Geoscience Reference
In-Depth Information
1960s-1970s, all led to an increase in the frequency of
dust storms in these locations. While the latter two
regions have produced the most dramatic recent
increase in dust storm activity because of human activ-
ities, the best-documented effect has been on the
United States Great Plains during the dust bowl years
of the 1930s Great Depression. Here, it can be shown
that dust storms have a cyclic occurrence. In the 1870s,
dust storms during drought drove out the first group of
farmers. While no major dust storm period was
recorded until the 1930s, drought has tended to occur
synchronously with maxima in the 18.6-year M N lunar
cycle. Dust storm degradation of arable land also
occurred in the mid-1950s and mid-1970s. While the
dust bowl years of the 1930s have become part of
American folklore, the drought years of the mid 1950s
and 1970s actually produced more soil damage. This is
despite the fact that reclamation and rehabilitation
programs had been initiated by the federal govern-
ment in response to the 1930s dust bowl. These
programs included the return of marginal land to
grassland, and the use of strip farming , crop rotation,
and mulching practices. These techniques build up soil
moisture and nutrients while protecting the surface
soil from wind deflation . In most cases, the dust storms
followed a period of rapid crop expansion during
favorable times of rainfall and commodity prices.
Maj or storm events
There have been many notable dust storms. For
instance, in April 1928 a dust storm affected the
whole of the Ukrainian steppe, an area in excess of
1 million km 2 . Up to 15 million tonnes of black cher-
nozem soil were removed and deposited over an area of
6 million km 2 in Romania and Poland. In the affected
area, soil was eroded to a depth of 12-25 cm in some
places. Particles 0.02-0.5 mm in size were entrained by
the wind. In March 1901, 1 million tonnes of red dust
from the Sahara were spread over an area from
western Europe to the Ural Mountains in Russia. In
the 1930s, the dust bowl of the United States Midwest
produced some of the most dramatic dust storms of the
twentieth century. Their effect was enhanced by
the fact that the area had not previously experienced
such extreme events. Most of the storms occurred in
late winter-early spring, just as the snow cover had
melted, and the frozen ground inhibiting soil deflation
had thawed. However, at this time of year, the polar
westerlies were still strong. The most severe storms
occurred between 1933 and 1938 on the southern
Plains and between 1933 and 1936 on the northern
Plains. The number and extent of storms during the
worst period of March 1936 is shown in Figure 3.31.
These storms covered an area from the Gulf of Mexico
1000 km
10 Dust storms
20 Dust storms
Fig. 3.31 Extent and frequency of dust storms in March 1936 in the United States (after Lockeretz, 1978).
Search WWH ::

Custom Search