Ice from Lake Cerknica, the largest intermittent lake in Europe, used to be exported all the way to Mumbai, India, but now the lake might eventually be left dry due to climate change. Water levels have been dropping for years, which increasingly affects the lake's ecosystem.

Lake Cerknica, caught between the Javorniki Hills and the Bloke plateau on one side, and Mount Slivnica on the other, stretches across 25 square kilometres, which makes it Europe's largest intermittent lake. It is probably also the best known intermittent lake in the world.

"No other intermittent lake has been present in the written records as much as Lake Cerknica. It was first mentioned in antiquity, and it has regularly appeared on maps of Europe or pictorial documents outlining the geography of the area since the 10th or 11th century. On some maps, it is even bigger than Slovenia, which means it has also served as a strong landmark," Matevž Podjed, director of Notranjska Regional Park, the public institution that manages the lake, told the Slovenian Press Agency.

Lake Cerknica is unique also because it is very well preserved. "Very few intermittent lakes in the world are in such a good condition as ours, because despite some attempts throughout history it has remained relatively intact and preserved. For example, there is no railway or motorway running alongside it, and it has not been drained. There have been some attempts, but nothing dramatic," he said.

Over the centuries and millennia, locals have learned to live with the lake and have not interfered with it too much, while at the same time making the best use of everything it has to offer - fishing, hunting, grazing and mowing.

It is a little-known fact that ice used to be extracted from the lake during cold winters and exported around the world, to places that did not have it because of warm climate. Ice from Lake Cerknica is said to have even reached Mumbai in India. "It was transported by wagons in large clusters to Trieste, and from there by ship all over the world. To prevent it from melting, it was properly insulated, either with spruce branches or sawdust," Podjed explained.

Why does the lake get dry?

The most unique feature of Lake Cerknica is that it is almost completely dry and covered in vegetation for a part of the year. This occurrence is a result of an underground system of reservoirs and sinkholes. When water inflow exceeds outflow, the water level rises and the Cerknica Field is flooded and turned into a lake.

The only source of water for the lake that runs on the surface is the Cerkniščica, a stream which fills the lake with the water from near-by hills, while all other are underground sources. "Some run for only a few metres, and others a few dozen metres. But the lake has no sinkhole on the surface, so it mostly fills and dries underground," Podjed said.

The water usually lingers for eight months a year. The lake gets filled up in two to three days when there is a lot of rain, and then gradually recedes when there is no rain, but it never disappears completely. The water level is the highest in springtime when snow is melting and in autumn during heavy rainfall.

However, in recent years, water levels have been lower, which has started to affect the lake's ecosystem, Podjed said. A recent study by the Karst Research Institute has shown that the wider Cerknica Lake area received almost 20% less rain in the last decade compared to the last fifty years.

Extremely dry and extremely wet spells are becoming more frequent. "For example, last year we had a dramatic drought in the spring months, which is why the lake was completely dry already in April, while this year, we've had a lot of rain in the spring and the water levels were quite high at that time."

The lake's receding causes the most problems to the fish, so the local fishermen's association come to the rescue. The fishermen move the fish to the part of the lake that never gets completely dry. But some fish simply disappear in the sinkholes and then somehow find their way back into the lake when the water rises again.

Visitor numbers swelling

Cerknica Lake, the Rak Škocjan landscape park and Križna Cave have been declared an important wetland and also protected under Natura 2000, being an important habitat for endangered birds. This may be one of the reasons why the area sees more visitors by the year.

The Notranjska Regional Park is trying to maintain order, so the required infrastructure is being arranged by the lake such as parking lots, bicycle paths and hiking trails. A visitors centre offers information about the lake and an exhibition.

Still, visitors are urged to leave their cars in Cerknica and come to the lake by bicycle. It is also very important that visitors remain on the marked paths, as the land covered by the lake is private property when dry. But when covered with water or ice it can be used by everyone. Boat rides are possible on about a third of the lake's surface and those who have a licence can also fish there.