HANOI (Reuters) - Seasonal flooding on the Mekong river that usually begins in early August has not yet started in Vietnam's southern delta rice basket, raising questions over the country's biggest rice crop of the year. The floods, which generally peak in the Mekong Delta in late October, are forecast to be late and low this year, the National Centre for Hydro and Meteorology Forecasts said in a report published on Wednesday. Water levels in the Delta's gauging stations in Tan Chau and Chau Doc were 0.5 metre (1.64 feet) below the average of the past few years, said the report, carried in the ruling Communist Party's mouthpiece daily newspaper, Nhan Dan (People).

The Mekong Delta produces more than half of Vietnam's rice output and accounts for 90 percent of rice exports from the world's second-largest rice-exporting country after Thailand.

The winter-spring crop is the top yielding crop of Vietnam's three annual harvests, and most of its grain is exported.

"The waters are much lower than in the past, because hydro power plants upstream have closed dams for accumulating water," said Vo Thanh, head of the weather bureau in the Mekong Delta province of An Giang, a key rice area.

But he said it was too early to predict a weak flooding season because more storms are expected in coming months and heavy rain could swell the Mekong, which flows from the Tibetan plateau through China, Laos, Thailand, Cambodia and Vietnam.

"So far Vietnam has had just two storms, so when more storms come, power plants will have to flush water and the Mekong Delta will be fine," Thanh said by telephone from Long Xuyen, the capital of An Giang province, 190 km (120 miles) southwest of Ho Chi Minh City.

High flooding brings fertile soil to the Delta's rice fields, and as seasonal floods precede the winter-spring crop, water levels are closely monitored to help with crop production and output forecasts.

Farmers in the Delta have harvested more than half of the current summer-autumn rice crop and the third crop would be protected by internal dyke systems in the event of high flooding, Thanh said.

Vietnam, with its long coastline, could be hit by up to seven of 12 storms forecast to emerge in the South China Sea this year, the national weather centre forecast early last month.

The government said 40 people had been killed or were listed as missing after the two storms that hit northern Vietnam last month. One made landfall in Vietnam while another only dumped heavy rain on mountainous provinces when it landed in China.

The storm season lasts through October and mostly affects the central and northern regions of Vietnam. The Mekong Delta rarely gets a direct hit from storms or typhoons.

(Editing by Alan Raybould)