Fjölrit RALA - 05.12.1999, Side 106

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Human dimension of land degradation in China
fied land of Yulin County is about 4678 km2 which is about 67% of the whole terri-
tory. The data indicates therefore that the desertification problem is very severe in
Yulin County. A key region in relation to desertification is located along the ancient
Great Wall.
In order to analyze the distribution pattem of desertified lands, the “Buffer” pro-
gram of the Arc/Info was used. The width of the buffer zones was selected as 1, 5, 10,
and 15 km along the main rivers of the study area. Analyzing the buffer file, the per-
centage of each desertification type could be related to buffer distances. However,
moderate, slight, and latent desertifíed land types occupied a small proportion of their
respective total area. Within the zone between 1 and 5 km, the area of intensive deser-
tified land takes up about 60% of the total area of this type. So, it was shown that in-
tensive desertifíed lands within 5 km wide zone along rivers take up about 81% of the
total area of this type, which indicates that intensive desertifíed lands are mainly dis-
tributed in this zone. It is also found that 90% of moderate, slight, and latent types of
desertified lands are concentrated within a range of 5-15 km along rivers. These re-
sults show that man first conducted land resource exploitation along river banks with
better natural conditions. Furthermore, they continued to farm grasslands close to the
river banks with fragile eco-environment. So that the intensive desertifícation has
taken place in this region. The conclusion drawn ffom the above is opposite to general
opinion that the farther away it is from rivers, the more intensive is the desertification.
This conclusion is very valuable to analyze deserted land pattem and to make regional
hamessing and planning decisions.
Desertification monitoring
Remote sensing data from
three dates, 1960, 1987, and
1992, were selected to con-
duct comparative analysis
for monitoring desertifica-
tion development. The GIS-
system was used to compare
the three desert distribution
maps. The results are shown
in Table 1.
According to Table 1, desertification area in the Yulin County in 1992 is smaller
than that of 1987, and is 7% larger than in 1960. This shows that large-scale cultiva-
tion had taken place at this site since the 1970’s and vegetation cover had not recov-
ered to the 1960 level, although over-cultivation activity has been restricted lately.
ANALYSIS OF LAND DESERTIFICATION CAUSES
The Yulin study area is located at a typical semi-arid temperate steppe zone with an
annual precipitation of over 400 mm. Therefore, land desertification should not have
been caused in this region. Historical documents show that there used to be extensive
grasslands in the northem Yulin County and dense forests in the southem part, an area
with plenty of water and lush grass 2000 years ago (She Hianhei 1981).
Table 1. Desert area statistics (%) of the Yulín County from
three different dates.
Type 1960 1987 1992
Shifting sand 35.3 21.6 19.4
Semi-fixed sand 15.1 18.8 21.2
Fixed sand 7.6 26.8 24.6
Total 58.0 67.4 65.2