Rivers in some of the world's most populated regions are losing water, many because of climate change, researchers reported on Tuesday.

Affected¹ rivers include the Ganges in India, the Niger in West Africa, and the Colorado in the southwestern United States.

When added to the effects from damming, irrigation and other water use, these changes could add up to a threat to future supplies of food and water, the researchers reported in the American Meteorological Society's Journal of Climate.

"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," Aiguo Dai of the National Center for Atmospheric² Research in Boulder³, Colorado, who led the study, said in a statement.

"Freshwater being a vital resource, the downward trends are a great concern."

Researchers looked at records of river flow in 925 big rivers from 1948 to 2004, finding significant changes in about a third of the world's largest rivers.

For instance, annual freshwater discharge into the Pacific Ocean fell by about 6 percent, or 526 cubic kilometers -- about the equivalent volume of water that flows out of the Mississippi River each year.

The Columbia River in the US Northwest lost about 14 percent of its volume from 1948 to 2004, largely because of reduced precipitation and higher water usage in the West.

Annual discharge from melting ice into the Arctic Ocean rose about 10 percent, or 460 cubic kilometers.

"Also, there is evidence that the rapid warming since the 1970s has caused an earlier onset⁴ of spring that induces earlier snowmelt and associated peak streamflow in the western United States and New England and earlier breakup of river-ice in Russian Arctic rivers and many Canadian rivers," the researchers wrote.

"As climate change inevitably⁵ continues in coming decades, we are likely to see greater impacts on many rivers and water resources that society has come to rely on," said NCAR scientist Kevin Trenberth, who worked on the study.

研究人员于本周二报告称，世界上人口最密集的一些地区的河流出现了流量下降的情况，而这大多是由气候变化所致。

受到影响的河流包括印度的恒河、西非的尼日尔河以及美国西南部的科罗拉多河等等。

研究人员在报告中称，如果再加上筑坝、灌溉、以及其它水资源利用的影响，这些变化会对这些地区未来的食物和水源供应构成威胁。该报告在美国气象学会的《气候》杂志上发表。

研究负责人、科罗拉多州波尔得美国国家大气研究中心的戴爱国（音）在一份声明中称：“河流的径流量减少正增大世界大部分地区淡水资源的压力，尤其是随着人口增长，淡水资源需求更大的地区。”

“淡水是一种重要的资源，所以大河流量的减少是一个十分令人担忧的问题。”

研究人员分析了全球925条大河在1948年至2004年间的流量记录，发现其中约三分之一的主要河流的流量都发生了显著变化。

例如，每年流入太平洋的淡水总量下降了约6%（合526立方千米），相当于于每年从密西西比河流出的淡水总量。

在1948年至2004年间，位于美国西北部地区的哥伦比亚河的流量减少了约14%，这主要是由于降水量减少以及美国西部用水量增加。

冰川融化流入北冰洋的水年度流量增加了约10%，合460立方千米。

研究人员在报告中写道：“也有证据表明，自20世纪70年代以来，全球变暖的加剧导致春天提前到来，这造成冰雪消融期提前，美国西部和新英格兰地区出现洪峰，以及俄罗斯北极地区和加拿大很多河流的河冰提早消融。

研究小组成员、美国国家大气研究中心的科学家凯文•特伦伯斯说：“气候变化的趋势在未来几十年内不可逆转，我们可能会看到全球变暖对社会赖以生存的很多河流和水资源产生更大