Natural Hazards: Causes and Effects - Disaster Management Center ...

1960; 500,000 in 1973. In 1975, in the heart of the region, where the heaviest concentrations of
villages are found, the population density reached 70-100 inhabitants per square kilometer.
Twenty-five percent of the regional space contained more than 50 inhabitants per square
kilometer.
The climate of the region is characterized by a totally dry period from November to March and
an annual rainfall of about 700-710 millimeters (after 50 years of observation), occurring almost
soley from June to September. The rainfall is subject to considerable interannual fluctuation,
and its timing is irregular during the wet season. Spread out over a short time span, these rains
play a decisive role in the volume of crops. This is particularly true of the frequency and
abundance of the precipitation that falls at season’s end, especially since the unabsorbent and
unfertile soils cannot cope with these irregular rainfalls.
Cotton growing evolved briskly, supplying what was then the mother country. The cultivation of
this crop has left a dismal souvenir; deforestation occurred as growers searched for new land to
cultivate. The yield of these cash crops fell considerably during the 1960s. Yatenga, which
supplied the market with 420 tons of raw cotton in 1930, 800 tons of groundnut and 270 tons of
raw cotton in 1939, delivered in 1970 fewer than five tons of cotton and marketed only a few
dozen tons of groundnuts.
Stockbreeding also played a role in the effort to make Yatenga participate in an exchange
economy. At the beginning of the century, the region was considered rich in cattle and horses.
These were the products sold in the southern regions, of Upper Volta, Ghana, and the Ivory
Coast, thus bringing into Yatenga the money needed for the payment of taxes and new duties. 5
Geographical Distribution
It is important to delineate the world distribution of deserts and to define the meaning of aridity.
Figure 8.1 illustrates not only the true deserts but the widespread semidesert areas of the world,
all of which can be categorized as the arid zone. Neglecting the very cold drylands and the
extreme deserts themselves, the latter not subject to further degradation, there remains an area
of potentially productive but threatened drylands covering 45 million square kilometers or 30
percent of the world’s land surface. These occur so widely that two-thirds of the 150 nations of
the world are affected. For statistical convenience the true climatic desert has been defined
cartographically by the 100 millimeters (c. four inches) annual precipitation line (termed an
isohyet). Calculations have shown that within this zone there is a permanent water deficit,
because potential evapotranspiration, the combination of water evaporation and water used by
plants and finally given off in water vapor, is so high that it exceeds the total rainfall by a factor
of at least 10. The coastal deserts of Chile/Peru and of Namibia (southwest Africa) are
exceptions to the general 100 millimeters rule; because of high humidity of the air in certain
seasons, the desert margin is marked by the 50 millimeters (c. two inches) isohyet.
The world’s semidesert can be defined as those areas lying between the 100-400 millimeters (c.
4-12 inches) isohyets, again with the exceptions of north Chile and Namibia, where the limits of
50-300 millimeters apply. These slightly moister parts of the arid zone include not only large
stretches of the Middle East and the notorious Sahel belt to the south of the Sahara, but also
the Kalahari, central Australia, much of inner Asia, Patagonia and the southwest portion of the
United States. The widespread range of the arid zone (from 50 degrees latitude to within some
10 degrees of the equator) is sufficient to demonstrate that the lack of precipitation is the
dominant climatic element, not the temperature. Thus we can distinguish between the hot