Drought story in the western US provides illustrative numbers.
Apparently, the flow of the Colorado River has been reduced to almost half its previous Dust Bowl levels. In general, I don't know what to make of this, but I do know that all the water gets used by humans by the time it reaches Mexico. The current rate over the last few years has been 5.4 million acre-feet per year at a gauging station in Arizona.
This number is interesting because it amounts to:
5.4e6 acre-feet * 43560 ft2/acre * 7.48 gallons/ft3 / year = 1.76e12 gallons/year
115 million barrels of water / day
Remember that the world uses 80 million barrels of oil per day.
This is like tapping the Colorado River for nearly all its worth in equivalent volume of oil. Picture a big spigot connected to the Hoover Dam.
The difference in practical terms? Evidently the SouthWest has learned to adjust to variable amounts of water over the years. But what will happen when fluctuations in oil become this severe?