In Plano, Mesquite, Rockwall and other cities, the rules say lawns shall be watered no more than one day a week.

But in Dallas, residents can sprinkle daily. Twice-weekly watering is OK in Grand Prairie and Carrollton. In Farmers Branch, Duncanville and DeSoto, it's water at will.

Those once-a-week cities get their water from the North Texas Municipal Water District. The others are among those served by Dallas Water Utilities.

And when it comes to water supplies, the two systems are in notably different shape.

As of Friday, the five reservoirs – Ray Roberts, Lewisville, Grapevine, Ray Hubbard and Tawakoni – that supply the Dallas system's water were a combined 67 percent full.

Lavon Lake, the North Texas district's water-collection reservoir and primary source, stands about 38 percent full. Lake Chapman to the east, another district source, is down to 22 percent.

Officials of the two systems say the differences can be blamed in large part on Mother Nature, for the varying amounts of rains and runoff in the watersheds that serve the reservoirs.

While all of north-central Texas has the nation's harshest drought conditions, according to the U.S. Drought Monitor, rainfall has been particularly scant in the areas north and east of Dallas that drain to Lavon and Chapman.

The Dallas lakes have fared better with Ray Hubbard, east of Dallas, and Ray Roberts, to the north, in the best shape at about 78 percent full.

Ray Hubbard's health is a matter of rain, officials say, not water from Lavon. Lavon lies upstream on the Trinity River's East Fork, but hasn't released floodwaters to Ray Hubbard since April 4, 2005, records show.

"We've been at the right place with the right reservoirs," said Charles Stringer, Dallas' assistant director for water operations.

Another factor may be the capacity and spread of the Dallas system, he said.

As the North Texas water district moves into the fourth month of Severe Stage 3 drought restrictions, Dallas is considering launching the Stage 1 voluntary phase of its drought plan.

Since June 1, harder-hit cities have shut off sprinkler systems, written thousands of citations and collected tens of thousands of dollars in fines. They are considering turning the screws further to avoid rationing.

If reservoir levels keep dropping, Dallas officials say, the city likely will request voluntary cutbacks.

A city prohibition on outdoor watering from 10 a.m. to 6 p.m. ends Sept. 30, and water system officials will propose extending those restrictions next year from April 1 to Oct. 31, Mr. Stringer said.

The Dallas system, which delivers water to 2.3 million people in 22 cities, has never had mandatory water curtailments since the 1950s drought lighted a fire under city leaders, he said.

A 1955 planning report called for four new reservoirs, three of which were built: Ray Hubbard, Tawakoni and Aubrey, renamed Ray Roberts.

A later study called for securing water rights in Lake Palestine, among other strategies. The city plans to begin drawing water from that East Texas reservoir between 2015 and 2020, Mr. Stringer said.

The North Texas water district serves 1.6 million people. Its sources include Lake Texoma and recycled wastewater, as well as Lavon and Chapman.

"Where they are today, Dallas was in the 1950s," he said. "We got burned real bad."