Mapping Marijuana Cultivation Sites and Water Storage in the Redwood Creek Watershed, Southern Humboldt County

Impending regulation of California's marijuana industry demands a quantifiable understanding of the extent of cultivation and adequacy of water storage on private lands long devoted to a dispersed, but commercial-scale, unregulated marijuana industry. Water storage is a critical factor, given both California's droughts and its climate: indoor plants are grown under lights year-round, and outdoor plants are grown during the dry months. This research aimed to quantify marijuana cultivation and water storage in the Redwood Creek watershed of southern Humboldt County, where major land use changes have taken place since the late 1960s. Using Google Earth imagery and geospatial analysis, an inspection of 369 assessor's parcels located within or partly within the watershed yielded 303 greenhouses, 100 outdoor cultivation scenes, 164 water tanks, and 51 installed ponds. Estimating the number of water-storage tanks was much more challenging than estimating the number of cultivation sites; it is virtually certain that numerous tanks are under the forest canopy and not visible. The only way to verify their number and holding capacity would be through either ground truthing or GIS analysis using point-cloud data. The latter is costly to collect and process; therefore, given that many landowners who have engaged in unregulated marijuana cultivation are loath to grant access to outside parties, at this juncture it may be difficult for academic researchers, state and county regulators, environmental inspectors, and other interested parties to measure some of the industry's existing environmental impacts.