New analysis: extreme flows in US flows increasing

Climate change has direct consequences on the water cycle through our environment. The warmer atmosphere retains more moisture, causing heavy rains to spill even more water than before. On the other hand, warmer air can suck even more moisture from the soil through evaporation, which makes droughts worse. These things should obviously lead to changes for the feeds. But the amount of water in streams varies enormously under normal conditions, and it can also be affected by more than just weather conditions. Finding trends in this data has proven difficult.

A new study led by Evan Dethier at Dartmouth College has set out to group the flows into physically meaningful categories, to see if consistent patterns emerge once apples are separated from oranges. This analysis reveals some trends, both in the extremes of high flow and low flow.

Go with the flow

Many efforts have found mixed trends between flows when analyzing annual peak flow records, where records tend to go back further than constant measurements. Attempts to find regional models have largely relied on grouping by arbitrary boxes or political boundaries, which have only a limited link with the landscape.

The new research looked at approximately 540 streamflow stations in the United States and Canada, all located in locations that have little human impact and have at least 60 years of data. To group the stations, they used their location, elevation, and seasonal flow pattern. The sites grouped into 15 different groups, 12 of which had a sufficient number of stations to attempt a trend analysis.

These groups were analyzed for changes in the frequency of extremely high and low flows on an annual and seasonal scale. The researchers calculated this for events of different rarities (50% chance per year, 20% chance per year, etc.) and different starting years, but the results were generally consistent.

Two main themes emerge from the results: one for regions where snowmelt results in peak flows, and another for regions where drought is widespread. Snowmelt regions include the Pacific Northwest, the Rockies (US and Canadian parts), the Midwest, the Appalachians, and the Northeast. In these locations, there is either a trend towards higher peak flows in the spring, or higher peak flows in the winter. This corresponds to the tendency for the early depletion of the snowpack with the warming of spring temperatures.

On the other hand, high flows in summer and fall have become more common in the northeastern, midwestern and Appalachian regions, consistent with precipitation trends.

Drying out

In areas generally prone to drought – the west coast and the southern part of the country – the streamflow trends are different. An increased frequency of summer and fall low flow events is common everywhere, with the west coast areas also seeing more high flow events. The researchers point out that low-flow events caused by drought tend to be broadly regional, which helps them show up in the analysis. In contrast, high flow events caused by storms tend to be much more localized. Yet with a sufficiently large data set, these trends can also become apparent.

When regions show statistically significant trends, the researchers say, the average change is a double the frequency of this type of streamflow event since 1950.

By better grouping similar streams together, this method helps to disentangle the impact of climate trends on the blue ribbons that connect the North American landscape. This means more actionable information at the local level. And as the researchers say, “Decisions about extreme high- and low-flow river events have billion-dollar consequences.”

Science Advances, 2020. DOI: 10.1126 / sciadv.aba5939 (About DOIs).