As Guangdong's Dongjiang (East River), the source of between 70 and 80 per cent of Hong Kong's water, comes under increasing pressure, I'm visiting High Island Reservoir to learn a little about how the city is trying to safeguard its supplies - and a surprise awaits deep within the massive east dam.

Hong Kong needs a fresh policy on water before the taps run dry

of Hong Kong was established, in 1841, simple bamboo aqueducts helped carry water from hillside streams to villages.

The colonial government seemed blasé about water initially, digging just four of five planned wells by 1852. It was spurred into action as the population rapidly grew and, in the early 1860s, Hong Kong's first major reservoir was built, at Pok Fu Lam. Even when newly completed, it could not meet demand - especially during the winter dry season - and work soon began on an expansion project, completed in 1877. Again, growth in demand had outpaced supply and the government continued on what would prove to be a long game of catch-up.

A larger reservoir was built at Tai Tam. While the water from Pok Fu Lam Reservoir was piped around hillsides, relying on gravity, the Tai Tam site required a two-metre-high, 2.2km tunnel and a pumping station. The tunnel proved especially challenging to build; physical labour combined with "elementary machinery" produced only a fifth of its length in nine months, according to Ho Pui-yin's book , prompting the government to bring in machinery from England to accelerate the work.

Construction of the Tai Tam project, from 1883 to 1888, cost HK$1.25 million - a huge sum for a government that was suffering a budget deficit. While it boosted supply, and helped the city expand eastwards from Central, the distribution system remained rudimentary - just 1,877 private houses had running water by 1881 - and there was so much carrying of water in buckets that physical strength was an important qualification for cooks.

After the New Territories were leased to Britain, in 1898, the government began building reservoirs in the hills above Kowloon. The economy improved, helping fund more reservoirs on Hong Kong Island - three at Tai Tam and one above Aberdeen, where the government also rebuilt a privately owned reservoir. However, reservoir construction remained expensive, and workers living on-site as Shing Mun Reservoir was being built, in the 1920s and 30s, received, writes Ho, a HK$6 per month food subsidy in lieu of overtime payments: a meagre reward given the accidents with explosives and health threats, including malaria.

Wasting water cheaper than bigger reservoirs: Paul Chan

Cross-harbour water pipes were laid, too, allowing Shing Mun Reservoir to supply a service reservoir - a smaller storage facility for treated water before it is pumped to users - below the Zoological and Botanical Gardens, under the present-day water fountain. Metered residential supplies became widespread.

But with Hong Kong's annual rainfall fluctuating, water supplies were uneven. A severe drought in 1929 led to extended periods without tap water, and people queued for supplies from steel tanks. Water was brought by steamer from Shanghai, Fuzhou and Japan, and by barges from near Zhuhai. Businesses suffered, leading to tens of thousands of people losing their jobs, and more than 70,000 leaving Hong Kong, writes Ho.

There was an even worse drought during 1963 and '64, the impact of which reverberated through the economy. Fights erupted at standpipes, bucket makers profited by converting kerosene containers to water containers and poor hygiene resulted in 115 cases of cholera.

By this time, work was under way on Plover Cove Reservoir - the first in the world to be built in a marine cove. According to the book , by Stella Thrower, the idea for it came to T.O. Morgan, director of water supplies, as he swam in the cove, east of Tai Po. The project was completed in 1973, trebling Hong Kong's water storage capacity. Work was then under way on the world's second reservoir in a coastal inlet: High Island, the volume of which, at 280 million cubic metres, is even greater than that of Plover Cove.

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"Ironically, by the time this reservoir became operational, in 1978, the government had abandoned its objective of achieving a high degree of self-sufficiency in water supply," says Dr Frederick Lee Yok-shiu, director of the Water Governance Research Programme at the University of Hong Kong's Faculty of Social Sciences. Water was being piped in from the mainland and stored in Plover Cove Reservoir, some occasionally being transferred to High Island. "These reservoirs are a bit like water tanks."

Being lakes cradled in forested hillsides, Hong Kong's reservoirs - the WSD now oversees 17 across the New Territories, Hong Kong and Lantau, as well as eight irrigation reservoirs - are much more than "water tanks", though.

"Catchments and reservoir areas represent about 30 per cent of 1,700 hikes we schedule each year for the group," says Shum Si-ki, founder of Hong Kong Hiking Meetup. "Many roads and paths by catchwater channels serve as links from one area of a reservoir to the next. This is especially true with the Tai Tam, Aberdeen, Kowloon and Shek Lei Pui reservoirs. My hikes usually involve bush hiking, and most entrances for my bush hikes are next to catchments as well."

Desalination an option to ensure free flow of Hong Kong's water supply

To moderate water flow and reduce silting, forests were planted and protected in catchment areas. While enhancing the appeal for hikers such as Shum, the restoration of woodland also benefited the plants and animal populations that had suffered during centuries of deforestation and a surge in tree felling during the Japanese occupation.

"If you think of biodiversity in general, one of the best things that ever happened was the establishment of water catchments - with set-aside land preserved from development," says David Dudgeon, chair professor of ecology and biodiversity at HKU. "The landscape could revive from the destruction of the second world war. In the 1970s, country parks were established [see sidebar] basically because of water catchments, not really for conservation."

Lee deals more in the facts and figures of water, even pondering why Hongkongers use so much to flush toilets.

"Here, seawater for flushing is free of charge, and on average [we use] 90 litres per day per person - that's a lot of water for flushing. In Macau, they use about 40 litres per day," he says. "It's a mystery to me as to why we use so much."

He has a strong focus, too, on the future of Hong Kong water supplies.

HK$9b plan to keep Hong Kong taps running in a drought

"In recent years, there have been assessments regarding to what extent we can extend the catchment area, and become less dependent on Dongjiang water," he says. "Singapore has essentially transformed its entire area into a catchment, but while it is technically possible to turn urbanised areas of Hong Kong into catchments, you need to take care not to mix sewage pipes and drainage pipes, and the government believes there's a limit to extending catchment areas."

Lee produces a table showing the allocation of Dongjiang water to various cities, with Hong Kong allocated about 115 per cent of the water it actually consumes. Hong Kong's consumption has fallen a little since the early 1990s, with manufacturing having moved on.

"In the future, consumption should level off, along with the population," says Lee.

Even so, Hong Kong should not take Dongjiang water for granted. Rising pressures in the region are reflected in plans for the transfer of water from the West River to eastern areas of the Pearl River basin, to serve parts of Guangzhou and nearby cities, perhaps including Shenzhen.

"This will be expensive," says Lee - who notes that the price Hong Kong pays for water from the Dongjiang rises every year. The city currently pays HK$5.48 a cubic metre, although that is a fixed price based on the full allocation, meaning the effective price per cubic metre is a little higher. The bill for 2016 is HK$4.49 billion and the price will have risen 36 per cent between 2013 and next year.

In 1975, Hong Kong opened one of the world's largest desalination plants, near Tuen Mun, but the Lok On Pai complex was mothballed in 1982, because it could not compete on price with water from the Dongjiang. Now, a reverse osmosis desalination plant is planned for Tseung Kwan O, which could meet 10 per cent of Hong Kong's needs - the first phase is scheduled to come online in 2020 - and Lee can see a time when sea water stripped of its salt becomes cheaper than imported fresh water. Yet he still believes we should protect the reservoir system, to maintain as stable a supply mix as possible.

A turning tide that could leave Hong Kong in deep water

Although the WSD receives complaints that the water it sells is too expensive, Lee has expressed the opposite view. Is Hong Kong's water too cheap?

"Oh definitely," he says. "It's now about 50 per cent subsidised, sending the wrong signal to consumers. I've written of the need to raise tariffs."

As debate about pricing and future sources swirls, Fan is one of the many aiming to ensure the WSD maintains a reliable and safe water supply. He's been with the department for 24 years, and his responsibilities span a swathe of the eastern New Territories, including service reservoirs, two treatment works and High Island Reservoir.

"We aim to keep equipment operating 24 hours a day," he says. "If we can't repair something within eight hours - which is the normal supply level in a service reservoir - we would have no choice but to inform the public about the problem. But mostly, we repair within this time."

Water flows are controlled by computers, in remote facilities, but sometimes there are problems - a signal failure during a lightning storm, for instance - and teams are dispatched to investigate.

"Sometimes, they may stay overnight [outside] to manually open and control valves," says Fan.

He is always on call, at times having to head out in bad weather, late at night. Sometimes, he and his colleagues have fixed one problem only for a second to have arisen, which also needs to be diagnosed and assigned to a mechanic or electrical engineer to repair. If a service reservoir runs dry, he might have to divert water from elsewhere, or ensure a tanker is ready to come to the rescue of a neighbourhood in need.

Fan and his colleagues take me to High Island's east dam. This is set on one of Hong Kong's wildest stretches of coastline, and blocks a channel that once separated High Island from the Sai Kung Peninsula. We head down near the shore, to walk along the coffer dam, which was built first - along with a sister dam to the west - so the channel could be emptied, allowing for construction of the main dams.

Beside us, dolos blocks protect against damage by the sea - especially when mighty waves are driven onshore by typhoons. Each of the blocks is shaped like a capital H with one leg turned at a right angle. Piled together, they loosely interlock and dissipate the power of waves.

"They made around 7,000 25-tonne dolos blocks," says Fan. They're so massive that a truck could carry just one at a time. I stand on a block and look down through a layer maybe five or six deep. "Every one is numbered," Fan remarks, pointing to a small label that corresponds to an item in a ledger.

Walking to the main dam, we pass jointed rock that formed in the last great eruption and collapse of a supervolcano, adding to the grandeur of the scene. The 70-metre-high dam soars above us. Its steep slope is covered in broken rocks and Fan tells me that every one that falls must be weighed and catalogued, to help monitor safety.

Through a door we enter a passageway so narrow, we walk in single file, then turn left, into the depths of the dam. Solar-powered fluorescent strips light our passage; at intervals there are short water pipes, about the width of my arm: measuring equipment can be fitted to these, to check whether there is internal leaking.

We turn left into a side room with bare concrete walls, ceiling and floor, and containing what looks like a squat oil barrel suspended on a metal frame, above a circular hole.

"This is the most important part of the dam," says Fan. "It's an inverted pendulum."

While I think of pendulums as swinging to and fro, as in a grandfather clock or terrorising the protagonist of an Edgar Allan Poe story, this one is stationary. A wire runs from the middle of the barrel down to an anchor point within the hole. Within the barrel, it's attached to a float suspended on oil. Tap the float, and it moves slightly, then returns to its place in the centre of the barrel. Should the dam become distorted, the float will move off centre, signalling a major problem.

Right now, though, the dam remains as solid as when first built, about 40 years ago, and will continue to play a key role in supplying us with that most precious yet underappreciated of resources: clean, fresh water.

Hong Kong's reservoirs good for people, bad for ecology

Although they are today considered to be the green lungs of an increasingly polluted city, Hong Kong's country parks were established primarily to act as catchment areas for the city's reservoirs, rather than for leisure purposes.

Most of the trees planted were not native to Hong Kong. Species such as Taiwan acacia and Brisbane box grow well on almost barren slopes but do not blend with local ecosystems.

"The non-native species have leaf litter that's more resistant to decomposition," says David Dudgeon, chair professor of ecology and biodiversity at the University of Hong Kong. "If you have native trees around streams, it's better for the stream life, and with more aquatic insects you get more birds."

Some plantations of exotic species have been thinned and native trees planted, but moves to improve the balance have so far been cautious.

Catchwater channels have become something of a problem, too. They may aid hikers, but animals tend to fall into them.

"Snakes, frogs and other animals … they can't get out - and drown or starve," says Dudgeon. "The channels could be re-engineered a bit."

In west Lantau, larger animals have been getting into difficulty. A cow and at least five barking deer have become trapped in catchment infrastructure lately; most died or were euthanised.

"We must do something to save the animals from this fate," says Kathy Daxon, chairwoman of the Tai O Community Cattle Group, which supports measures such as simple fencing along catchments.

Dudgeon notes that most dams are at least 100 metres above sea level, and "in biodiversity terms, the lowlands are fairly trashed".

"Streams are often channelised and prone to pollution, and the reservoir system captures so much water that few streams flow all year," he says. "We might think of not capturing all water, allowing ecological flows so there is always some flow downstream, to maintain ecosystems. We've done some work on this in my lab, and found it can help, with a bigger effect in the dry season."

Dr Ng Cho-nam, of HKU's Faculty of Social Sciences, with research interests including environmental policy, also advocates ecological flows - noting that this practice has been adopted in China.

"Our river management is like one river, two systems," he says. "In the system run by the Water Supplies Department, they take water away from rivers. Lower down, the Drainage Services Department drains away water - making channels wider, deeper, straighter. So, rivers may have no water."

There has been some transition to ecological designs for lowland river channels - the encouragement of mudflats and planting of mangroves at the mouth of the Kam Tin River, for example - but Ng would also like to see the Water Supplies Department adopt a new approach to managing reservoirs, so they are not just single use "water tanks".

"I'm trying to convince them to realise they can serve multiple functions - like landscape, ecology, etc. You go to reservoirs in the UK and there are ducks and swans, people windsurf. In Hong Kong, they're empty. Twenty years ago, I helped review a consultants' study on multiple uses, but it was shelved. The department says, 'We provide safe drinking water.'"