significantly on density and diversity of herbaceous