The cost of Dongjiang water from the mainland has doubled in the past decade, according to official documents.
The Dongjiang river is one of the major water sources of Hong Kong, but critics have often highlighted the increasing cost in the water supply agreements signed with the mainland.
The raw cost of Dongjiang water increased from HK$2.494 billion in 2007 to HK$4.491 billion in 2016.
Dividing by the actual quantity supplied, the raw unit cost was HK$3.489 per cubic metre in 2007, but it rose to HK$7.14 per cubic metre in 2016.
The government has estimated that the cost for 2017 would be HK$4.478 billion.
The Water Supplies Department said the current Dongjiang water supply agreement covers the period up to the end of 2017.
It started negotiating with Guangdong authorities on the next agreement in February, and aims to sign the agreement by the end of this year.
Double cost of rainwater
Lawmakers received a written response from the government after they filed written questions to the Finance Committee over departmental spending in the annual budget.
The raw Dongjiang water requires treatment before it is supplied to residents.
In 2015-16, the unit production cost of Dongjiang water was HK$9.5 per cubic metre.
The cost was higher than the HK$4.3 per cubic metre production cost for locally-collected rainwater and the HK$5-6 per cubic metre cost of reclaimed water, which is used for irrigation and toilet flushing.
But it was cheaper than producing desalinated water, which would cost HK$12-13 per cubic metre.
Inadequate local supply
The Water Supplies Department said the local yield is inadequate for meeting the fresh water demand in Hong Kong: “It also fluctuates significantly and is unreliable.”
In the past decade, the quantities of water collected locally ranged from 103 million cubic metres in 2011 to 385 million cubic metres in 2016.
The department said that Hong Kong could depend on an annual supply of up to 820 million cubic metres under the agreement with Guangdong.
“If the annual supply ceiling is lowered, Hong Kong will be exposed to a risk of inadequate water supply in the event of drought,” it said.
“In fact, we imported [Dongjiang] water close to the ceiling in 2011 as the rainfall in that year fell short of the normal level by 40 per cent.”