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Alongside such clearly defined cyclic variations
in the total runoff from the world's rivers, it is also quite apparent that there is no
discernible trend within the entire 65-year period. This pertains to all continents,
particularly so if no attention is paid to the data for the two recent decades for Africa
and South America where there does seem to be an increase in river runoff in South America
and a decrease in Africa. If there is in fact no noticeable increase in global river
runoff over such a long-term period, this must raise doubts about the widely-held
hypothesis of an elevation in the World Ocean since the beginning of the present century
due to an intensive melting of continental mountain glaciers. River runoff is very unevenly distributed throughout the year in almost all regions of the world. About 60-70% of runoff is generated during flood periods and therefore values for renewable water resources vary noticeably during a year. |
Current estimates are that the major part of European runoff
takes place during April to July (46%), in June to September in Asia (54%), during
September to December (44%) in Africa, 45% during April to July in South America and 40%
during January and April in Australia/Oceania. On average, the wet season for the entire
land surface lasts from May to August, during which time total global runoff amounts to
some 45% of the annual total. Unevenness in the distribution of river runoff during a year leads to the necessity for regulation by creating reservoirs of different types. Of most importance for water supply is what we call base-flow runoff, a stable volume, varying little both during a year and from year to year, whose use is possible without artificial regulation. Its value is approximately 37% of the total volume of global river runoff, or about 16,000 km3 per year. |
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