Geological Factors Governing the Vulnerability of Land to Degradation: - The Case of Hosaina – Werabe Area, Using Remote Sensing and Gis
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Date
2006-07
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Addis Ababa Universty
Abstract
The study area, Hosaina-Werabe, is situated in the western margin of the main
Ethiopian rift valley. It is structurally controlled and tectonically active. It is covered
by loose ash falls, weak pumice layers, and less welded tuff layers, which are
vulnerable to land degradation activities, and are intercalated with relatively stronger
ignimbrite layers. Whereas the lithologies and geological structures make the area
vulnerable to land degradation, the steep slopes at fault escarpments and volcanic
domes accelerate the erosive power of overland flows and /or rivers. The main land
degradation process is erosion by water.
The land degradation activity in the area is assessed using integrated remote sensing
and GIS techniques and field survey. The rate of land degradation, in areal bases, has
been estimated using Landsat satellite images of the years 1973, 1984, 2000 G. C. on
a small sample site of about 247 sq km. area, which was a subset from the study area.
The average rate of land degradation from the year 1984 to 1973 is 0.7 sq. km / year
and from the year 1984 to 2000, it is 1.0 sq. km / year. The result shows a general
increasing trend of land degradation.
The areal extent of degraded land is also increasing over time. In the year 1973, it was
7.3 %, in the year 1984, it rose to 10.4 % and it was extended to 16.7 % in the year
2000 G. C Cross-tabulation of two year images at a time, indicated that few parts of the formerly
degraded areas were seen to be covered by vegetation. However, newly formed
degraded lands have shown prominent areal expansion through time.
Land degradation vulnerability factor maps were produced from topographic maps,
existing maps, Landsat satellite data and SRTM (Shuttle Radar Topographic Mission)
data. GIS analyses of distance and area measurements, slope, and reclassification
were done for each factor based on their relation with land degradation activity. These
layers were standardized and weights were given in a hierarchical order. Finally
multi-criteria analysis was done that indicated areas susceptible to land degradation. It
was reclassified in order to produce three classes of vulnerability map for the study
area. Accordingly, factors that are responsible for making the area vulnerable to land
degradation are, in decreasing order of importance: drainage, slope, geological
structure, geology (lithology), soil and land use/ land cover.
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Keywords
Vulnerability of Land