1 foot of elevation - could mean 80,000 acre-feet or 320,000 acre-feet

      There have been a few news articles recently (i.e. here and here) about an announcement by Central Arizona Project (CAP, interestingly there's no mention of the idea on the CAP website) that they are considering leaving a portion of their allocation of Colorado River water in Lake Mead this coming year in an effort to maintain a higher lake level and hopefully avoid the potential for a declaration of shortage.  

      How does this work, you might ask?  Part of the agreement from 2007 that allocated who will bear the brunt of shortages on the river also included some complex rules for what is called Intentionally Created Surplus (ICS).  The general idea is that the lower basin states could develop arrangements whereby water that they don't really need in a given year - usually by engaging in some activity that actually conserves water that would have been used, i.e. fallowing farmland for a year - is left in Lake Mead, to be withdrawn in some future year.  I don't know if Arizona's actions are considered ICS because nothing extraordinary is being done to save this water - it's basically water that Arizona doesn't currently need and for various reasons it might make more sense to leave it in Mead rather than pump it into the canal.  But California entities (MWD) have taken advantage of this arrangement in the past.  Arizona - or more specifically, the CAP - has not.  CAP has been doing everything it can to take all its water.

      Here's a little background.  Arizona is allocated 2.8 million acre-feet (MAF) from the Colorado.  1.6 MAF goes to the CAP and the rest is used generally along the river for irrigation.  But the thing is, entities that contract for CAP water don't currently need 1.6 MAF - so much of that water is put in the ground for storage, to be pumped out at a later date, like when there is a shortage on the river.  Here is how the 2010 uses of CAP water break down, according to documents on their website:

• 2010 scheduled subcontract deliveries – 425,000 ac ft (this is mostly the water that is used by cities)
• 2010 scheduled deliveries to agriculture (technically excess water, but specifically allocated to the Ag pool) – 400,000 ac ft
• Other excess scheduled deliveries (includes deliveries for firming*; water purchased to offset pumping; probably some water purchased, taken and used directly for industrial purposes; and water purchased and recharged for generation of credits by private entities) – 474,000 ac ft
• Deliveries to Indian reservations (primarily Ag uses) – 104,000 ac ft
• Deliveries of Indian water for off-reservation uses (mostly recharge and leases to non-tribal entities) – 240,000 ac ft  
• this adds up to slightly more than 1.6 MAF because some of the water delivered was water previously stored that was recovered (pumped out of ground) and delivered 

This means that somewhere around 1 maf of water taken off the river by CAP was actually needed and used, while the rest was taken and stored underground, primarily so the state would not lose that water and to provide insurance against future shortages (*this is what is termed "firming" the water -taking lower priority water and changing it's character so it will be available during a shortage that otherwise would have curtailed the availability of that water).

By leaving this water in Lake Mead this year, Arizona hopes to 1) forestall a declaration of shortage and 2) make that water available to be taken out of the river in a future year when the reservoir storage is higher.  This strategy makes a lot of sense for several reasons.

1) The water will hopefully be available in the future.

2) The in-state entity that handles most of the firming for the state (by recharging excess water or using it to replace groundwater pumping that would otherwise happen to supply irrigation water) is the Arizona Water Banking Authority (AWBA), an entity created specifically to help the state take its full allocation of Colorado River water sooner than we could actually put it to use in more traditional ways.  In past years the AWBA has been given money from state budgets to carry out those activities, but some of that money has been taken away, leaving them with a reduced ability to carry out that function.  But the main reason it makes sense at this point is:

3)  If it really looks like Lake Mead is heading for that magic elevation of 1075 ft Arizona stands to lose out on 320,000 ac ft of water from the Colorado, which is a whole lot worse than temporarily losing out on 80,000 ac ft.

The whole reason Arizona has been taking all that extra water and putting it in the ground in the Central parts of the state was because we knew that shortages were gonna come some day and it would be much better to be able to pump that water out of the ground in-state than try to get any additional water from the Colorado.  We're last in line at that tap anyway.  But that was a good strategy as long as the level of Lake Mead stayed high enough that we should be able to take our full allotment.  When shortages are looming - possibly in the next 2 years - a new strategy is called for.  That's when you try to limit your losses as best as you can.

But, looking at the snow pack in the upper basin, it may be a very good year for flows into Lake Powell, which means more water will flow down to Mead in the summer and possibly forestall that shortage a little longer.  We'll see.

updated 1/8/11 for content and clarity