A sweeping deal to reduce Lower Colorado River Basin water use will get its most detailed public airing to date at tomorrow’s (Nov. 7, 2016) meeting of the Metropolitan Water District of Southern California’s Water Planning and Stewardship Committee.

If the deal goes through – and there are hurdles yet to clear – Arizona, California, and Nevada would all agree to further cuts in their use of Colorado River water. It would be the third major agreement among the states and the federal government in the last two decades, deals that have repeatedly sidestepped the risk of litigation over unsettled legal questions about who is entitled to how much of the river’s increasingly scarce water.

Under the deal Nevada and Arizona would take deeper cuts than those already locked in by a 2007 multi-state agreement. In addition, California (which has senior water rights and therefore the strongest bargaining position) would also agree to join in the reductions. Arizona, which pumps Colorado River water uphill from Lake Havasu to the farms and cities of the Phoenix-Tucson region, would take by far the biggest cuts. But having California join in the reductions is a huge breakthrough in finding a workable solution to the Colorado River’s problems.

The deal’s been talked about for some time, so much of what’s in the MWD staff presentations is not new. But Monday’s public meeting begins the next step in the process, as the many agencies involved begin the process of formally approving the deal, so it’s worth digging down into the details at this point.

Why is this needed? Because water use has been consistently greater than water supply since the turn of the century. Lake Mead, which stores water for a vast region of the Southwestern United States, has dropped more than 140 feet since the late 1990s, and is now at its lowest levels since it was first filled in the 1930s.

It is not yet at critical levels, and modeling done in 2007 suggested that the risk of the supply/use imbalance pushing Lake Mead to critical levels was relatively low. But in the last decade, things have just gotten worse and worse. From the staff presentation to be delivered to Met board members tomorrow:

Elevation 1,020 is in the zone where things get really bad, a threshold beyond which it is difficult to maintain the basic structure and function of the communities depending on Colorado River water in Nevada, Arizona, and California. In 2007, the federal study used to support the interim shortage guidelines negotiated by the states suggested a bit more than a one in ten chance of reaching 1,020 by year 2026. By the summer of 2015, the modeling put that at something like one in five. But a “stress test” modeling run, using data that more closely matches the recent drought and the impact of climate change, puts the risk now at more than 40 percent absent action to slow Mead’s decline by reducing water use ahead of trouble.

That action, to slow Mead’s decline, makes up the most important piece of the deal.

The 2007 shortage sharing guidelines took the first steps in that direction, with Nevada and Arizona agreeing to specified cuts as Mead dropped. Those first cuts would kick in when Mead begins a year below elevation 1,075 feet above sea level, which hasn’t happened yet.

As outlined in this table being presented to Met board members tomorrow, the new agreement includes deeper cuts, sooner:

One of the tricks here is that agreement at the state level to reduce the amount of water taken from the Colorado River must then be sorted out at more local levels, among the various water users with claims on the Colorado River. California appears to have a deal among those using Colorado River water to apportion the cuts as follows:

That would mean, for example, that the Imperial Irrigation District would have to absorb 60 percent of the 200,000 acre foot cut that happens if/when Lake Mead drops to elevation 1,045, or 120,000 acre feet of water.

But here’s where the deal gets tricky. The amount of water not taken (by IID or whomever) would get its own line item in the Lake Mead accounting system, sort of like a piggy bank. Here are the rules for managing that water and in some cases getting it back out of the piggy bank:

What this means, in essence, is that if it gets wet again, users will be able to get their conserved DCP water back out of Lake Mead. And in the meantime, they have the option of using some of it on a short term basis.

There is some urgency to get this thing done. Folks in the current administration have made it clear that they want to have a deal in place before Jan. 20. Even with if Hillary Clinton wins next Tuesday’s election and we have relative continuity, a new administration would want to take a fresh look at a deal this important, which would mean significant delay. A change in party would mean even more uncertainty and likely delay.

With the California part of the deal looking relatively solid, there are two major areas of uncertainty remaining. (Nevada, with essentially one major water user, the Southern Nevada Water Authority serving greater Las Vegas, faces little controversy.)

The first is Arizona. There’s a scramble underway to figure out how shortages there would be allocated, and as near as I can tell no agreement yet on the details. (If you know different, text me, let’s talk!) Arizona faces an additional complication in that any agreement requires ratification by the state legislature. I’m told that could happen in early January if the parties within Arizona can come to agreement.

The second is the question of Mexico. Diplomats from both countries are in the midst of delicate negotiation over the final terms of an agreement that they also hope to have done before the Obama administration leaves office. This will determine the extent to which Mexico also shares in shortages as Lake Mead drops. It’s not at all clear to me where this stands, other than that a lot of work remains between now and Jan. 20.

By tradition, big deals like this are nailed down and publicly signed and sealed and speeched upon by the Secretary of the Interior at the annual Colorado River Water Users Association in Las Vegas, which this year begins Dec. 14. Here’s what to watch for: If Interior Secretary Sally Jewell is there for the usual Friday federal session, we’ve got a deal. If Deputy Secretary Mike Connor is on the agenda to speak in her stead, be nervous.