When rivers keep flowing during long dry spells, much of that water isn’t coming from rain or snowmelt — it’s coming from underground.
That slow, steady contribution is called baseflow, and a new national study suggests it’s becoming more vulnerable to drought across large parts of the Southwest.
Researchers analyzed more than four decades of stream and watershed data from across the United States, tracking how often and how long baseflow dropped to unusually low levels. Their findings show a clear pattern in arid and semi-arid regions: groundwater-fed flows are declining more frequently and staying low for longer periods.
In contrast, some parts of the eastern U.S. saw fewer of these “baseflow droughts,” highlighting how local climate and geology shape water availability in very different ways.
Why baseflow matters
Baseflow helps keep creeks, rivers, and streams alive when rainfall is scarce. It supports fish and wildlife, supplies water for farms and communities, and helps regulate stream temperatures during hot, dry months.
The U.S. Geological Survey describes baseflow as the portion of streamflow that comes from groundwater seeping into river channels — often the only source of water during prolonged dry weather.
What’s driving the change
The study points to rising temperatures and longer dry spells as major contributors to baseflow drought in the Southwest. Hotter conditions increase evaporation and reduce the amount of water that soaks into the ground, leaving less groundwater available to feed streams later.
But climate isn’t the only factor. Soil types, rock layers, and mountain terrain also influence how well water moves underground and how quickly aquifers can recharge.
Implications for water planning
Water management has traditionally focused on what’s visible — rivers, reservoirs, and snowpack. Researchers say these findings highlight the need to pay closer attention to what’s happening below the surface.
Groundwater trends can shape how resilient a watershed is during drought, especially in regions already facing long-term water shortages.
The National Oceanic and Atmospheric Administration notes that groundwater plays a growing role in drought response as surface water supplies become more variable.
Looking ahead
As the climate continues to warm, the study suggests that tracking baseflow could become a key tool for understanding future water risks — especially across the Southwest, where rivers depend heavily on groundwater to keep flowing through long, dry seasons.
