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Year-to-year variability within the regions can
be quite significant and considerably exceed averaged data for the continents. This is
especially so in the arid and semi-arid regions where the actual values are small. Here,
values for individual years can be 1.5 to 2 times less than the averages over a long
period, whereas for wet regions this difference is within 15 to 25%. The figure (top
right) shows some long-term variations in renewable water resources for Africa and Europe.
It demonstrates a cyclic nature and in practically all regions the dry periods can last
for three to six years, during which time many regions experience severe water supply
problems. Note the decrease in water resources that has occurred since the 1970s in a number of regions of Africa. For the Sahel, this decrease has been particularly hazardous since water resources there have reduced almost by half during the last 15 years. The possibilities of using water resources for economic needs are determined not only by their year-to-year but also seasonal and monthly |
variability. Many regions are characterised by
an extremely uneven river runoff distribution, with 60-80% of annual runoff occurring
during the flood season of some three to four months.For example, 64% of annual runoff
occurs during the three flood months in the north and south of the European part of the
former Soviet Union. An estimate of renewable water resources has also been made for some 60 separate countries. Those selected include developed and developing countries as well as those with a transitional economy, and the largest and the smallest in both area and population. The greatest renewable water resources are concentrated in six principal countries of the world: Brazil, Russia, Canada, United States of America, China, and India, which have more than 40% of total annual river runoff formed within their territories. Runoff distribution during a year is evened out to a considerable extent in these countries because of their vast land mass and diverse climatic conditions. |
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