What's behind California's historic drought?

Taps are running dry in California's Central Valley as the worst drought in more than a century is forcing farmers to drill ever deeper underground for water.

In the rush to grab groundwater for agriculture, shallow household wells are drying up and those who can't afford to dig deeper are facing a future without running water.

The BBC's Alastair Leithead explains just what's behind the drought, who it's affecting, and how scientists measure the impact.

Produced by the BBC's Alastair Leithead, Regan Morris, Jack Garland, and David Botti