Wait, Mexico’s share is (as seems from both your comment and the link) specified in absolute terms rather than as a percentage like the US states? Leaving no allowance for that amount not existing?
Minute 319 [1] determines how water allocations will change during conditions of drought or surplus. This is also covered a little at the end of the Wikipedia article.
In a max drought scenario (Lake Mead surface elevation <1,025 feet) it looks like delivery requirements would be reduced by 125,000 acre-feet, with different delivery thresholds above that range.
I believe that the water to be supplied to Mexico via the river was of such poor quality that we ended up either trucking or directly purifying a set volume of water downstream to meet our treaty obligations.
More specific link:
https://en.wikipedia.org/wiki/Colorado_River_Compact#Provisi...