The first clear indication usually comes in late autumn and winter, when we typically see more rainfall and experts can assess whether that water is moving through rock layers into underground stores, known as aquifers.
Dr Kevin Collins, senior lecturer in environment and systems at The Open University, says even when autumn rains arrive, people often assume the drought is over.
But sustained rainfall is needed over months rather than a few heavy rainfalls.
Hydrologists should know by February or March whether aquifers and human-made reservoirs have been replenished. But recovery from the environmental impacts can take years.
A dry winter will make the current drought worse and heighten government concerns.
Prof Tim Palmer of Oxford University says it will be “catastrophic” for the UK next year if the rains do not come.
There is some hope. Over Europe, there are signals for a wetter-than-average autumn, potentially influenced by global ocean changes, including the developing El Niño weather phenomenon. However, there remains considerable uncertainty.
“The government needs to start planning for a worst-case scenario where the rains don’t return in sufficient amounts this winter, and next summer is similar to the last two,” Prof Palmer says.