Water resources in Finland
Climatically, Finland is located in a transition zone between a maritime and a continental climate. The country belongs mostly to the boreal zone. Due to the influence of the Gulf stream and the Baltic Sea, the climate in Finland is in many respects more favorable than in most other regions located between the 60th and 70th latitudes.
During the period 1961-1990, the Finnish territory received a mean precipitation of 660 mm. Of this amount, 341 mm was evaporated, while 318 mm flowed into the seas or passed out over the national borders. The water storage was increased by 1 mm during this period. The mentioned value 318 mm corresponds to a mean discharge of 3400 m3/s.
There is rather good data on the Finnish water resources, by which we here mean the presently available total amount of surface water and groundwater. Of the most important phases of the water circulation, some records of precipitation and runoff (discharge) began already in the 19th century. Efficient knowledge on groundwater and evaporation has been obtained only since the 1970s.
Finland belongs to the so called humid zone; the major water bodies do not normally dry out. A typical feature in Finland is the abundance of water bodies - lakes and rivers. The water situation may, however, greatly differ from year to year. Within a year, there is a considerable difference between the winter (when the precipitation is stored in the snow cover) and the summer (when a major part of the rainwater evaporates).
The waters are in Southern Finland least abundant in early autumn, in Northern Finland in late winter. An trend towards more abundant precipitation has increased the wintertime discharges and water levels in the coastal areas, while the snow cover has increased in the eastern and northern parts of the country.
The number of lakes larger than 1 ha is estimated to be around 56 000. These lakes are included in the lake database which will soon be completed. The total shoreline length of the lakes is 130 000 km. The total length of all rivers is estimated to be around 20 000 km.
There are 74 main water systems with a catchment area larger than 200 km2. The numerical water system database was completed in 1993. At the moment, update of the Finnish drainage basin system is going on. As a result of this project, a detailed drainage basin system will be released within couple of years.