Kevin Trenberth says there’s less water flowing in world’s rivers

Today, less water is flowing through the world’s largest rivers, compared to 50 years ago. That’s according to Kevin Trenberth, of the National Center for Atmospheric Research. He explained why this decreased river flow is a problem.

Kevin Trenberth: Firstly, there’s always increasing demand for water. There’s increasing population, there’s increasing uses of water from agriculture, industry, and so on. And then we have the climate factor, which is what our study is about.

In the 2009 study, Trenberth and his colleagues identified the influence of climate change on rainfall as the main cause of river decline.

Kevin Trenberth: There’s a shift in the patterns and the nature of precipitation. In general, when precipitation occurs, it’s a bit heavier than it used to be. That means the frequency is also changing. So we get – especially in the United States – heavier rainfall with a little bit further in between events.

He said that the dry periods between these heavy rains can promote drought. Trenberth added that when and where rainfall does occur, the risk of flooding increases. He said such fluctuations can create an even drier climate, further impacting river flows.

Kevin Trenberth: In general, around the world, we found that there were decreases in important rivers like the Yellow River, the Ganges, where a lot of people live, and the Niger, in West Africa.

He added that in the southwestern U.S., the Colorado River’s water flow has declined. Trenberth said because human populations depend on these large rivers in so many ways, we need to carefully manage our water supply.

Trenberth and his co-author, Aiguo Dai, used climate models and historical records to create the data used in the study. Their study was published in 2009 in the Journal of Climate.

Kevin Trenberth: We used a detailed land surface model where we specified the precipitation and estimates of the radiation in the clouds, and the winds, and humidity, and so on, to simulate what river flow should be. And so we used that as a control to compare that with actual river flow. So we can identify precipitation as the main cause of changing rivers around the world.

Trenberth and his colleagues analyzed the water flow of the world’s 925 largest ocean-reaching rivers, finding an overall decrease in water flow. Human withdrawal of water has an impact on river flow, but the scientists believe human influence is small compared to the climate change factor.