Annual hydrological report 2001
Lake- and soil water resouces decreased during 2001
The year 2001 began with an abundance of water and ended unusually dry. Snowfall was low in the winter of 2000-2001, the onset of spring was early and spring floods, which normally replenishes the water reserves, were low. Evaporation during the summer
was high, due to exceptionally warm weather in July-September, particularly in eastern regions. Temperatures in Lapland were near-normal, and precipitation was very heavy during August. The winter season began at approximately the normal time in most
parts of the country. December was very cold and precipitation was low.
Precipitation during 2001 was approximately normal in most parts of Finland, except for some easternmost areas where low precipitation was recorded.
Snow water equivalent was low in late winter throughout the country, which was a major reason for the decrease in water reserves which occurred overall during the year. Particularly low levels of snow were recorded in the region around Inari in
northern Lapland. At the end of the year 2001 the water equivalent of the snow cover did not differ significantly from the mean value.
Water levels of surface water bodies at the beginning of 2001 were 10-50 cm above the seasonal mean, due to heavy precipitation towards the end of the year 2000. By the end of 2001, however, water levels in most parts of the country were below their
mean, i.e. they had decreased during the year by several tens of centimetres, particularly in eastern Finland. An exception was formed by the large, regulated lakes of northern Lapland, in which the water reserves were above the mean as a result of heavy
local precipitation during the summer. Near-normal water reserves were also recorded in the basin of the river Kokemäenjoki at the end of the year. At the beginning of 2001 lake water reserves were about 15% above the seasonal mean and at the end of the
year they were 20% below the mean; the percentages were calculated on the basis of the available storage capacity.
Spring floods were rather low throughout the country. Some flooding due to ice dams did howver occur in Ostrobothnia and northern Finland in April-may, but overall flood damages (250 000 EUR) were significantly lower than in 2000 (1.7 million EUR).
Discharges were above the seasonal mean at the beginning of the year 2000. By the end of the year, discharges were below the mean in all rivers of Finland. Late summer discharges were very low in smaller rivers of southern and central Finland until the
onset of autumn rainfall. During the period of heavy frost at the end of the year, discharges again rapidly decreased in these small rivers. In the rivers of northern Lapland, e.g. the river Teno, the heavy precipitation during August caused discharges to
increase for a short time to exceptionally high levels.
Evaporation and temperature
The decrease in water reserves was accelerated by unusually high evaporation between July and September in southern and central Finland. The weather was very hot for a rather long period, and water temperatures after a cool period in June were several
degrees above the seasonal mean in July-September. Particularly high evaporation was recorded in the lakes of eastern Finland. In Lapland, both water temperatures and evaporation were close to the seasonal mean.
At the end of the year groundwater levels in most parts of Finland were somewhat below those recorded one year earlier. The worst situation was in eastern Finland, where groundwater level deficits of several tens of centimetres were recorded. Elsewhere
the deviation from the mean was smaller. Water levels remained below the mean throughout the year 2001 in many large groundwater formations reacting only slowly to climatic conditions, and in which the current water deficit had originated towards the end
of the 1990s.
Groundfrost in the late winter and early spring of 2001 was shallow. During the following winter season, heavy frosts caused the development of a rather deep layer of groundfrost at the very end of the year.
Break-up of ice in spring occurred 1-2 weeks ahead of the normal annual schedule, during April in the south and during May further north. Water temperatures after the warm summer decreased rapidly during the autumn and freezing over of watercourses was
approximately on schedule in October-December. The last lake to freeze over was the Tehinselkä basin of lake Päijänne on 21.12. At the very end of the year ice thicknesses in some areas were greater than normal due to the heavy frosts during December.