Groundwater pumped to irrigate cannabis crops in California's Emerald Triangle can cause some streams that normally run year-round to dry up entirely during summer, say researchers from Simon Fraser University, the University of California, Berkeley and the United States Department of Agriculture.
A new study reveals how groundwater pumping in the region's rugged mountain headwaters may reduce streamflow that support salmon habitat and feed larger river systems downstream.
Key findings
- Researchers analyzed 580 water-use scenarios.
- They found some seasonal streams could stop flowing up to five weeks earlier.
- Some streams that normally flow year-round may dry up completely during summer.
- Impacts were greatest during dry years and drought conditions.
- Effects were detected even when cannabis cultivation occupied a small share of a watershed.
- Reduced streamflow can affect salmon habitat and downstream river systems.
"People often think of groundwater or aquifers as a separate resource from streams. But in many landscapes, they're connected parts of the same system," says Jesse Hahm, assistant professor of geography and co-author of the study.
"During heat waves and droughts, demand for irrigation often increases at the same time stream ecosystems are under the greatest stress. What matters is not only how much water is used, but when and where it is taken," he explains.
"We found that in these upland, mountainous landscapes, pumping water from underground can affect streamflow in ways that may not be immediately obvious."
Published in the Journal of Hydrology, the study examined how groundwater pumping affects headwater streams in two watersheds in California's Emerald Triangle, a globally recognized cannabis-growing region that is increasingly vulnerable to water shortages.
Using a new modelling approach, the team analyzed 580 scenarios to understand how groundwater pumping and environmental conditions converge to influence streamflow.
Researchers found that under realistic agricultural water demands, groundwater pumping can cause seasonal streams to stop flowing up to five weeks earlier and, in some cases, dry out streams that would normally flow year-round.
The effects were strongest in dry years and in watersheds with limited underground water-storage capacity. Even where cultivated land represented only a small share of a watershed, the researchers found groundwater pumping could still have measurable impacts on streamflow.
These findings help fill a major knowledge gap as communities across western North America increasingly rely on groundwater during hotter, drier summers driven by climate change.
"Snow traditionally acts as a natural reservoir, slowly replenishing groundwater through the spring and early summer. But warmer winters and reduced snowpack are changing the way water is stored and released into the landscape," says Hahm.
"Understanding how groundwater pumping influences streamflow higher up in our water systems can help decision-makers balance agricultural needs with protecting fish habitat and other environmental values."
