Each year as the monsoon draws to an end, Thais thank the water goddess for sustaining life. But with deadly floods now plaguing the kingdom, many feel she has been a little too generous of late.

The centuries-old Loi Krathong festival is usually an occasion for celebration with ornately-decorated traditional banana-leaf lanterns set adrift in rivers, canals and lakes under the night sky.

This year it is a reminder of the kingdom's bitter-sweet relationship with water, which has left more than 500 people dead and wreaked havoc in the worst flooding to befall the nation in half a century.

The festival, which also aims to seek forgiveness for polluting the precious resource, comes as questions mount over successive governments' management of water development and urban planning.

"I apologised to the goddess of water, thanked her for giving us water to use and wished all the bad luck to go away," said Usanee Krapukthong, who launched her lantern on a lake in a Bangkok park on Thursday.

"I wished my house and also the whole country would be dry," said the 37-year-old expectant mother. "Normally I do it at a river, but this year I escaped the floods to come here."

Her home has been submerged by more than one metre (three feet) of water for the past two weeks, like many others in badly-hit districts in the north and west of the Thai capital.

Unlike Usanee, however, many residents have refused to evacuate despite the risk of disease associated with the polluted floodwater.

The authorities in the capital city of 12 million asked people not to launch their lanterns in canals and streets that have been transformed into rivers, to avoid blocking drains and adding to the rubbish problem.

Many -- though not all -- heeded their call.

"This year, I'm not floating my krathongs on the rivers or canals because I don't want to block them up. It's better to do it on ponds or lakes like here," said Teerapong Meeiam, 33.

Some Thais chose to write their wish on a virtual lantern on the Internet.

Others opted for lakes in the city's parks, which stayed open late for the occasion, "to preserve beautiful Thai tradition ... and float away our suffering," according to Bangkok governor Sukhumbhand Paribatra.

Despite the brave faces, it was a much more subdued affair this year.

"The atmosphere is kind of downbeat -- people seem to be focused on the floods. But we have to understand the victims too," said Bangkok resident Sittipan Ampan, 19.

Luxury hotels along the city's swollen main Chao Phraya cancelled their gala dinners and firework displays for the festival, traditionally held under the full moon of the 12th lunar month.

"People are worried about whether their homes will be flooded or when the floods will recede, and what's next," said a worker at one of the city's hotels, which have seen a slump in tourist arrivals.

Vendors of the lanterns, typically made from hard bread, wood or plastic, decorated with candles and flowers, are also having a bad year.

"This year is not very good. They aren't many people here," said Lamai Kiewsa-Ard. "I lost my job because of the floods so I'm trying to earn some money."

According to the University of the Thai Chamber of Commerce, spending on the Loi Krathong festival was expected to fall by about one-sixth this year to 8.1 billion baht ($270 million), the lowest in four years.

"In recent years it has become a hugely commercial festival," said Olivier Evrard, an anthropologist at the French public research institute IRD in Bangkok.

But Loi Krathong, which fuses animist, Hindu and Buddhist tradition, is about more than just money.

"According to popular belief, the krathong is also a way to wash away all wrongs, to purify oneself," Evrard added.

Loi Krathong is almost as important to Thai culture as the Songkran water festival which marks the start of the Buddhist new year in April, when people call for the onset of the rains after the dry season.

"This year, Thais find it impossible to celebrate, which risks adding to the trauma they are enduring," said Evrard.

On the tranquil lake in Lumphini Park, located in the heart of Bangkok, still untouched by the slowly advancing floodwaters, hundreds of Thais looked on with an air of disappointment as many of the lanterns drifted back to shore.

"It's OK, we still have next year," said Sittipan.