In California, cannabis farming continues to expand. The use of California’s water, however, grows more contentious. To address increased demand and dwindling supply, we need to improve understanding of irrigation practices and adjust regulations to incentivize best practices. Widespread reliance on groundwater wells for farm irrigation may lead to depleted streamflow and harms to fish and wildlife. To address these potential impacts, we need to know where and why groundwater wells are most prevalent. Using state-level cannabis permitting data, we address four important information gaps. (1) What is the prevalence of groundwater wells as an irrigation source for regulated cannabis farms statewide? (2) To what extent does groundwater use occur outside of regulated groundwater basins? (3) What are the most useful predictors of whether a farm will rely on groundwater for irrigation? (4) What is the prevalence of well use from unpermitted cannabis farms.

Across most of California’s top-producing counties, 75% of permitted cannabis farmers irrigate from groundwater wells. More than one quarter of these farms reside in groundwater basins that are not regulated by the state. Larger farms were more likely to use groundwater wells. A machine learning model suggests that the majority (60%) of unpermitted farms are likely to use groundwater wells, assuming they follow the same patterns as the regulated industry. Proactive steps can address the impacts of groundwater use in cannabis regulations in California. Outside of large groundwater basins, regulations should be based on further research into the effects of groundwater use on streamflow.