Water management strategy responds to climate change
The United Nations Intergovernmental Panel on Climate Change confirmed in its Fifth Assessment Report that the rise in global average temperatures since the mid-20th century has been caused by an increase in the concentration of anthropogenic greenhouse gases, thus providing further indication of global warming. Hong Kong cannot avoid being affected by climate change, and the Hong Kong Observatory has forecast that this trend will intensify, with increasingly frequent occurrences of extreme weather and a higher chance of long-lasting droughts or heavy rain. An increase in dry weather would inevitably affect our local yield and also the Dongjiang water resources. Coupled with our increasing demand for water, these factors pose a number of challenges for management of our water resources.
In 2008, the Water Supplies Department (WSD) promulgated the Total Water Management Strategy to ensure a stable water supply for Hong Kong people. This strategy adopted a multi-pronged approach that focuses on containing the growth of water demand as well as the exploration of alternative sources of water supply. First, we emphasise and promote water conservation to contain the growth of water demand. Second, we find it worthy to explore the application of seawater desalination for a supplemental water source in view of our abundant seawater and the mature development of the reverse osmosis membrane technology over the past decade. Third, we will study the feasibility of supplying reclaimed water for toilet flushing and other non-potable uses in the northeast New Territories New Development Areas, Sheung Shui and Fanling. Finally, we will work closely with local universities and the Hong Kong Observatory to conduct studies on climate change to deepen our understanding of its impact on water resources and draw up appropriate contingency plans.
Although the WSD has been working hard to manage water resources effectively, as mentioned above, the continuous heavy rain brought by climate change in recent years has caused occasional reservoir overflow and some of the water has to be discharged into the sea. We understand that water resources are precious and should not be wasted. However, before exploring ways to minimise overflow, we should learn more about how a reservoir is designed and constructed.
Generally speaking, there is no need to build reservoirs if the daily rainfall collected by the catchments is adequate for the daily demand in the designated areas. In reality however, Hong Kong’s rainfall varies greatly from the rainy season to the dry season. Therefore, we need to construct reservoirs as buffers to cope with the imbalance in water supply and demand in different seasons. In this connection, when designing the water catchments and capacities of reservoirs, our main concern is whether the amount of water collected in the catchments and stored in the reservoirs can cope with the demand of the designated areas in dry years.
Furthermore, the construction cost of a reservoir is huge and its cost-effectiveness needs to be considered. If we focus on years with exceptionally large amounts of rainfall and construct an over-sized reservoir, the storage capacity would be wasted most of the time, which is not an ideal way to utilise our land. Taking such factors into account, some reservoirs will overflow when the inflow during periods of heavy and continuous rainfall exceeds the outflow and the remaining storage capacity of the reservoir. When designing reservoirs, engineers need to calculate the potential amount of overflow and put in place the required overflow facilities. This allows overflow water to be discharged into the sea safely and keep the dams intact.
To minimise the amount of overflow we have considered various options, including expanding the reservoirs and interconnecting them. However, the assessment shows that these options are not cost-effective, as the investment and operational cost to minimise overflow would far exceed the cost of collecting rainwater or even that of purchasing Dongjiang water. In addition, the dams and water supply facilities of some reservoirs, such as the main dam of Kowloon Reservoir and the dams of the Tai Tam Group of Reservoirs, have been declared as monuments and therefore not suitable for expansion. Reservoir expansion may also affect the resources and ecological environment in the downstream areas.
Hong Kong is a world city and various sectors rely heavily on a stable water supply. As the chance of having long-lasting droughts has increased due to climate change, we have to maintain the total water storage of our reservoirs at an appropriate level to ensure sufficient water supply under extreme drought conditions. Our two largest reservoirs, Plover Cove Reservoir and High Island Reservoir, serve as our main storage pools for such a contingency. Following the implementation of the WSD’s water regulation plan in recent years, overflow in these two reservoirs has become rare. Cases of overflow have mostly occurred in those reservoirs in Kowloon and Hong Kong Island that were built between the late 19th century and the early 20th century. These reservoirs were designed with small capacities in consideration of the water demand at that time, and consequently there is a higher chance of overflow from these reservoirs during the rainy season. Nevertheless, we will continue to work out methods of minimising the amount of overflow in small-scale reservoirs.
In 2006 we changed our purchasing method for Dongjiang water and adopted a “package deal lump sum” approach. Under this approach, we make monthly proposals to the Guangdong authorities on the supply quantity of Dongjiang water according to our actual needs, thereby allowing greater flexibility for us to control our reservoirs’ storage, minimising overflow in large-scale reservoirs more effectively, avoiding the waste of water resources and saving pumping costs. Beforehand, from 1994 to 2005, the average annual amount of overflow of Hong Kong reservoirs was about 101 million cubic metres. Since the adoption of the package deal lump sum approach, the average annual amount of overflow has been reduced by 71 per cent to about 29 million cubic metres per annum from 2006 to 2012. If we exclude the exceptionally heavy rainfall year of 2008, the annual amount of overflow since then was between 0.3 and 25 million cubic metres, with the average annual amount of about 14 million cubic metres, which equals 1.5 per cent of the annual total water consumption in Hong Kong. All these prove that the approach has been quite effective.
While we need to adopt multi-pronged measures to ensure the sustainability of our water resources, your participation in conserving water is of utmost importance. We look forward to joining hands with you to lead Hong Kong to become a water-saving city.
8 December, 2013