Hot Summers Push Colorado River Flows Back to Average, Even After Big Snow Years

Plants in the headwaters of the Colorado River pull water from shallow groundwater already on its way to the stream, and that habit makes summer temperature as strong a control on flow as the winter snowpack is. Harry Stone and Reed M. Maxwell, both of Princeton University, report the finding in Communications Earth & Environment, published Sept. 14.

Water planning in snowmelt country has leaned on winter precipitation as the quantity to watch. Growing-season temperature, not just winter snow, governs how much water reaches the stream, the study argues. The researchers describe the plant-and-groundwater route as previously unaccounted for, noting that as warming continues and droughts intensify, this mechanism will deepen flow declines beyond those from reduced snow alone.

The evidence begins with daily rises and falls in soil moisture, groundwater and streamflow recorded in the headwaters. Those patterns indicate that roots intercept groundwater to keep the plants releasing water through their leaves once the surface soil dries. Stone and Maxwell then extended the analysis across 18 headwater catchments, the mountain basins that feed the river. Summer temperature, they found, regulates streamflow on its own, with effects that rival those of snowpack. Warm summers bring flows from high-snowpack years down to average levels. Cool summers substantially raise flows from moderate-snowpack years.

The paper does not report the decline in Colorado River flow, which is already documented, but a route by which warming deepens it. The authors point to steep terrain and the long delay between snowfall and summer flow as reasons the link had stayed unclear. A separate study in the same journal, published the same day, examined drought, streamflow and water quality in the Upper Colorado.

The work was funded by the U.S. Department of Agriculture, the National Science Foundation and the Department of Defense's Strategic Environmental Research and Development Program.