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The objective of this study was to examine vegetation and vegetation change in Eritrea over a period from the mid 1980s to 2002 using satellite remote sensing, and relate observed changes to the recent history of drought and war in the region. Specific objectives were (1) to examine vegetation change in Eritrea (NE Africa) from the mid 1980s (1984, 1985 and 1986) to 1999, 2000 and 2002; (2) to map vegetation into three vegetation categories: desert, highland shrubs, and forest and (3) to analyze whether vegetation classes and/or changes differed among three vegetation remote sensing indices. Eritrea has an area of 2,234 km2 and occupies 1151 km along the Red Sea coast, with elevations ranging from -75m below sea level to 3018m. The climate is primarily arid to semi-arid, and vegetation is predominantly semi-arid and woodland. Population is sparse in the coast but dense in the central highlands. Eritrea experienced drought in the mid 1980s and again from 2000 to 2004. It also experienced two periods of war, from 1961 to 1991 and from 1998 to 2000. Vegetation was expected to increase during the 1990s in the absence of war and drought, but to decrease during periods of war and drought in the mid 1980s and 1999-2002. The study was conducted using eight pairs of Landsat TM and ETM+ images over eight study areas spanning the range of topography, climate, and population density in Eritrea. Normalized Difference Vegetation Index (NDVI), Soil Adjusted Vegetation Index (SAVI), and the Tasseled Cap Greenness (TC Greenness) indices were used to detect vegetation. The factors used to explain vegetation change were Moisture Stress Index (MSI), Tasseled Cap Wetness (TC Wetness), annual rainfall amount, topography, drought, rivers, deforestation, and land use. Analysis of Variance (ANOVA) was used to test for differences among the three vegetation indices. The classification confirmed the presence of deserts in the coastal areas, highland shrubs in the central highlands and forests in Southwest Eritrea, the Southern Highlands, and in some portions of Eastern Eritrea. However, vegetation change in Eritrea over the study period varied according to location and time period. NDVI, SAVI, and TC Greenness values were generally higher in Southern Highlands, South Western Lowlands and Western Escarpment during 1999 than in 1984 and 1986; this finding was consistent with the expectation that vegetation had recovered after the war and drought of the late 1980s. However, vegetation decreased in the 2000 to 2004 period in the Coastal plains, North East Sahel and West Sahel mainly due to drought. There was no significant difference among NDVI, SAVI, and the TC Greenness index when the assessment was conducted at large scale, while one small scale study area showed a difference between NDVI and TC Greenness. Tasseled Greenness Cap seemed to be the most reliable of the three indices as it has a more reliable soil correction factor. Satellite image analysis using vegetation indices provided useful indicators of vegetation change that could be related to climate and war in Eritrea. |
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