They do mention that it's "small" pot farms. I've seen some pretty small grow operations, quarter of an acre. I'm predisposed to believe nothing the media or politicians say about pot. It's been a useful whipping boy my whole life and I don't think were done blaming everything on drugs. Might be like post race Obama, just because it's legalized doesn't mean we can't blame it for poor water management or anything else that sounds plausible.
I believe this in the sense that I know what kind of water I know an outdoor operation can use. Like even 5 outdoor plants you could be looking at 10-15 or so gallons a week per plant. So if you're a small time grower in ca and are doing 25-99 plants outside during the summer, your water consumption can really add up. A buddy of mine was in San Bernardino and had a small indoor operation of around 30k watts and it used all his well water up around twice per 3 month cycle and had to buy water which ended up being insanely expensive. And outdoor plants generally get bigger and need more water than their indoor counterparts, so I could maybe halfway believe this. Them and the moonshiners lol, that's who must be using all our water. All 4% of the water used by people other than the government subsidized farmers, who get the other 96%... (I was listening to an article recently that gave these numbers but I think that is what household use of the states water goes like.)