Enlarge /. Flooding in Midland, Michigan in May 2020 after heavy rainfall caused two dams to fail.
Climate change has direct consequences for the cycle of water through our environment. The warmer atmosphere holds more moisture and allows intense rainstorms to drain even more water than before. On the other hand, warmer air, through evaporation, can suck even more moisture out of the soil and worsen the drought. Obviously, these things should result in changes to streams. However, the amount of water in streams varies greatly under normal conditions and can be influenced by more than just the weather. It has proven difficult to find trends in this data.
In a new study, led by Evan Dethier at Dartmouth College, currents were divided into physically meaningful categories to determine whether consistent patterns emerged after the separation of apples and oranges. This analysis shows some trends – both at extreme and low flow rates.
Go with the "flow"
Many efforts have found mixed trends between currents in analyzing records of annual peak currents, where records tend to go back further than constant measurements. Attempts to look for regional patterns have been based largely on grouping around arbitrary fields or political boundaries that have limited relation to the landscape.
The new study included approximately 540 Streamflow stations in the United States and Canada, all from locations with low human impact and data for at least 60 years. To divide the stations into groups, they used their location, altitude, and seasonal flow pattern. The locations were grouped into 15 different groups, 12 of which contained a sufficiently large number of stations to attempt a trend analysis.
These groups were analyzed for changes in the frequency of extremely high and low electricity flows on an annual and seasonal basis. The researchers calculated this for events with different rarities (50 percent chance per year, 20 percent chance per year, etc.) and from different starting years, but the results were generally consistent.
There are two general themes that pop out of the results: one for areas where snowmelt causes peak currents and one for areas where drought is prevalent. Snowmelt regions include the Pacific Northwest, the Rocky Mountains (in both the US and Canada), the Midwest, the Appalachians, and the Northeast. In these places there is either a trend towards higher peak currents in spring or higher peak currents in winter. This is in line with the trend towards earlier depletion of the snowpack with warming spring temperatures.
Regardless, high electricity flows have become more frequent in summer and autumn in the Northeast, Midwest, and Appalachian regions, in line with precipitation trends.
Enlarge /. Changes in extremes depending on the season (quarter of the circle) for each region. Dark colors represent the highest statistical security.
In the areas that are frequently hit by drought – the west coast and the southern plains of the country – current trends look different. An increased frequency of summer and autumn low flow events is common everywhere, with more high flow events occurring on the west coast as well. The researchers point out that low flow drought events are usually largely regional in nature, which helps them stand out in the analysis. In contrast, high flow storm-related events tend to be much more local. With a sufficiently large data set, however, these trends can also become visible.
Where regions show statistically significant trends, the average change, according to the researchers, is a doubling in the frequency of this type of flow event since 1950.
By better grouping similar flows, this method helps filter out the effects of climate trends on the blue ribbons that hold the North American landscape together. That means more actionable information at the local level. And as the researchers put it, "Decisions about extreme river events, with high and low flow, have billions of dollars in consequences."
Science Advances, 2020. DOI: 10.1126 / sciadv.aba5939 (About DOIs).