I have a classic Western postcard tacked to the bulletin board above my computer. It shows two men in a field holding shovels over their heads, locked in mock battle. Behind them runs an irrigation ditch. The caption reads: "Discussing Western Water Rights, A Western Pastime."

The postcard makes me laugh because I know firsthand how worked up people can get over water. At an annual ditch meeting two years ago, my western Colorado neighbors seemed on the verge of an insurrection when the volunteer board members sheepishly announced that a leak in the local reservoir had not been fixed. The reservoir, which supplies our late-season water, would not fill, and the ditches would run dry by the end of July. Our green patches of grass, alfalfa and corn would quickly become as brown, bare and cracked as the desert lands that surround them.

In years past, my neighbors might have shrugged off one shortened growing season; once the reservoir was fixed, after all, the ditches would flow copiously all summer long with snowmelt from the mountains behind town. But that was then. Nowadays, something weird is going on with the weather. The snow pack -- the source of nearly all of our water -- has become unpredictable, with most years on the lean side. No matter how much snow flies in the winter, it seems to melt off earlier every spring.

Climate-change scientists confirm what old-timers know by observation: The West's water supply is shrinking. A difficult period of triage lies ahead. If our cities get their way, the rural areas and the Indian tribes will end up handing over their water. It's already happening in places like Southern California and Las Vegas, where deals are being cut to pump groundwater and divert traditional agricultural waters to the urban areas.

Here in Colorado, water managers have long assumed that the state has enough Colorado River water to accommodate future growth on both the Front Range and the Western Slope. But now, forward-looking realists like Eric Kuhn, who runs the Colorado River Water Conservation District, are tossing those assumptions out the window. They now accept that the Colorado River's flow, which has been overestimated ever since it was divvied up between seven states during a wet period in 1922, will shrink even more as the globe warms. Combine this physical reality with the legal reality that Colorado and the other Upper Basin states (Utah, Wyoming and New Mexico) must allow 7.5 million acre feet of water to flow downriver each year to the Lower Basin states of California, Arizona and Nevada, and that large pool of undeveloped water disappears like a mirage in the desert.

Mr. Kuhn told High Country News reporter Matt Jenkins that he believes Colorado has legal access to just one-tenth of the water it thought it had. This puts the burgeoning Front Range metropolis of greater Denver, which anticipates nearly 3 million new residents in the next 25 years, in a tight spot. Denver will need to push its residents to conserve more, even as it looks to my side of the Continental Divide to buy up existing senior water rights, held mostly by farmers and ranchers. And there are other thirsty players. The nascent oil shale industry also needs large amounts of water if its hopes for a boom are to be realized.

The future looks grim for my neighbors and me; our lands may soon dry up in order to keep the faucets running in Denver and the wheels of the energy industry turning.

But necessity, as we know, is the mother of invention. It is heartening to see major players in the region acknowledge that the Colorado River will not deliver what we once thought it would. The seven states have already adopted a drought plan, though it probably doesn't go far enough. We need to pay attention to managers like Eric Kuhn, who has proposed a plan for Colorado that includes strict conservation measures in the cities and creative water banking in the rural areas. To adhere to a stricter water diet, everybody will have to cut back on something.

The Western Pastime of discussing water rights will continue unabated, but the postcards depicting it my have to change; instead of shovels, rivals will need to wield calculators and climate-sensitive computer models. And they better bring their lawyers, too.

Paul Larmer is a contributor to Writers on the Range and the publisher of High Country News (plarmer@hcn.org) in Paonia, Colorado. It has covered the American West for the past four decades.