Detection of post-drought environmental conditions in the Tombouctou region

Abstract

As a test of the sensitivity of Landsat Thematic Mapper (TM) data to evaluate small-scale environmental change and post-drought recovery, a time series of TM images of the small, well-mapped region around Tombouctou, Mali was analysed for change for the interval 1986-1988. Actual increases in vegetation measured during field work differed for specific landforms, ranging from 5 per cent on floodplain silts to a 40 per cent increase in ephemeral grass cover on stable dunes. This region is part of the Saharan-Sahelian fringe, where severe drought and environmental degradation occurred between 1976 and 1985. Reflectance curves derived from the TM images for six specific landform units between March 1986 and March 1988 show that seasonal reflectance changes are statistically significant, and that overall reflectance decreased over the study interval. Dunes, abandoned wadis and inactive floodplain areas near the Niger river show vegetation regrowth, while active floodplain and densely-vegetated distributary channels showed little change. Albedo proved to be a reasonable simple indicator of both seasonal change and change over the term of study. Neither the Normalized Difference Vegetation Index (NDVI), the Soil-Adjusted Vegetation Index (SAVI) nor the Leaf Vegetation Index (V_L) produced results consistent with field observation. Normalized reflectances and albedo both show appropriate seasonal trends and an overall trend towards increased vegetation that is consistent with field observations over the study interval.