Tom Yulsman and Brendon Bosworth have an excellent piece over at Climate Central looking at drought and water supply on my favorite river*. They capture what I think is one of the key elements in understanding what’s happening on the Colorado: the realization that, independent of the drought that has plagued the river for more than a decade, the lower basin is using more water than their entitlement under normal conditions:

Traditionally, Lake Mead has been topped off with extra water from Lake Powell, unused by the upper basin states. But during the recent drought, water managers decided to provide only normal deliveries from the Lake Powell bank account. For Lake Mead, this has meant users have been removing more water than has been flowing in. And so inevitably, Lake Mead has dropped, from almost full in 2000 to about 42 percent full in mid-2010.

“What you’re really seeing here is a combination of drought and an overuse problem amongst the three lower basin states of California, Nevada and Arizona,” Udall says. “That overuse problem historically has been covered up by a little extra water that flows down from Colorado and the upper basin states. But over the last 11 years, with the most serious drought on record, that water hasn’t been there, and so the overuse problem has become readily apparent.”

* Sorry Rio Grande. What can I say? The Colorado and I have history.

With the mondo snow pack and good runoff forecast, Arizona may no longer need to leave some of its annual share of Colorado River water in Lake Mead, Shaun McKinnon reports:

The CAP had been discussing a plan to forfeit as much as 80,000 acre-feet of its allocation this year to help keep water levels at Lake Mead from falling below an elevation of 1,075 feet above sea level, the first drought trigger.

At that level, Arizona would lose 320,000 acre-feet of its 2.8 million acre-foot allocation for at least a year.

An acre-foot is 325,851 gallons, enough to serve two average households for one year.

The water left in Lake Mead was to be taken from a pool of excess water available for underground recharge and would not affect any cities or farmers. The intent of the plan was to give up a little water to avoid bigger losses.

The bureau’s forecast should render the plan unnecessary. Hydrologists now predict above-average runoff – as high as 120 percent of the 30-year average – on the upper Colorado from April through July, when most of the snow melts and flows into the river.

What a difference a wet, wet month in the Colorado Basin makes.

In December, the long range forecasts for the big reservoirs at Lake Powell (upstream) and Lake Mead were flirting with trouble. Lake Mead’s levels for the end of the 2010-11 water year were forecast to drop to 1076.68 feet above sea level, close to the magic 1075, the level that would trigger the Colorado River’s first shortage declaration in history. But a series of December storms that brought major inflows between Powell and Mead, along with significant above-average precip across much of the rest of the basin, has changed everything.

The latest Bureau of Reclamation “24-month study” (pdf) shows substantial improvement, sufficient that there is now a 76 percent chance that water managers in the Lower Basin will hear that magic word they’ve been longing for: “equalization”. In short, equalization means that there’s enough flowing into Lake Powell in the Upper Basin that a big slug of bonus water could be released through the Grand Canyon to help raise the levels of Lake Mead.

Projected 2010-11 storage in Mead, Powell, data courtesy USBR

I hope this graph will help illustrate what’s going on (click through to blow it up). The dotted lines are the early December, pre-storm projections – Mead in blue below, Powell in black. The solid lines are the current median projection, based on the latest runoff forecast from the Colorado Basin River Forecast Center. You can see that the total estimated storage in each reservoir rises under the new forecast, but that Mead rises a lot more than Powell. That’s because the river’s operating rules favor Mead under the current situation. Both reservoirs get a slug of the bonus water, but Mead gets a bigger share in a process called “equalization” – an attempt by the river’s managers to essentially balance the amount of water in storage in the two reservoirs.

It is worth remembering that the fix here requires extra water to be released from Lake Powell, above and beyond the requirements of the 1922 Colorado River Compact and related elements of the “Law of the River“. The compact and related legal structures require the release of 8.235 million acre feet per year. The new equalization forecast calls for a release of 11.36 maf, a bonus of more than 3 million acre feet. This is a reminder that the fix for the Lower Basin’s problems of a dwindling Lake Mead requires more water than the law prescribes under normal operations.

udpate: full column here (sub/ad req)

When I first began seriously trying to sort out New Mexico’s place in the scheme of Colorado River water management, I kept banging unsuccessfully up against what seemed like a simple question: If long term drought or permanent climate change reduces the river’s flow, how would the shortage be sorted out? Who, in other words, would have to use less water?

I take a crack at an answer in tomorrow’s newspaper (I’ll add a link when it’s up, in the meantime here’s a blog post teaser), via a new analysis by Doug Kenney at the University of Colorado (pdf of the study here).

As I explain in the column, Kenney makes a straightforward argument: that to the extent climate change or long term drought reduce total flows in the Colorado, the four upper basin states are responsible for reducing their use. California, Arizona and Nevada might have problems of their own (they’re overusing what Colorado River water they get as it is now), but they would not face a climate change-induced cut.

The framers of the Colorado River Compact, it has long been known, made a fundamental mistake in assuming that the Colorado River would reliably supply 16.5 million acre feet of water per year, and dividing it up accordingly – 7.5 million acre feet for the upper basin states, 7.5 million for the lower basin states and the rest for Mexico (actually that “rest for Mexico bit” came later, but it’s part and parcel of the underlying idea of a stationary supply).

Turns out there’s less.

Southern Nevada Water Authority Tunnel Boring Machine, being used to excavate a new, more reliable Lake Mead water intake for Las Vegas water supply as Colorado River levels decline

In the 11 years since 2000, the Colorado has averaged just 12 million acre feet per year, according to the U.S. Bureau of Reclamation. In a sensible world, the shortage would be divided equally among the parties – say, in round numbers. 5.5 maf for the lower basin, 5.5 maf for the upper basin, and 1 maf for Mexico.

But the compact was not written that way. In addition to the 7.5-7.5 split, the Compact has this to say about the upper basin’s water management obligations:

The States of the Upper Division will not cause the flow of the river at Lee Ferry to be depleted below an aggregate of 75,000,000 acre-feet for any period of ten consecutive years reckoned in continuing progressive series beginning with the first day of October next succeeding the ratification of this compact.

As I explain in tomorrow’s column, that “will not cause … to be depleted” line is the sort of thing clever water lawyers will salivate over, sufficiently rich with ambiguity that legal hijinks are sure to ensue. But the risk is that the court will agree with Kenney’s interpretation (and that of most legal scholars) – that the upper basin must deliver 7.5 maf a year no matter what.

That changes the arithmetic. Even if one assumes a reduced share for Mexico, in a 12 maf per year world, you get:

• Mexico – 1 maf
• Lower basin – 7.5 maf
• Upper basin – 3.5 maf

As Kenney puts it:

[T]he Upper Basin apportionment is essentially the last priority on the river, and as average flow volumes decline, this apportionment bears the full brunt of the “squeeze” of reduced water availability.

Some additional links:

• The Colorado River Compact (from the USBR’s great Law of the River web  page)
• The 12 maf average for 2000-11 comes from the USBR’s draft Annual Operating Plan for the 2010-11 water year (pdf)
• Kenney’s report (pdf), which is a must-read for anyone interested in the institutional issues surrounding the problems we face on a river that is already over-subscribed, even before climate change