Water storage and irrigation practices associated with cannabis production drive seasonal patterns of water extraction and use in Northern California watersheds

Concerns have been raised over the impacts of cannabis farms on the environment and water resources in particular, yet data on cultivation practices and water use patterns and have been limited. Estimates of water use for cannabis cultivation have previously relied on extrapolated values of plant water demand, which are unable to account for differences in cultivation practices, variation across the growing season, or the role of water storage in altering seasonal extraction patterns. The current study uses data reported by enrollees in Californias North Coast Regional Water Quality Control Board (Regional Water Board) Cannabis Program to model how variation in cultivation practices and the use of stored water affect the timing and amount of water extracted from the environment. We found that the supplemental use of stored water resulted in a seasonal pattern of water extraction (i.e. water withdrawals from the environment) that was distinct from water demand (i.e. water applied to plants). Although water input to storage in the off-season months (November through March) reduced water extraction in the growing season (April through October), farms generally did not have sufficient storage to completely forbear from surface water extraction during the growing season. Beginning in 2019, forbearance will be required during this period for those in the regulated cannabis industry. The two most important predictors of storage sufficiency (type of storage infrastructure and seasonality of water source) also had reliable effects on seasonal extraction patterns, further emphasizing the link between water storage and extraction profiles. These findings suggest that resource managers and policy makers should consider the ways in which cultivation practices drive water extraction patterns and how these practices may be influenced by participation in the regulated cannabis industry.