In contrast, the scientists reported greater stream flows over sparsely populated areas near the Arctic Ocean, where snow and ice are rapidly melting.
"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," says NCAR scientist Aiguo Dai, the lead author of the journal paper. "Freshwater being a vital resource, the downward trends are a great concern."
Many factors may affect river discharge, including dams and the diversion of water for agriculture and industry.
The researchers found, however, that the reduced flows in many cases appear to be related to global climate change, which is altering precipitation patterns and increasing the rate of evaporation.
The results are consistent with previous research by Dai and others showing widespread drying and increased drought over many land areas.
The study raises wider ecological and climate concerns.
Discharge from the world's great rivers results in deposits of dissolved nutrients and minerals into the oceans. The freshwater flow also affects global ocean circulation patterns, which are driven by changes in salinity and temperature, and play a vital role in regulating the world's climate.
Although the recent changes in freshwater discharge are relatively small and may only have impacts around major river mouths, Dai said the freshwater balance in the global oceans and over land needs to be monitored for long-term changes.
