World’s arteries shrinking

WASHINGTON – The flow of water in the world’s largest rivers has declined over the past half-century, with significant changes found in about a third of the big rivers.

An analysis of 925 major rivers from 1948 to 2004 showed an overall decline in total discharge.
The reduction in inflow to the Pacific Ocean alone was about equal to shutting off the Mississippi River, according to the new study appearing in the May 15 edition of the American Meteorological Society’s Journal of Climate.
The only area showing a significant increase in flow was the Arctic, where warming conditions are increasing the snow and ice melt, said researchers led by Aiguo Dai of the National Centre for Atmospheric Research in Boulder, Colo.
‘Freshwater resources will likely decline in the coming decades over many densely populated areas at mid- to low latitudes, largely due to climate changes, Dai said. ‘Rapid disappearing mountain glaciers in the Tibetan plateau and other places will make matters worse.’
Added co-author Kevin Trenberth, ‘As climate change inevitably continues in coming decades, we are likely to see greater impacts on many rivers and water resources that society has come to rely on.’
While Dai cited climate change as a major factor in the changes, the paper noted that other factors are also involved, including dams and the diversion of water for agriculture and industry.
Nonetheless, he said, ‘long-term changes in stream flow should be a major concern under global warming’.
Among rivers showing declines in flow, several serve large populations.
These include the Yellow River in northern China, the Ganges in India, the Niger in West Africa and the Colorado in the southwestern United States.
On the other hand, areas with rising stream flow near the Arctic Ocean tend to have small populations.
There was considerable year-to-year variation in the flow of many rivers, but the overall trend showed annual freshwater discharge into the Pacific Ocean fell by about 6 per cent, or 526 cubic kilometres of water. That’s close to the 552-cubic kilometre average annual flow of the Mississippi, the researchers reported.
The annual flow into the Indian Ocean dropped by about 3 per cent, or 140 cubic kilometres. In contrast, annual river discharge into the Arctic Ocean rose about 10 per cent, or 460 cubic kilometres. There was little change in inflow to the Atlantic Ocean, where increases in the Mississippi and Parana rivers were balanced out by decreases in the Amazon River.
A cubic kilometre is a cube one kilometre on each side. A kilometre is about six-tenths of a mile.
Discharge of river water into the oceans deposits sediment near the river mouth and also affects worldwide ocean circulation patterns, which are driven by variations in water temperature and salinity.
In the US, the flow of the Mississippi River increased by 22 per cent over the period because of increased precipitation across the Midwest.
Major rivers showing declines in flow included the Amazon, Congo, Changjiang (Yangtze), Mekong, Ganges, Irrawaddy, Amur, Mackenzie, Xijiang, Columbia and Niger.
Declines in the Niger River in the 1970s and 1980s in particular reflected the Sahel Drought, the paper said.
– Nampa-AP