Plants around the Colorado River are tapping into groundwater reserves during droughts, reducing the amount of water reaching the already strained river system. This finding, from a new study using ground-level sensors in Colorado, confirms a “drought paradox”: plants don’t reduce water use during dry spells – they simply find it elsewhere.
The Hidden Drain on Water Supplies
Over 1.4 billion people worldwide depend on water from snowmelt-driven rivers, and the Colorado River supplies more than 10% of the US population. But as temperatures rise, less water is flowing into these rivers, and scientists are now discovering why : plants are maintaining transpiration rates even when surface moisture vanishes. They’re accessing shallow groundwater that would otherwise feed the river.
Researchers from Princeton University tracked water movement across an 81-hectare area of the East River watershed, which drains into the Colorado River, during 2023 and 2024. The study shows that plants consistently accessed water reserves regardless of weather conditions. This means that even years with large snowpacks see reduced runoff because of increased plant water use.
Why This Matters Now
The Colorado River Basin has warmed by 1.4°C (2.5°F) over the last century, and water flows have dropped by 35% in the last seven years. This study helps explain why. As temperatures climb, plants don’t simply slow down transpiration; they intensify it by drawing from groundwater. This is happening even with increased snowmelt, meaning snowpack alone is no longer a reliable indicator of future water availability.
The implications are stark: we need to revise our water budgets. Current models may underestimate how much water is actually available because they don’t fully account for this hidden plant-driven drain.
What’s Next?
New water-sharing rules for the Colorado River Basin are scheduled for next year, but negotiations are stalled. Declining flows and rising temperatures will only make these talks harder. Understanding the full extent of evapotranspiration—including groundwater extraction by plants—is now critical for accurate water management.
“A better water budget that takes into account increases in summer transpiration is a really important factor when figuring out how much water there is in the basin, before we start to divide it up,” says Reed Maxwell, an environmental engineer at Princeton.
The study is a preprint, meaning it has not yet undergone peer review, but experts agree that its direct measurements strengthen the growing evidence that warmer temperatures are exacerbating water shortages in the region.
