SPATIO-TEMPORAL VARIATIONS OF TOP SOIL COARSENING AS A PEDO-GEOMORPHIC INDICATOR OF LAND DEGRADATION AND DESERTIFICATION STATUS IN A PART OF SEMI-ARID ZONE OF NIGERIA

Abstract

Soil degradation is commonly mentioned as an indicator of desertification, not much is focused on specific form of soil degradation such as top soil coarsening which is rarely studied with respect to desertification. The major aim of this study therefore, is to assess the spatio-temporal patterns of top soil coarsening as a pedo-geomorphic indicator of desertification status (extent, rate and magnitude) in the semi-arid zone of Nigeria. This achieved using top soil grain size index (GSI) evaluated from remotely sensed reflectance of three (3) bands R, B and G of multi-temporal Landsat images (TM, 1987; ETM+, 2000 and OLI/TIR, 2015) and computed using a ratio algorithm. The multi-temporal sets of GSI (1987, 2000 and 2015) were generated from corresponding temporal Landsat images and  top soil grain size sensitivity index (GSSI) was evaluated from the sensitivity analysis of the geometric mean of the three sets of GSI. Each of the temporal GSIs and the GSSI were segmented into five (Very High, High, Moderate, Low and Very Low) sensitivity to desertification areas. Result show that there is a generalized temporal trend of increasing intensity of top soil fine sand content or coarsening and hence desertification in the study area. Although the intensity varies among all the respective sensitivity areas mapped. For instance the highest intensity of 21.15% was recorded in the Very High Sensitivity Area, followed by Moderate (18.39%), High (17.50%), Low (12.82%) and Very Low (7.53%). The spatial extents  of the distribution of  top soil coarsening or fine sand content show a decline in the Very High and High Sensitivity Areas at annual rates of 1.46 and 1.39 km2 respectively, while in the Moderate, Low and Very Low Sensitivity Areas expansion at annual rates 1.17, 1.38 and 1.38 Km² respectively. Dynamic rate for the period (1987-2015) among sensitivity areas stood at Very High (3.56%), High (1.36%), Moderate (3.32%), Low (8.63%) and Very Low (43.74%). The single factor ANOVA analysis at 0.05 level of significance showed that there is significant variation in the change in top soil coarsening over the period (1987-2013) as well as among the various desertification sensitivity areas.