NAXOS, Greece -- The biggest reservoir on the Greek island of Naxos has dried up, useful only to the turtles that cruise its muddy shallows. Downstream, sea water has seeped into empty irrigation wells, harming the island's prized potato crop.

Further south, on Karpathos island, authorities have imposed restrictions on topping up swimming pools, while in the northern island of Thasos, officials are seeking a desalination unit to make sea water drinkable.

Most of Greece has seen little or no rain in months. Now, as the country's islands prepare to host a record number of summer tourists, the strain on water supplies has rarely been heavier, officials, farmers and scientists said.

"There has been an intense shortage of rainfall across the Mediterranean and, on Naxos particularly, our surface reservoirs are empty," said the island's mayor, Dimitris Lianos.

Millions of tourists visit Greece each year to enjoy its ancient sites, pristine beaches and turquoise waters.

But climate change impacts, including higher temperatures, erratic rainfall and wildfires threaten the future of the country's biggest economic driver.

This year feels especially fraught. After its warmest winter on record, wildfires began unusually early, some in areas where there would normally be snow. At least six tourists, including well-known British television presenter Michael Mosley, died last month as heatwaves swept the country.

Climate experts fear the worst is yet to come. Andrea Toreti, the coordinator of the European and global drought observatory of the Copernicus Emergency Management Service, said once the effects of drought become visible, it is too late to take action.

"We need to avoid thinking in an emergency mode, (instead) looking at prevention and preparedness," Toreti said.

Sickly crops

The water shortage is stark in Naxos, a mountainous island of 20,000 people in one of the most popular - and dry - parts of the Aegean Sea. Tens of thousands of tourists flock to its shores each day during summer.

The island's two reservoirs hold 220,000 cubic meters (7.7 million cubic feet) of useable water, a third of last year's level and the equivalent of just a few dozen Olympic swimming pools.

Authorities have secured three portable desalination units that will treat sea water to make it safe to drink, and which mayor Lianos said should cover the shortfall for houses, hotels and pools.

But farmers will not receive any of the treated water and have to rely on wells that have been contaminated by sea water aquifers. Farmers said that this contamination occurs when the wells are empty enough for the salty water to creep in.

Stelios Vathrakokoilis grows Naxos' famous potatoes, which are loved in Greece for their buttery taste and are protected from imitation under EU rules. His yields will be more than halved this year because of the salty irrigation water, he said.

"It's a big disappointment because we humans didn't succeed in anticipating that climate change would knock on our doors too," he said as a handful of workers harvested potatoes nearby.

Short supply

Countries across the Mediterranean, including Spain and Italy, are looking for ways to back up their water supplies by using desalination, but suppliers said units were in short supply this summer due to soaring demand.

Even in Thasos, which is much greener than rocky Naxos, officials said they wanted to buy a unit for future use.

Greece-based manufacturer Sychem could not fully meet customer demand this summer because of a shortage of key components and longer building times, Chief Executive Alexandros Yfantis said. New units should be available after September.

"Since the problem is all around, any temporary equipment has been already leased," Yfantis said.