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Climate Change Leads to Changes in Rainfall Patterns
Anticipating Future Droughts and Floods in Hong Kong

Green Country Vol. 114 (Jun 2015)

Climate change caused by global warming, as a result of greenhouse gases emitted by humans, has become the greatest environmental problem. In May, The Environment Bureau of The Government of the Hong Kong Special Administrative Region unveiled the "Energy Saving Plan for the Built Environment" in hope of increasing energy efficiency and reducing greenhouse gas emissions. Apart from reducing the problem of emissions at source, the need to deal with resulting problems in future is also a pressing issue in addressing climate change. With climate change, the rainfall patterns in Hong Kong will begin to change. By the next century, it is predicted that the number of rainy days will decline greatly, yet heavy rainstorms will be more frequent. Is Hong Kong ready for such extreme weather?

Based on climate statistics from Hong Kong Observatory, annual total rainfall rose at an average rate of 38mm per decade over the last 60 years, while heavy rain days (days with hourly rainfall greater than 30mm) increased at an average rate of 0.3 days per decade. The Observatory forecasts that annual rainfall in Hong Kong by the late 21st century will rise by about 180mm. Despite this, the annual number of rainy days is expected to drop from 104 days at present to 77 days on average. Future numbers of days with 5mm of rain or less will also decrease, from the current 104 days to 77 days, while the number of days with 70mm or more of rain will increase. On average, the return periods of daily rainfall higher than 300mm are expected to decrease from 9 years to 4 years. All these forecasts point to the conclusion that rainy days will become more concentrated, in a shorter period of time, and the intensity of rainfall will also greatly increase, with more frequent rainstorms.

Currently, 70% of Hong Kong’s potable water supply comes from the Dongjiang (East River), and the remainder is from local reservoirs. Whether the water is from Dongjiang or reservoirs in Hong Kong, they all depend on rainwater. Compared with areas that rely on meltwater from glaciers at high altitudes for potable water, the effect of changing rainfall patterns in Hong Kong is important. High levels of rainfall in a short period of time can easily cause water in the Dongjiang catchment area and reservoirs in Hong Kong to overflow, wasting drinking water. On the other hand, the concentration of rainy days means a lowering frequency of rainfall, which reduces the chances of the Dongjiang catchment area and reservoirs in Hong Kong to collect potable water.

Years of drought will increase

The rainfall in Hong Kong is very dependent on large scale weather systems, and the wet season is between May and September. During the change of seasons between spring and summer, the cold and warm air masses meet creating a mei-yu front that brings more than 700mm of rainfall in the months of May and June, which makes up one-third of the annual rainfall. Typhoons bring more than 1,000mm of rainfall to Hong Kong between July and September, which is roughly half of the annual rainfall. With the influence of global warming, meteorologists in Taiwan recently found that cold air masses from the north have weakened, and the Mei-Yu rainbelt of Southeast Asia has shifted to 30°N from the original position between 20°N to 30°N (between the north of Hainan Island and the Yangtze River Delta). Hong Kong is at 22.3°N, and the shift of the Mei-yu rainbelt may reduce the level of rainfall in Hong Kong between May and June. In terms of typhoons, the numbers of tropical cyclones entering into Hong Kong’s 500 km area radius have seen a significant decline, so the level of rainfall in Hong Kong brought by typhoons will also decline. Hence, the rainfall patterns in Hong Kong may experience great changes in future.

Some may think that even with the changes of Hong Kong’s rainfall patterns, the problem can be solved by adjusting the amount of water supplied from the Dongjiang to Hong Kong. However, since the Dongjiang is not far from Hong Kong, it too will not be drought free if Hong Kong is affected, and the flow of water will very likely be reduced. To make matters worse, there is growing demand for water in mainland China cities, and currently there is also tight demand for potable water in many cities along the banks of the Dongjiang. Hong Kong must find ways to cope with the water crisis accompanying climate change.

New Challenges to Rainwater Collection

Hong Kong Observatory estimates that extremely wet years will be more frequent in the future. Extremely wet years mean that that annual rainfall is more 3,000mm. The current average annual rainfall in Hong Kong is around 2,300mm. In the last century, there were three extremely wet years. It is forecast that the number will increase to around 12 years in the next century. Although the rainfall patterns may affect the collection of potable water, the scale and frequency of floods may increase.

Hong Kong has hundreds of rivers and streams of varying sizes. Presently, numerous residential buildings and shops are located close to river channels. According to the government, channelisation of natural rivers by deepening and widening the channels can increase velocity of discharges into the sea. However, this is an ill judged “solution”, as the natural channels and vegetation will disappear. Permeability of the soil will decrease, only to bring more severe flooding. Widening a channel is a waste of land, as there are only a few storm days within a year, and the use of land within the channel is extremely low. Also, discharging urban rainwater into the sea without making good use of it is by no means good water management, as it will intensify Hong Kong’s water crisis given the forecast reduction in number of rain days.

We cannot control the frequency or the level of rainfall during rainstorms, but there are methods of reducing the effects. Presently, the urban drainage system diverts rainwater to nearby river channels. In other words, the river channels will receive a large amount of rainwater during a rainstorm, which can easily cause river water levels to rise, resulting in floods that will affect residents along the river. In contrast, the preservation of a certain ratio of soil can increase the infiltration ability of urban land. Large areas of vegetation can slow the surface runoff, relieving the pressure of floodwater. Preserving urban soil can be achieved with large parks without concrete covering the ground, large areas of grass, green zones, football fields etc. On days without rainstorms, these facilities can also be used by people for recreation.

For developed areas, where a large park or grass cover cannot be added, the government should divert floodwater to river channels far from residents, by building bypass floodways or drainage tunnels. To relieve Lai Chi Kok, Cheung Sha Wan and Sham Shui Po from flood problems during rainstorms, the Drainage Service Department implemented the “Lai Chi Kok Transfer Scheme” by using tunnels to divert and discharge rainwater out to Victoria Harbour, reducing the chance of floods in the area instead of having stormwater drainage improvement works in heavily populated areas. However, this only addresses the problem of flooding, as potable water is wasted as it is discharged into Victoria Harbour.

During the Lai Chi Kok Transfer Scheme, the government also carried out a project involving a “Water Tunnel between Kowloon Byewash Reservoir and Lower Shing Mun Reservoir” with the construction of a 2.8km tunnel that diverts 3/4 of overflow water from Kowloon Byewash Reservoir to Lower Shing Mun Reservoir, while 1/4 of the overflow water is discharged to Victoria Harbour using two water tunnels of the Lai Chi Kok Transfer Scheme. The Kowloon Bywash Reservoir has a small carrying capacity and has frequently overflowed during the rainy season in the past, which also places pressure on the existing drainage system. After implementation, the Water Tunnel between Kowloon Byewash Reservoir and Lower Shing Mun Reservoir scheme, apart from collecting the original overflow rainwater, has also indirectly relieved the problem of urban flooding, more directly addressing the problem of changing rainfall patterns in Hong Kong.

Currently, there is only one “Inter-reservoirs Transfer Scheme” that is not large scale, and a large majority of rainwater in the urban areas is discharged out into the sea. The government should conduct a study on the collection of rainwater for potable water usage for future new development areas or large reclaimed areas, such as the Northwest New Territories Development Area, and West Kowloon Reclamation area. As these new development areas are expansive, plans must be made in advance for rainwater collection facilities to be incorporated in the planning process.

Desalination for Emergency Use

The Waster Service Department has planned to construct a desalination plant at Tseung Kwan O, but the cost for the facilities is high and there will be a high energy usage, which will emit large amounts of carbon dioxide, contributing more to the climate change, creating a vicious cycle. Therefore, desalination has its strengths, but can only be considered for emergency use. In addition, through a series of treatments and disinfecting processes, our daily polluted water can be made clean, yet this so-called “reclaimed water” is mostly discharged into the sea. The government should consider further promoting and improving reclaimed water and expand its use to toilet-flushing and street-washing etc. More work should be done in the area of water conservation, as Hong Kong people have a poor understanding of the crisis in drinking water, thinking that the water supply from Dongjiang River is limitless, while the low charges for water use can easily contribute to abuse and waste. The Water Service Department has implemented a “Total Water Management Study” in recent years, which is undoubtedly a step in the right direction. The government should also address the aging, leaking and bursting problems of potable water pipes, which have long attracted criticism.

Underlying problems of climate change can be very broad and far reaching. We need to prepared, especially when some infrastructure changes and constructions cannot be implemented immediately and often require long-term arrangements and planning. Potable water is a basic resource that we depend on for survival. Therefore, given forecasts for changes in rainfall patterns, we must act now. After the Public Consulation on “Hong Kong’s Climate Change Strategy and Action Agenda”, we hope the government produces and implements substantial work proposals and measures.

Text | Henry Lui & Dr. Cheng Luk-ki