Plants are drinking up water that would usually flow into the Colorado River, a new study shows. This is important for areas like Arizona and California, which rely on snowmelt for water.
Globally, over 1.4 billion people depend on water from rivers fed by mountain snow. In the U.S., more than 10% of people get a lot of their water from the Colorado River.
As the climate warms, less water is moving into the river. Understanding this shift is crucial for managing water in a hotter future.
Most of the water in these mountain ecosystems is lost through evaporation and plant transpiration, known as evapotranspiration. Scientists previously believed that in dry spells, plants primarily tapped into shallow soil moisture from recent rain or snow, meaning less water should reach the rivers. But recent studies reveal a surprising twist: plants often maintain or even increase their transpiration during dry times.
To explore this, environmental engineers Reed Maxwell and Harry Stone set up sensors across a 200-acre area in Colorado’s East River watershed. They measured water movement through the snowmelt-to-streamflow process over two years: one with a high snowpack and another that was cooler and wetter.
Their findings indicated that even in dry conditions, evapotranspiration was high. This suggests that plants are reaching deeper into groundwater reserves, using water that would otherwise flow into the river.
Maxwell noted, “Dry summer or wet summer, they’re getting their water, but from different sources.”
Historical data shows that summer temperatures significantly influence streamflow, regardless of the previous winter’s snowfall. The efficiency of converting snowmelt into runoff has decreased over the last century, meaning less water is collected in reservoirs than before. Climate change is likely a factor in this change.
The research links higher temperatures to rising evaporation demands from plants. “Even with a big snowmelt, high summer temperatures can shrink overall water output,” Maxwell explained.
Brad Udall, a senior research scientist at Colorado State University, praised the study for its direct measurements of evapotranspiration. He pointed out that as temperatures have risen by 2.5°F over the past century, water flow in the Colorado River Basin has dropped by 35% in just the last seven years. He anticipates a potential 40% decline in flow by mid-century.
New rules for water sharing between the upper and lower river basins are set to take effect next year, but there’s still no agreement on how they should look. With rising temperatures and diminishing water flows, negotiations will be more challenging.
Overall, this research is just one piece in understanding water dynamics in the Upper Colorado River Basin. As summers get hotter and drier, we must rethink our assumptions about water availability, even in years with heavy snowfalls. A more accurate water budget will help manage resources fairly as conditions change.
For further reading on water issues in the Colorado River Basin, you can check the U.S. Bureau of Reclamation for updates on management practices.
