Cannabis groundwater pumping dries California’s Emerald Triangle streams

Some streams in California’s Emerald Triangle have run daily, every year, since well before records were ever kept.

New research indicates this is beginning to shift, for reasons that may seem unexpected. There has been no single severe drought to hold solely responsible, nor has anyone directly diverted the water.

The cause is instead hidden below ground: water extracted through wells to sustain cannabis crops during the dry season.

Watercourses that once flowed throughout the year are now drying out entirely in summer. Intermittent streams are ceasing to flow several weeks sooner than they once did.

In mountain landscapes, however, water impacts do not remain in one place. They move downstream, reaching salmon habitat and the rivers on which all downstream ecosystems rely.

Jesse Hahm led the research with colleagues from Simon Fraser University, the University of California, Berkeley, and the United States Department of Agriculture.

Farming in a region not built for this

Within cannabis-growing communities, the Emerald Triangle is renowned for producing some of the world’s finest marijuana. It is also becoming increasingly known for water shortages.

Farmers in the area rely extensively on groundwater to keep crops growing through the dry months, a practice that is not unusual in Californian agriculture. The distinctive factor is the landscape.

This is not level, irrigated farming in a valley. It is steep, remote and mountainous headwater terrain, where water is distributed very differently from the way it is across flat land.

“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,” Hahm said.

Testing 580 scenarios

To examine how pumping and weather interact in this setting, the researchers developed a new modelling method and applied it to 580 distinct scenarios.

They changed the time of year, the quantity of water extracted and the extent to which the ground had already dried out before a well was switched on.

Such a comprehensive set of conditions is necessary in a landscape so varied that it does not respond identically from one situation to the next.

The findings were clear. At realistic levels of agricultural water use, pumping caused seasonal streams to stop flowing up to five weeks earlier than they would otherwise have done.

Under the most severe conditions, streams that had flowed every summer for as long as people could recall stopped running completely.

Five weeks may not sound significant, but it can determine whether a young salmon survives its first summer in the water. For such a small fish, that period can mean everything.

Not much farmland to do damage

Perhaps most troublingly, the impact was not limited to catchments in which cannabis farming occupies most of the land.

The researchers recorded meaningful reductions in streamflow even where cultivated land represented only a small portion of a catchment.

The most severe effects occurred during dry years and in catchments where the ground has limited ability to store water underground, leaving very little in reserve once pumping increases.

The timing of pumping proved to be as important as the amount extracted.

“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,” Hahm said.

Damage invisible from the streambank

One difficulty is that these changes take place somewhere few people would think to inspect. They develop below ground, out of view, long before a dwindling stream reveals the process underway beneath it.

“We found that in these upland, mountainous landscapes, pumping water from underground can affect streamflow in ways that may not be immediately obvious,” Hahm noted.

This lack of visibility is becoming increasingly important.

Throughout western North America, communities are relying more heavily on groundwater each year as summers become longer and drier.

Previously, snowpack performed the understated but vital task of sustaining these systems, melting gradually through spring while replenishing streams and aquifers. That process is now starting to fail.

“Snow traditionally acts as a natural reservoir, slowly replenishing groundwater through the spring and early summer,” Hahm noted.

“But warmer winters and reduced snowpack are changing the way water is stored and released into the landscape.”

Broader implications of the study

The message of the study extends far beyond cannabis.

Any crop with high water demand, grown in a mountain catchment with limited underground storage, faces the same conflict-particularly as climate change reduces the snowpack that previously kept those reserves replenished.

Replacing cannabis with wine grapes or almonds produces the same result.

What the research offers decision-makers is something they have largely lacked: a way to anticipate a stream’s future before it dries up, rather than assessing it only afterwards.

“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,” Hahm concluded.