Wildfires are nothing new in California. But in the third year of a historic drought, the tinder-dry western US state is battling near-record numbers of blazes.

And the normal fire season has only just begun.

Nearly 7,500 firefighters are currently struggling to douse the so-called King Fire east of Sacramento which has forced almost 3,000 people to evacuate as it rages across an area bigger than the city of Las Vegas.

But while this is fairly typical for an ordinary year, it is far from the first of the season.

"Already this year California responded to nearly 5,000 wildires, where in an average year that number would be closer to 3,900," said Daniel Berlant of California's Department of Forestry and Fire Protection, or CalFire.

"There has been a significant increase in fire activity due to the fact that conditions are so dry from the drought," he told AFP, noting that "historically, California experiences its largest and most damaging wildfires in the fall months.

"So we're just now getting to the peak of fire season."

Blazes have been erupting for months. In May, thousands of residents had to leave their homes due to a surge of fires which triggered the partial evacuation of a military base and a tourist amusement park.

In July and again in August wildfires forced more than 13,000 evacuation orders near California's landmark Yosemite National Park, disrupting vacations for some of the millions of tourists who visit every year from the United States and abroad.

In all there have been 1,000 more wildfires than average, and 700 more than last year, which was already the worst for a decade, according to CalFire.

"In our west, wildfire season now stretches most of the year," President Barack Obama said in a speech on climate change to the UN General Assembly earlier this week.

- Reservoirs at historic lows -

Talk of climate change certainly rings true in California, which is baking in the third straight year of an intense drought -- the worst for up to a century, according to Governor Jerry Brown.

The drought has devastated farming in the Central Valley, known as the nation's food basket, but which is struggling to grow crops and raise cattle on parched soil.

Water reservoirs are at historic lows. They are typically filled in the spring by melting snow from the Sierra Nevada mountain range. But last winter was one of the driest on record.

Most of the fires so far this year have been in the north of the state. But as the summer ends firefighters in southern California are bracing for worse to come as the real season gets under way.

After a whole summer virtually without rain, forests and canyons are as dry as they can get -- just in time for the so-called Santa Ana winds which blow down from the desert in the fall and winter.

"As we look into the next couple of months, unfortunately we do not see any significant rain... that means that conditions are only going to get drier," said CalFire's Berlant.

"As we get into October that's typically when we see Santa Ana wind events and so those strong winds, coupled with the already tinder-dry conditions, lead to an elevated fire danger," he added.

Other experts say much will depend on how soon the Santa Ana winds blow up, and how much rain falls in the critical next few months.

NASA climatologist William Patzert told the LA Times: "It's a race we run every fall: what comes first, the rains or the Santa Anas... The dice are loaded this year for Santa Anas.

"And who knows how intense or benign it will be."