Guernsey Water says planning ahead helped it cope with "incredibly high demand" during the June heatwave.
Last week, Guernsey, along with the rest of Europe, experienced an intense heatwave.
The island's highest-ever temperature of 36.4°C was recorded on Thursday, 25 June.
At the time, Guernsey Water asked islanders to be mindful of their water usage.
Operations Manager John Holt says the daily demand increased by around 11%.
"We had four days of incredibly high demand, over 15 million litres each day, when we'd normally expect summer usage to average at about 13.5 million litres a day.
"We have two reserves of water, effectively. We have raw water, which is the water that everyone can see at St Saviour's reservoir, Longue Hougue.
"But when we have these very high demand periods, the stress is more on the treated water resources.
"With the forecasting available, we'd planned ahead and managed to increase our stored water to balance that out.
"The hot spells are becoming more predictable and more extreme.
"People are using water wisely, and we would encourage people to continue to just think before they use water."
Another spell of hot weather is predicted for next week.
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