'Two million people have left town,' says my taxi driver as we negotiate the central Bangkok traffic. This means there will be only about five million left for the battle. Many of Bangkok's seven million residents leave town each year to return to their home villages and towns for Songkran, Thailand's traditional New Year celebrations, which run from April 13 to 15. But those who remain aren't 'stay-at-homes'. In fact, most of the young ones come out in force to join in the world's biggest water fight. The exodus doesn't seem to have thinned the Thai capital's traffic noticeably. The taxi finally drops me at the entrance to Khao San Road, still Bangkok's most popular area for backpacker tourists. Today the 500-metre-long street is closed to vehicles. The roadside stalls that normally sell pirated DVDs and fake IDs ('Wanna be an FBI agent?') are absent, replaced by huge piles of bottled water for sale. Beside them are hefty bins of chilled water. Mischievous-looking teenagers loiter nearby, sporting water-cannon pump guns. Zap! The first jet of iced water hits the back of my neck. Given this is the peak of the hot season, the chill is not unpleasant. As a farang, or foreigner, I know I am a 'mark'. Sure enough, one squirt is followed by another, then a ladle of chilled water from a tub. I had been warned and have come prepared, wearing nothing that can't be drenched and carrying only a waterproof camera. Very soon it looks as though I have taken my morning shower fully clothed then absent-mindedly stepped out for a stroll. Songkran derives from the Sanskrit term for the beginning of the new solar year; its associated celebration may have arrived in Thailand from India 2,500 years ago. Before the festival, people give their homes a thorough spring clean in the belief that anything old and useless should be discarded because it may bring bad luck. Traditional Thais dress in new clothes and release fish and birds for spiritual merit. Many visit their local wat, or temple, to offer food to the monks and to sprinkle scented water on Buddha images. At home, families sprinkle water on the hands of elders in a show of respect. If you're in an area like Khao San Road and not particularly 'elder', prepare for respect with a damp ending. If you're a farang, the respect is even greater: you'll receive twice the soaking. It's all done in fun, but in the melee, the plea, 'Don't shoot me, I'm just a tourist!' becomes an irresistible invitation for Thai teens to open up like firing squads on newly minted backpackers. Half-way down Khao San Road, a young girl stops me and daubs my cheek with liquid clay that she, like many others, is carrying in a small bowl. Two of her friends then step up for this once-a-year chance to 'decorate' the face of a stranger, not to mention the fun of painting white spots on a farang's face. The crowd grows and a loose procession forms; we move past a row of musicians and Thai dancers; only these and police in uniform are spared an icy dousing or a clay daubing. We move around the block in a sodden parade. Youths in passing pick-up trucks fill their water guns from large bins and drench pedestrians. These 'water gunships' are targeted by other teenagers zipping past on motorcycles, with the pillion passengers dishing out as good as they get. Different regions of Thailand have their own games and songs to celebrate Songkran. In the provinces, scattered family members come together for this annual reunion and to feast. In a ceremony known as Rod Nam Dam Huaw, the whole neighbourhood might line up to pour water on the hands of the community's oldest members, offering their blessings for the coming year. The main event for children, tourists and the unwary is the 'big splash' of April 13, New Year's Day, known as Saad Nam. Streets across the kingdom are lined with teenagers bearing an arsenal of hoses, buckets and water pistols. Bangkok in particular is soon awash and everyone is expected to take their punishment with good humour. Songkran is also observed in Myanmar, Laos and Cambodia, but the festivities are perhaps nowhere as exuberant as in Chiang Mai, where celebrants wade into the mid-city Ping River, scooping up water in buckets. A Queen of the Water is chosen and the city becomes home to a three-day, water-cooled carnival of music and dance. Staying in Bangkok and taking my chances with the bucket brigade, I see a city given over to a grand procession honouring Buddha, with the kingdom's 76 provinces represented - appropriately amid all the water - by vividly coloured floats. There is even a Miss Songkran beauty pageant ... and no, it's not a wet T-shirt contest. A dancing fountain runs the length of Khao San Road. Latin music booms from loudspeakers and, this being Thailand, a procession of monks glides serenely through the middle of the wet riot, blessing it all while remaining miraculously dry. Despite the head-to-toe drenching, my face has been finger-painted with clay scores of times. I make my way back to my hotel looking like a dissolving clay zombie. The reception staff don't bat an eyelid, meeting scores of such dripping, ghostly apparitions at Songkran every year. By evening I assume the aquatic shoot-out must have abated and head down to the shopping strip of Silom Road. At the entrance to Patpong Night Bazaar, I find my way blocked by a young lad with a devilish grin and a small garbage bin. Full of water. Oh no. To beg for mercy is to earn a double drowning. Happy Songkran, mister! Getting there: Cathay Pacific ( www.cathaypacific.com ) and Thai Airways ( www.thaiair.com ) fly direct from Hong Kong to Bangkok.