“In June, we’ve actually seen a third of the flow and so the models have just not necessarily predicted that,” said Tania Flasck, director of water operations.
“When we’ve actually measured the levels we’ve seen it’s dropped down a lot, a lot further and faster than we’ve anticipated.”
Households are being asked to use watering cans or buckets for activities such as watering gardens, filling paddling pools or washing cars.
Southern Water said investment in infrastructure, including the UK’s first new reservoir for 30 years, would help stop it taking water from the Test and Itchen rivers by 2040.
Paul Vignaux, executive director of the Test and Itchen Association, said: “…we have no reservoirs here and all the water comes from the river or from the aquifer.
“And effectively, if you take water from our rivers and aquifers, you’re taking water from nature.”