Extremely low streamflows are becoming less frequent over time, according to a recent U.S. Geological Survey (USGS) study, in collaboration with researchers in several other countries, that analyzed historical streamflow trends across multiple continents and seasons. The study indicated that wetter summers and warmer winters likely kept rivers flowing over the long-term period of study in most of the watersheds investigated.

“This research should help natural resource managers better understand and anticipate the nuanced changes that may come with climate change,” said lead study author Glenn Hodgkins, a research hydrologist with the USGS New England Water Science Center.

Changes to streamflow can directly impact water and ecosystem quality, drinking water supplies, and the amount of water needed for hydropower.  When the volume of water in the river or stream falls well below average it is considered a “low flow.” 

This study analyzed extreme low flows, or statistically-rare low-flow events in North American, European, South American, and Australian watersheds not influenced by manmade infrastructure, like dams, over 40-year and 80-year periods. Published in Water Resources Research, this journal article outlines one of the first studies to investigate extreme low-flow trends for multiple continents that separately examines summer and winter seasons. 

“It's important to study winter low flows separately from summer low flows, as they're caused by different processes, which is an important consideration for future climate and streamflow models,” said Hodgkins. 

Extended periods of well below normal precipitation, or drought, prompt extreme low flows in summer, whereas extended periods of cold temperatures that keep water trapped in ice, snow, and frozen soil prompt extreme low flows in winter.

Over the 80-year period of study, extreme low flows became less frequent in both summer and winter for the majority of watersheds. Results were mixed for the 40-year period.

“Higher temperatures and longer growing seasons in the summer could lead to lower streamflows as a result of more evaporation and water use by plants. However, it’s not showing up in the streams that we looked at over the 80-year period – things just got wetter and extreme low flows became generally less frequent,” said Hodgkins. Warmer temperatures in winter, however, leading to less extreme low flow events, is in line with what would be expected to occur as a result of increased winter temperatures in those regions from climate change, according to Hodgkins.

For the shorter, 40-year period, USGS scientists found that extreme low-flow trends toward drier conditions were primarily driven by natural climatic cycles, namely the Pacific Decadal Oscillation (PDO). A naturally-occurring 20-to-30-year cycle, the PDO is related to variations in surface water temperatures of the Pacific Ocean and precipitation totals in parts of North America. 

Both the climate-driven long-term seasonal trends and the shorter-term influence of natural ocean and atmosphere patterns on extreme low flows can inform both climate change models and water resource management.

Learn more about this project here.