The water level in Johor's Linggiu Reservoir has dropped to a historic low and Singapore's water supply could be affected if the dry weather continues, Minister for the Environment and Water Resources Masagos Zulkifli said in a Facebook post last night.

Due to low rainfall, the water level in Linggiu Reservoir has dropped from 55 per cent in August to 43 per cent now, said Mr Masagos, who was at the reservoir yesterday morning. The Linggiu Reservoir regulates the flow of Johor River, from which Singapore and Johor draw water for treatment.

Mr Masagos noted that although Singapore can still draw 250 mgd - millions of gallons per day - from the Johor River "on most days", the low rainfall has affected both Singapore and Johor's water supply.

"Water rationing exercises are already ongoing in many parts of Johor. If the dry weather continues, it will eventually also affect us," he added.

He hopes that the annual northeast monsoon will bring more rain over the Linggiu Reservoir catchment area. "I have asked PUB to give me another update on the water situation in December, when the monsoon sets in."

Singapore's strategy of diversifying its water supply "has served us well", said Mr Masagos.

The PUB has been running desalination and Newater plants at high capacity to keep local reservoir stocks healthy. But if the water stock in Linggiu Reservoir does not recover, Singapore may have to do more to conserve water, such as restricting water use for non-critical activities such as washing cars, water fountains and watering plants, he said.

The Minister also urged all Singaporeans to do their part to conserve water.