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For many years, everyone familiar with the Colorado River knew it was oversubscribed and that shortages caused by droughts would force periodic cuts in water supplied to the Central Arizona Project and other users.

But it's become clear that the river's outlook is worse than even the worst-case scenario predicted by a federally funded study written 12 years ago, according to one of the study's authors.

The principal reason is that the river's three Lower Basin states — Arizona, Nevada and California — have grown faster and used more water than forecasters predicted in the early 1990s.

Arizona's population, now about 6.2 million, is up more than 65 percent since 1990. From 1995 through 2005, the seven river basin states' population grew nearly 11 million, or about 25 percent.

For Tucson, Phoenix, Central Arizona farmers, Indian tribes and others dependent on CAP water, that means the river stands a good chance of running short more quickly than experts were predicting as recently as the start of this decade.

More water has been taken from Lake Mead at the Arizona-Nevada border than was expected, and Lakes Mead and Powell have stayed at significantly lower levels than expected.

Exactly how soon shortages might hit is something scientists and public officials continue to discuss.

But there is probably a 10 percent chance of shortages in four to five years, and a 25 percent chance the river will run short between 2020 and 2025, according to two prominent water officials in Arizona.

This week, the Bureau of Reclamation, which operates the reservoirs, will release a draft report outlining proposed standards for determining when there is a shortage and how shortages will be managed.

"I have no doubt that within the next five to 10 years, we will be in a shortage," said David Modeer, Tucson Water's director and a member of the three-county board that manages the CAP. "It does not look good. If everything everyone is saying about the longtime climate or global warming is true, it looks worse for the future."

CAP's deputy general manager, Larry Dozier, is somewhat more optimistic, but he agreed that there is a significant chance of shortages by 2011 or 2012, and a 25 percent to 30 percent chance by 2025.

The initial shortages would not reduce deliveries of CAP water to Tucson and other cities; it would primarily affect non-Indian farms that are lower on the pecking order for getting CAP water. If the supply stayed short for five or six years, however, Tucson's share could be reduced.

But Dozier said the pain from shortages for cities and tribes should be eased, if not eliminated, because the state has been buying excess CAP water and storing it in the ground for the past decade. The goal is to bank enough water underground to prevent cities and tribes from losing any of their CAP supply through 2100, he said.

The Southwest's precipitation, runoff into the river and reservoir levels have lagged far below normal for most of this decade. Currently, Lake Powell is only 48 percent full, although that's slightly better than the 35 percent level of early 2005, which followed five straight years of below-normal flow into the reservoir.

But the river's plight is not just a result of dry weather, according to one of the 1995 study's authors, Ben Harding, an engineer for the private consulting firm Hydrosphere in Boulder, Colo.

Harding called this a "system drought," caused by the huge scale of the Colorado's reservoir system that 25 million people rely on for water, and by population growth that has come to rely on it.

"The bigger the reservoirs that you build, the bigger the system you build, the more sensitive it becomes to droughts (and) the longer it takes to recover," Harding said last week. "You have a bigger hole to fill."

But the forecasts don't take into account the possibility of global warming. In a report released last week, the National Academy of Sciences predicted climate change will likely mean even less water for the Colorado in the future.

The academy didn't predict how soon or how much the Colorado would run short. But reduced stream flow from global warming would make future droughts last longer, be more severe and occur more often.

The river has long been over-allocated, with the seven basin states and Mexico having divvied up the rights more than 50 years ago to 16.5 million acre-feet of water. More recent tree ring studies pegged the actual annual average flow at about 14.8 million acre-feet.

The 1995 federal study, which looked at the effects of what it called a "severe, sustained drought," was still relatively upbeat for the Lower Basin states. Those states would lose no more than 3 percent of their river supplies even in the worst drought year, said the study, which was primarily by the Army Corps of Engineers and the United States Geological Survey.

Instead, this is what happened:

● In seven of the 12 years since then, Lakes Powell and Mead carried less water than had been predicted for the worst possible drought.

● In 11 years, cities, farms and other water users took more water from Lake Mead than had been expected in the worst-case drought.

● The river's flow at Lee's Ferry, just east of the Grand Canyon, has been less than the worst-case scenario forecast during six of those years.

Although the two big reservoirs have recovered a bit from their 2004 lows, they're still far from full.

In that analysis, Harding recalled that the study's authors assumed the worst-case drought would be a very rare event and probably many years off, and that the high cost of pumping CAP water 300 miles uphill to Tucson from the Colorado would reduce the demand for the project's water.

"The current drought, however, has caught water managers unprepared," Harding wrote.

The National Academy of Sciences report also underscored links between population growth and water demands. It said that it would take 15 years of normal precipitation and runoff conditions to refill Powell and Mead.

Regardless of rainfall levels, higher population and water demand will make filling the reservoirs more difficult, the report said.

In another report that reached a similar conclusion, U.S. Bureau of Reclamation official Terry Fulp wrote in 2005 that reservoir levels will continue to decline in the near term, and the system will take longer to recover than it did after previous droughts, largely because of greater demands today.