Warmer temperatures brought about by climate change could see up to 7.3 million cubic metres more of Hong Kong’s potable water – about half of the Shing Mun Reservoir – literally evaporate every year by the end of the century, an environmental group has warned. Rainfall helps offset evaporation from reservoirs in most years, but Green Power said the lack of necessary research would jeopardise the city’s long-term water supplies as global warming caused more extreme weather patterns such as droughts and floods, which can contaminate freshwater sources. “We won’t take this seriously now but if droughts prevail over southern China, our water supply from the Dongjiang could be affected,” said Cheng Luk-ki, Green Power’s head of scientific research and conservation. He was referring to the mainland river system that provides Hong Kong with more than three-quarters of its freshwater needs. The problem of reservoirs drying up has been deemed pressing enough for other cities to warrant action. Last year, drought- plagued Los Angeles filled its Van Norman reservoirs with “shade balls” – plastic spheres designed to block sunlight in the hope of reducing water vapour escaping into the atmosphere. Israel has been doing something similar. Even rainy Singapore recently tried out a food-safe powder – which forms a thin, single-molecule film over the surface of water – at Bedok reservoir to curb evaporation, which costs the city state 20 per cent of potable water annually. The United Nations Environmental Programme has stated that more water evaporates from reservoirs globally than is consumed by humans, a trend that dates back to the 1970s. Cheng admitted there was no political urgency for Hong Kong to act. The city’s water tariffs are heavily subsidised and it pays a lump sum for a guaranteed supply from the Dongjiang. According to Cheng, Hong Kong’s reservoir system loses an estimated 30.68 million cubic metres of water annually through evaporation, equivalent to 3.2 per cent of annual potable needs and 5.2 per cent of reservoir capacity. The rate is calculated by applying the annual mean evaporation rate recorded at the Observatory’s King’s Park facility, which is currently about 1,227mm, or roughly half of annual mean rainfall. Cheng was able to work out an annual evaporation rate for reservoirs of 49mm for every one degree Celsius temperature rise. And with temperatures projected by the Observatory to rise by anywhere between one and six degrees Celsius by the century’s end as a result of more greenhouse gases in the atmosphere, Cheng calculated a worrying annual evaporation rate of 49 to 249mm across the city’s 25 sq km of reservoirs. In terms of volume, this would amount to a range of 1.2 to 7.3 million cubic metres lost per year or roughly 10 to 55 per cent of the capacity of the Shing Mun Reservoir in the New Territories. In terms of money – based on today’s production costs for potable water – this would mean HK$9.1 million to HK$54.9 million a year. “What we don’t realise is that all this water lost has a value to it and these are hidden costs,” Cheng said. Dr Frederick Lee Yok-shiu, director of the University of Hong Kong’s Water Governance Research Programme, said the amount cited by the group was not significant but in the long term, the Water Supplies Department would need to explore more holistic approaches to water management. The department did not respond to enquiries by press time. Tomorrow marks UN World Water Day, when countries focus on water-related issues and how they can impact societies and economies.