It’s looking increasingly likely that some of this year’s “bonus water”, extra releases from Glen Canyon Dam to bolster supplies in Lake Mead outside Las Vegas, is evaporating with our dwindling Rocky Mountain snowpack.

Hoover Dam, February 2015

The folks at the Bureau of Reclamation have been careful since the beginning in telling us that the bonus water was no guarantee, but lots of people (myself included) have been treating the extra water as a given. It is not.

This is one of those Colorado River management issues for which the details are buried in arcane operating rules, but it boils down to this: In a “normal” year, the Bureau of Reclamation releases 8.23 million acre feet from Glen Canyon Dam, down through the Grand Canyon, and into Lake Mead. If conditions are good enough upstream, the rules allocate a bit of bonus water, and lots of people were expecting that to mean 9 million acre feet this year. But with a dwindling snowpack, that is looking less likely.

Here’s how the Bureau tried to warn us about this possibility last August:

Based on the August 24-Month Study, which is the Bureau of Reclamation’s monthly operational study, the water release from Lake Powell to Lake Mead for water year 2015 will be 8.23 million acre-feet (maf). This is an increase from the 2014 release of 7.48 maf, which was the lowest release since Lake Powell filled in the 1960s….

Under the 2007 Interim Guidelines, another review of the conditions at Lake Powell and Lake Mead will occur in April 2015. Based on an analysis of those projections in the April 24-Month Study, Lake Powell’s water releases could be increased to 9.0 maf for water year 2015.

But because the underlying studies and operating plans pegged that 9 million acre feet as the most likely outcome, a lot of us jumped to conclusions. Bad on us, because the preliminary numbers that are publicly available suggest it’ll be well below that 9 million when we see the final reports next week.