How much water do the Finnish lakes hold?

On a map, Finland looks like a country with a lot of water. Lakes cover around one tenth of the country’s surface area. How much water do they actually hold and does this volume vary?

Finland has around 57,000 lakes of more than one hectare in size. The number of lakes and ponds smaller than this is much greater, up to 300,000 according to the largest estimates. Finnish lakes are relatively shallow, however, with an average depth of only seven meters, and they do not hold very much water. Lake Ladoga in Russia alone has almost four times more water than all the lakes in Finland put together.

The total volume of water in Finnish lakes is approx. 235 cubic kilometres, or around 235 billion tons. However, this volume varies seasonally and from year to year, and it may be 20 billion tons more or less in particularly wet or dry periods.

The underlying reasons for these variations are precipitation, evaporation and runoff. The average annual precipitation in Finland is 660 millimetres, which equals around 220 billion tons of rain falling across the entire surface area of the country. Consequently, the yearly volume of rain is roughly similar to the amount of water stored in the lakes. Around one third of the precipitation comes as snow. The snow builds up on land, and when spring arrives, there is plenty of water everywhere.

Water from the melting snow runs downhill, and the flow rates in rivers go up. In summer, on the other hand, evaporation increases. At the yearly level, around one half of the precipitation falling on Finland’s surface area evaporates, while the other half runs into the Baltic Sea. While some of this precipitation is filtered into the ground and forms groundwater, some of the groundwater also bubbles up and flows out of the ground.

Image: Pixabay