The former depends on factors both unpredictable and out of our control. Even the best climate models cannot tell us exactly how much the levels of water throughout the basin are likely to decline in the decades to come.
The latter is likely to be a combination of many approaches, basic and far-fetched, simple and high-tech.
Some of the more fanciful ideas mentioned in the study, and more or less dismissed, include building mammoth pipelines from Kansas City to Denver or hauling icebergs down from the Arctic.
Other steps include more aggressive desalinization and other ways to reclaim water that is now unusable. Or we could, in effect, use the same water twice, by working harder to direct the "gray water" that flows out of our bathtubs, sinks and washing machines toward such uses as irrigation.
Many of those steps, though, are either too expensive, too energy-intensive or just beyond our technical, financial or political reach. And so it comes back to the most basic, and most effective, step to take when there are too many people chasing after too little water.
And that is conservation.
It will require some thought, some investment and, sometimes, some changes in our lifestyles. Or at least the appearance of our front lawns.
But the simple acceptance of the fact that there is not as much water around here as we might like, and that it will never change, should be enough to push people to use less, to install low-flow fixtures, to festoon our lawns with the kinds of plants that can handle an arid climate, and to price water and water connections at levels that reflect just how precious the stuff is.
Because our greatest threat isn't drought. It's greed.