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Wednesday, 14th October 2009

Hong Kong's future rainfall

I gave a speech on climate change and its effects on Hong Kong in last week's International Conference on Climate Change 2009 held in Hong Kong. This is to share with you the part regarding the projected rainfall for Hong Kong for the rest of the century.

The figure below presents results recently published by my colleagues. They have been based on the 4th Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC) in 2007. In essence, the average annual rainfall in Hong Kong is expected to increase by about 10% by the century's end (Hong Kong's yearly rainfall is about 2 380mm).

Figure 1
Figure 1   Past and projected change in annual rainfall for Hong Kong

The figure also shows that Hong Kong's rainfall pattern is expected to become more and more variable, meaning that there will be more extremely wet and extremely dry years. I believe by now many academics as well as the local scientific and engineering communities are trying to figure out what the implications are and what needs to be done from their respective purviews.

One thing that I would like to bring your attention to is the predicted rainfall in Hong Kong decrease of up to 5% over the next few decades, till the 2040s. A similar decrease is also expected for south China (including Guangdong). This is of immediate concern, as it is set against projected increases in water consumption both in Hong Kong and in Guangdong.

As you may already be aware, about 70% to 80% of fresh water supply in Hong Kong comes from Guangdong. While Hong Kongs water demand is expected to increase from about 950 to 1100 million cubic metres (mcm) by 2020 and to 1300 mcm by 2030, Guangdong has its own anticipated growth and development which will result in greater water demand, from the present 46 to 52 billion cubic metres (bcm) by 2020. But with less rainfall expected for the period, where will the water come from?

Unfortunately, despite the best intentions saving water in the city is not enough. We drink only a couple of litres a day, and the total direct daily use may be about 100 litres per person. In contrast, it takes more than 10 000 litres to produce a kilogram bolt of cloth. The food we eat each day requires 2 000 to 5 000 litres to produce. It takes 15000 litres to produce a kilogram of beef, compared with 2 000 litres for a kilogram of vegetable. The situation will get worse as the world's wealth grows, because affluence means that people are eating more meat.

So what can we do? A simple answer is to find ways to collect more water. After all, we are only extracting for our own use less than 10% of precipitation in the world. There are many ways. Examples include: more reservoirs and dams, but these may have ecological effect; more recycling and de-salination, but these involve costs.

One useful suggestion is to increase the water efficiency in agriculture: agriculture currently uses up 75% of the world's water. Some developing countries are using twice as much water in producing crops as developed countries. A significant portion of the world's cotton, a very thirsty crop, is produced in the rather dry central Asia. We can look to Australia as a good example in water efficiency. The country suffers from its worst drought in the past ten years. In response, some Australian farmers have switched to less thirsty crops and are able to keep farm output stable. In some cases water productivity has even doubled.

At the individual level we can also do our part to save water. It only involves a simple switch in lifestyle --- make sure we wear our clothes many times before disposal, and more importantly, change our eating habit to less meat and more vegetable. This is healthier too.

B.Y. Lee


a)   Water Supplies Department 2007 - Total Water Management in Hong Kong.
b)   Water Resources Department of Guangdong Province, 2008 - Water Resources Report 2007.
c)   People's Government of Guangdong Province, 2007 - 廣東省水資源綜合利用 "十一五" 規劃
d)   The Economist, 11 April 2009

Last revision date: <17 Jan 2013>