(Adds details on road closure, updates fire acreage)
By Laila Kearney
SAN FRANCISCO, Aug 28 (Reuters) - Yosemite National Park will close another key route into the park on Wednesday that could keep some visitors from reaching the premier outdoor destination over the Labor Day weekend, according to a park spokesman.
The planned shutdown of Tioga Road comes a day after the so-called Rim Fire burned deeper into the park and reached the shores of a reservoir that serves as the primary water supply for San Francisco some 200 miles (320 km) to the west.
The blaze has charred nearly 187,500 acres (over 75,800 hectares) - an area larger than the land mass of Chicago - since it erupted on Aug. 17. Containment lines have been established around 23 percent of the fire's perimeter.
It ranks as the biggest California wildfire since October 2007 and the sixth-largest in state history, according to the records of Cal Fire, a state government site. (http://www.nps.gov/yose/planyourvisit/rimfire.htm)
The fire is burning mainly in the Stanislaus National Forest west of Yosemite, but it has scorched more than 40,000 acres of the park, and firefighters were making an extra push to stop it from spreading inside Yosemite.
The fire last week forced the closure of a stretch of Highway 120 that leads to the west side of the 750,000-acre (300,000-hectare) park and is the main entrance from the San Francisco Bay area.
Tioga Road, the second of the four access routes into the park, was set to close at noon local time to allow fire crews to build containment lines along the road before the blaze approaches, said Yosemite spokesman Tom Medema.
"That will limit the access for visitors to and from the east side of the park, quite possibly over Labor Day weekend, which will have a significant economic impact on the area and (be) an inconvenience for visitors," he said.
Some 4 million people visit Yosemite each year, most of them during the peak months of June through August.
Firefighters plan to burn containment lines from the Hetch Hetchy Reservoir in the remote northwestern section of the park south to Tioga Road to stop the fire from moving further east into the park, Medema said.
By Wednesday afternoon, any remaining campers from the Yosemite Creek Campground and Tamarack Flat Campgrounds will be evacuated, he said. The park also closed the Crane Flat Campground, according to Medema.
HOT, DRY WEATHER EXPECTED
The blaze has been among the fastest-moving of dozens of large wildfires raging across the drought-parched U.S. West that have strained national firefighting resources.
Cooler temperatures, higher humidity and calmer winds had been expected to help the firefighting effort Tuesday night, said Alison Hesterly, spokeswoman for the California Department of Forestry and Fire Protection.
Later on Wednesday, temperatures were expected to be hot and dry, hitting a maximum of 94 Fahrenheit (34 Celsius) in the area with a minimum of 15 percent humidity, she said.
"If we reach the maximum temperature and the minimum humidity, we're expecting continued erratic fire behavior," she added.
On Tuesday, a firefighting force of some 4,100 personnel, backed by teams of bulldozers and water-dropping helicopters, continued to make headway in their drive to encircle and suppress the flames.
After advancing on the Hetch Hetchy Reservoir for several days, flames advanced to the artificial lake on Tuesday, officials said. The San Francisco Public Utilities Commission said there was little risk to the reservoir because of the rocky terrain and lack of brush surrounding it.
Officials said ash had drifted onto the surface of the reservoir, but testing of samples showed water quality remained healthy.
If the water should become fouled by too much ash and soot and require filtration, it can be diverted through a treatment plant near San Francisco before being delivered to customers, officials from the commission said.
Firefighters hacking through dense, dry brush and trees to create clearings in the rugged terrain on Tuesday rushed to improve buffer zones around some 4,500 homes outside Yosemite that were threatened by the blaze on its northwestern flank on Tuesday, said Cal Fire spokesman Daniel Berlant.
Most of those dwellings have been ordered evacuated or were under advisories urging residents to leave voluntarily or be ready to flee at a moment's notice. The fire has already destroyed dozens of homes and cabins, Berlant said, but no serious injuries have been reported.
The cause of the blaze remained under investigation. (Editing by Cynthia Johnston and Prudence Crowther)