Drought has plunged Paisley and Renfrewshire’s water supply into an emergency.
The twin reservoirs have lost a third of their capacity during weeks of the heatwave.
The Express can reveal that an astonishing 346,500 tons of water is missing.
Renfrewshire is served by the two operational sources above Howwood.
Now Buddies have been asked to double their efforts to save water.
The Scottish Environmental Protection Agency has said water in the zone is at “moderate scarcity”.
Scottish Water is asking people to use H2O efficiently, especially the use of hoses, sprinklers and paddling pools.
The Barcraigs and Rowbank reservoirs two miles above Howwood span hundreds of acres.
But the barren moorland streams which feed them have dried-up at a rate not seen in years.
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The present showers forecast will not put a dent in the low supply.
Barcraigs and Rowbank are more than 100 years old and are now 33 per cent below the brim.
Barcraigs can hold 7,783,000 cubic metres of water and Rowbank 2,250,000 cubic metres.
Laura Boyce, of Scottish Water, said: “Demand has been high due to warm, dry weather and we have appealed to our customers to use water efficiently.
“No hoses, sprinklers, pressure washes and paddling pools should be used because of some challenges in maintaining normal supplies
“Across Scotland, we are having to produce 200 million litres of extra water per day nationwide to maintain normal supplies.”
SEPA said the problem has been sparked by an extremely dry April and only 45 per cent of June rainfall.
The agency said forecasts for the second half of August also look to be very dry.
Chief executive Terry A’Hearn said: “Drier conditions are expected to become more prevalent later in August.
“Everyone knows that water is a vital resource.
“We need to get used to the idea that, even in a water-abundant nation like Scotland, it is a finite resource – as shown by the increasing severity of the water scarcity picture in large areas of the country.”
Scottish Water are asking Buddies to use a watering can instead of a sprinkler and a bucket to wash the car instead of a jet washer, which uses 36 litres.
They are even asking folk to turn the tap off when brushing teeth and use washing machines and dishwashers only when fully loaded.
In some areas, the supplier is having to use emergency storage and moving water between networks, including using tankers.
Kes Juskowiak, Scottish Water’s Water operations general manager, said: “These are very challenging conditions because of the warm, dry weather we’ve been experiencing and the forecast for the coming days is for more of the same.
“Current levels are below average for this time of year, but the main issue is demand for water from customers, which has increased considerably during the warm weather.
“It’s not isolated to one group, but rather is the result of how we all use water in warm weather, particularly in the garden.”
Barcraig reservoir was opened in 1916 by the Paisley Corporation Water Works right next to the much smaller Rowbank.