SPATIAL VARIABILITY OF SOIL-SALINITY AT DIFFERENT SCALES IN THE MANGROVE RICE AGROECOSYSTEM IN WEST-AFRICA

Spatial variability of soil salinity in coastal low lands results from a complex interaction of climate, river hydrology, topography and tidal flooding. The aim of this study was to determine the significant effects of these causal factors at different scales in the West African mangrove environment. The driving forces are the penetration of tidal saline waters and subsequent water evaporation in the flood plain, of which the magnitude is controlled by the causal factors. A hierarchical framework of the different factors was designed. Four river basins were selected: the Gambia, the Casamance (Senegal), the Geba (Guinea Bissau) and the Great Scarcies (Sierra Leone). Within each river basin, three strips of land (80 m wide, 500 to 1800 m long), perpendicular to the river at different distances from the mouth, were selected. In the dry season of 1991, soil samples were taken from the strips using a 40 m x 20 m grid at five soil depths to be analyzed for salinity. The contribution of the different sources to salinity spatial variability was analyzed with a nested ANOVA. Geostatistics were used to model spatial variability at micro-scale. As a result, main environments at macro scale (between river basins), sub-environments at meso scale (within river basins) and salinity classes at micro scale (within catena) were defined. Nested regression and geostatistics were found complementary to disentangle the complexity of the factors influencing salinity spatial variability.