Given the sturm und drang over Lake Mead’s dropping levels and their implications for the water future of Las Vegas, the preliminary numbers in the U.S. Bureau of Reclamation’s 2010 Annual Operating Plan are worthy of note.

The latest posted version of the plan is marked “final draft”, but I’m told it’s essentially complete, and won’t change in any significant way.

The bottom line: there is a good chance (essentially 50-50, according to the U.S. Bureau of Reclamation) that inflows to Lake Powell, the major upstream reservoir on the Colorado, will be sufficient to permit extra water, above and beyond Colorado River Compact levels, to be released through the Grand Canyon and into Lake Mead.

The details are arcane. Click through if you want to muddle along with my explanation.

Under the compact, the Upper Basin states are required to deliver 7.5 million acre feet per year to the Lower Basin states. Add in Mexico’s share, plus (minus?) some mathematical pixie dust associated with flow on the Paria River, and you have the magic number of 8.23 million acre feet, which is what they try to deliver every year from Lake Powell. (The Compact allows shortages in low flow years to be made up for with extra water in wet years, as long as the delivery obligation is met over any given 10-year stretch.)

For now, that’s the basic plan: 8.23 maf will be delivered downstream from Lake Powell, through the Grand Canyon and into Lake Mead. But the operating plans calls for the Bureau of Reclamation to revisit this plan come April 1. Currently, the surface of Lake Powell is 3,633.52 feet above sea level. If it looks like they’re having a good year, sufficient to push Powell up to 3,642 feet by the end of the 2010 water year, that will trigger an “equalization” release. The idea here is to balance out storage in the two reservoirs. If on April 1 it looks like that’ll happen, then the Powell release plan will be upped to 10.585 maf. (Or maybe 10.667 maf. What’s 80,000 acre feet of water among friends?) If that happens, Mead will end water year 2010 (Sept. 30, 2010) 11 feet higher than it was Sept. 30, 2009. The whole “equalization” thing is spelled out in the Record of Decision for the 2007 shortage sharing agreement, complete with the trigger levels for deciding whether to hold water upstream in Powell and let it slosh down to keep Mead’s users happy.

Got it? If you got this far and it didn’t make sense, ask questions, as the whole selfish point of this blog post is to refine my own understanding and ability to explain this.