“We’re screwed” is the overbearing subtext of so many environmental documentaries that it’s a relief to hear someone just come out and say it in Last Call at the Oasis, a many-lensed look at the coming water-supply crisis. The problem is that a shruggy “we’re screwed” is also an understandable response to docs, like this one, that are loaded down with doomsaying and statistics. Director Jessica Yu (In the Realms of the Unreal) uses Las Vegas as an entry point to her examination of our nature-blind habit of relying on human ingenuity to keep a growing population irrigated. An anecdotal approach keeps the pace tight: Yu moves from Vegas to Australia to Palestine to the ongoing Midwestern crusades of Erin Brockovich-Ellis, building a keen sense of a water shortage’s fearsome ripple effect. If the information flows calmly and clearly, cumulatively the story feels a little scattered. Occasionally the subject hopping confuses the issue: Are we talking about overuse or poor urban planning? Drought or climate change? Pollution or the public’s magical thinking? Ultimately, it’s all connected, and with as fascinating and far-ranging an issue as this one, you can’t fault the director for wanting to fit it all in.