Relations between soil colour and waterlogging duration in a representative hillside of the West African granito-gneissic bedrock

The purpose of this study was to analyse if the colour of soils on granito-gneissic bedrock of West Africa could give some simple indicators of the duration of soil waterlogging. A toposequence on a representative hillside in central Togo (lat 8°38′–8°39′N, long 1°00′–1°01′E) was studied by means of 19 hydropedological stations. Soil and piezometric conditions were studied on these stations for three annual cycles (1989–1992) during which rainfall conditions were representative of the mean rainfall conditions for that hillside. Data collected in each station were used to calculate the mean annual rate of soil waterlogging (WLG) at 10-cm intervals from the surface down to a depth of 7 m. At the same intervals, 10 numerical colour variables were calculated from the colours of the uncemented soil phases: Munsell value, chroma, angular hue and redness rating of the principal uncemented phase; mean Munsell value, mean chroma, mean angular hue and mean redness rating; barycentric chroma and barycentric angular hue. Statistical relationships were established between WLG and each of the variables derived from soil colour (10×574 pairs). The two most significant relations were those between WLG on one hand, and mean angular hue and mean redness rating on the other. These two relationships provided the basis for logistic models for predicting the mean annual rate of soil waterlogging. The operational limits of these models are discussed.