With Lake Mead water levels dropping, Pat Mulroy is in a race against Mother Nature.

That’s how the risks facing by Las Vegas, Nev., were framed this week in a piece in The Henderson Press. But as one of the members of my water geek brain trust, Phil King, likes to point out, reservoir levels are a wonderful way of tracking drought and/or plenty in a single indicator because they integrate both how much water nature puts into the lake and how much water humans take out.

Lake Mead is dropping, and Pat Mulroy is racing, in significant part because the people of Las Vegas, Phoenix and Los Angeles, and most especially the farmers of the Imperial Valley, use a lot of water. In fact, as I’ve pointed out previously, Lake Mead’s been getting all the water it’s supposed to get under the Colorado River Compact, year after year, since forever. Sometimes it even gets bonus water above and beyond that. But Lake Mead keeps dropping.

Mulroy’s in part racing against Mother Nature, but in large measure she’s also racing against herself.