We need to consider two factors. First, most of the vacant sites are small and not suitable for developing into housing complexes that can accommodate a considerable number of residents and provide the necessary community facilities and open space conforming to proper planning standards.

Second, and perhaps more importantly, we must always have some land reserved to cater for unforeseeable community or development needs, which change over time as the public's aspirations change. For example, we should not overlook expansion of local clinics and community facilities to meet the needs of the growing population and to satisfy the aspirations for better-quality housing.

There are other options that we should explore. Intensified land utilisation is certainly one of them. The case of Ming Wah Dai Ha, a rental estate in Shau Kei Wan currently under redevelopment, is a good example. Originally built in the early 1960s, the buildings are ageing, and so are their residents. The Housing Society is now redeveloping the estate, optimising the available plot ratio. When the redevelopment is complete, not only will it provide a third more housing units, it will also become an integrated community offering homes for the elderly and related services, as well as commercial and community facilities. In recycling developed land, we can optimise land use, either by fully utilising the approved plot ratio or possibly increasing it.

Are country parks also an option? They are more than just places for hiking, barbecues or other leisure activities at the weekend. They are our most valuable conservation assets and perfect locations for outdoor education. Most importantly, they are a vital part of our water supply system.

There are 24 country parks in Hong Kong, 11 of which are home to our 17 reservoirs. These 11 parks, totalling 69 per cent of the total area of all our 24 country parks, form a network of catchments for collecting rainwater into the reservoirs. These parks were designated with an important purpose of protecting the catchments from contamination. Although we now rely mainly on the Dongjiang for our water supply, rainwater collected from the catchments still provides a significant percentage - between 11 per cent and 27 per cent in the past three years - of Hong Kong's consumption . The reservoirs have another important function - they store the imported Dongjiang water that is surplus to immediate needs. Any development in these parks would contaminate the catchment areas and therefore water in the reservoirs.

We should also be aware of the increasing demand for water in Guangdong due to the rapid economic development and population increase in the province. In the long run, we must increase our water self-reliance. The Water Supplies Department is working on a feasibility study into seawater desalination with the aim of, by the 2020s, initially providing for 5 per cent of our water consumption needs. In the meantime, it would be most inadvisable to even think of encroaching on our catchment areas.

One may ask, "What about the other 31 per cent of our country park area?" It is true that this is not part of our water supply system. When there is no alternative to meet our development needs, as our population grows and people aspire to a better living environment, there may be a time when we will need to consider using some of the fringe areas of those parks, for example, the northern and southern strips of Lantau.

It is always a matter of balance between meeting human needs and protecting the natural environment for sustainable development - "meeting the needs of the present without compromising the ability of future generations to meet their own needs", as defined by the Brundtland Commission.

The government is pushing ahead with the northeast New Territories and Hung Shui Kiu new development areas and has proposed several other ways to create more land. We should concentrate on these projects, which deal with our medium- and long-term needs for land. Meanwhile, instead of spending time seeking isolated solutions, we should review our land-use strategy to seek co-ordinated ways to optimise land use in conjunction with our population and housing strategies, and people's aspirations.

A few days ago, the Hong Kong Trail, which crosses five country parks on the island, was named by Lonely Planet as one of the top 10 city hikes in the world. It confirms that our country parks are a world-class treasure, particularly since they are so accessible from the busy city centre.

Some 40 per cent of Hong Kong is country-park land; no other major city can match that. These parks are Hongkongers' great escape; where city folks can head for fresh air, nature and tranquil scenes. When severe acute respiratory syndrome struck in 2003, thousands flocked to country parks, a safe haven away from the virus-infected city, leading to enhanced appreciation of the parks' great value to our well-being. In 2011, they had 13 million recorded visitors. They are the jewel in the crown of Hong Kong, and we must spare no effort to safeguard them.

To many people, especially low-wage earners, making a living in the city is suffocating, debilitating and hardly bearable. But our spacious country parks welcome all, irrespective of means. There, we can relax; breathe in nature's fragrances; hike in the midst of beautiful landscapes; see plants, birds, butterflies and bugs, all the while nourishing the seeds of happiness in our hearts. Simply put, we get recharged. Feeling good, we then head back to work for another productive time. Seen in this light, country parks serve us well as Hong Kong's eternal spring of energy and happiness.

Unfortunately, suddenly, the whole city is talking about building houses in the country parks. It is portrayed as a quick fix to our chronic housing shortage. Anyone speaking up against the idea is relegated to the rank of bird-brained "environmentalist", or denigrated for a lack of sympathy for those in dire need of housing.

Population projections of bygone years clearly indicated the growing demand for housing. But the government sat on its hands for a decade or so. Officials are the apathetic ones, not "environmentalists".

The argument for turning country-park land into sites for housing is basically: "It's a vast area with few people - what a waste!" The idea also works well as a distraction, taking our gaze away from the core issue - the optimal use of all types of land in Hong Kong, including that in the New Territories.

In town, people are looking at maps and doing their homework. There are so many specific suggestions: use 1 per cent of the country-park area; push back the country-park boundary by 30 metres; develop Wong Nai Chung Gap and the Kowloon reservoirs for low-rise homes (for the rich?); a "great wall" of buildings along the coast of southern Lantau (again, for the rich?).

Within a week, potential country-park invaders are turning up everywhere. It's like the emergence and spread of cancerous cells. It takes only one mutation to create an army of cancerous cells. In this case, that mutation was Development Secretary Paul Chan Mo-po's blog article on September 8.

It is disappointing that key government officials cannot grasp the fundamental values embedded in the country-park concept. Once one piece of country-park land is surrendered, there may well be no end to the attrition.

As the proverb says: give them an inch and they'll take a mile. And so it would go on. To conserve our country parks, we have to prevent the first inch being given up. Thus, the first line of defence is effectively the last line; we have to fight the battle fiercely.

Those in power need to recognise that the vast country parks are there for good reason. They provide precious and invaluable services: giving families happy times together; enabling young people to build their character with hikes among the hills; energising tired souls; uplifting the depressed; providing a stage for people to enjoy group activities and develop collective strength through social interactions. I could go on.

Suffice to say that there is much more to our country parks than meets the eye. The space is not empty; it is full of meaning and its value to people is subtle but profound.

Just think of the billions of dollars people spend on fitness and training classes every year. Country parks contribute to our well-being at least as much, free of charge. They are a great natural resource for all, and should not be depleted permanently in favour of the one-off gains for a select few. That would be very poor economics.

The sad fact is that those who work in air-conditioned offices and never venture into country parks are unable to appreciate their aesthetic and spiritual value. They merely see the parks in dollar terms, as valuable land lying empty and a wasted resource. It's easy, using their logic, to seek to push the government to release this "wasted" land for building housing, thus bringing enormous profits.

But this push runs against the wishes of ordinary people who treasure country parks as somewhere where their spirit can be recharged.

For the man in the street, pressed down by the burden of working for a living, country parks offer a gasp of air. They will not let cancerous cells eat into the parks.