I was looking at the wikipedia article on the 1922 Colorado River Compact (CRC) to allocate 7.5 MAF of water to both the upper and lower basin states, or 15 MAF in total. (Mexico's "take" of 1.5 MAF was added in 1943.) Then I saw that the 2007 Interim Guidelines for allocations under water shortage conditions will reduce flows to lower basin states down to 7.167 MAF ("light shortage") or 7 MAF ("heavy shortage"). What blows my mind is that these sums, assuming upper basin and Mexican rights are left intact only reduces withdrawals -- under "heavy shortage" -- to 16 MAF in total. This doesn't make sense to me, given the Colorado's long term average total flow of around 14 MAF.
Current agreements, in other words, continue to assume there's more water there than exists! That's a recipe for conflict or disappointment, some time in the future.
Bottom Line: John McCain was right. The CRC needs to be renegotiated into percentage shares (to allow for annual variations) to prevent accounting dilemmas. THEN, the CRC needs to be amended to allow water trading among states on -- better -- among holders of appropriative rights to make sure that the scarce and totally depleted waters of this river are put to highest and best use.