RENO, Nev. (AP) - Reno-area water officials say a wet winter in the Sierra Nevada is crucial to restoring diminished water supplies.

Back-to-back skimpy winters have left reservoirs along the Truckee River water shed low.

"This winter is everything," said Chad Blanchard, chief deputy in the federal water master's office in Reno.

Officials say unless the fall brings unusual wet weather, Lake Tahoe, the river's largest water source, could drop below its natural rim by early December - something that hasn't happened since January 2005.

While officials say the Reno area won't run out of drinking water, another dry winter likely would mean minimum flows in the Truckee River set by law may not be met.

"It's getting real hard to sugarcoat things," said Bill Hauck, water supply coordinator for the Truckee Meadows Water Authority. "We will be entirely dependent on a good winter this year."

What will be affected is the ability to meet the so-called "Floriston rates," a century-old law designed to guarantee a minimum flow of Truckee River water.

That flow is measured at Farad, Calif., just upstream from the Nevada line. The law designed to ensure municipal, agricultural and power generation demands from river water can be satisfied requires that flows of at least 500 cubic feet per second be maintained through the summer, dropping to between 300 and 400 cfs over the winter, depending on available storage at Lake Tahoe.

The Lake Tahoe dam at Tahoe City allows for the storage of up to 6 feet of water above the lake's natural rim. Most of that water is now gone, with the lake level at less than a foot above the rim.

Depending on factors such as fall precipitation and evaporation rates, Tahoe should drop below its rim this year by early December, Blanchard said.

At that point, water managers will depend almost exclusively on water stored at Boca Reservoir to maintain minimum river flows.

Boca's currently at roughly two-thirds capacity but once it is tapped to maintain Floriston rates, "it will drop dramatically," Blanchard said.

Without added precipitation, achieving the desired rates of river flow will be impossible come mid December. A big winter would help fill Tahoe and other reservoirs and allow water managers to again meet the Floriston rates, hopefully through next summer and fall.

But a dry winter could mean those flows won't be possible all year, with the next shot at seeing them restored during the following winter of 2009-2010.

"It could be pretty challenging," Blanchard said. "If we have another bad year, we'll just get to the point we won't make Floriston rates over the summer.

"We haven't been in that situation for some time."

The last time was in 1994 at the height of a lengthy drought.

Even with another dry winter, the Truckee Meadows Water Authority will continue to be able to provide water demanded by the 93,000 homes and businesses the utility serves, Hauk said.

It would do so by drawing more on its 33 groundwater wells and by tapping water stored for times of drought at places like Independence Lake and Donner Lake.

"We would likely have to dip into that," Hauck said. "We would certainly still be able to meet customer demand."

(Copyright 2008 by The Associated Press. All Rights Reserved.)