Monthly hydrological report January 2012

Snow covers increased rapidly in the southern coastal zone; thin ice layers throughout the country

The beginning of January was mild, but the end of the month rather cold. Precipitation was high in the south but low in the north. Snow covers grew throughout the country, particularly along the southern coastline. Along with the cooling weather, both surface and groundwater levels began to decrease in all regions. Heavy frosts caused hanging ice dams in many rivers without an ice cover. Both groundfrost and lake ice thicknesses were weaker than normally.

Precipitation

Precipitation during January was high in the south but low in the north of the country. In southern and central regions the precipitation of 40–80 mm was above the seasonal mean, and in some southern coastal areas the precipitation even exceeded 80 mm. In northern Finland the precipitation was generally 20–40 mm, on both sides of the seasonal mean; in northern Lapland figures below 20 mm were recorded.

Snow cover

The water equivalent of the snow increased considerably throughout the country, particularly around the middle of the month along the southern coastline and in Kaakkois-Suomi, where the increase was as much as 30 mm in 10 days. Elsewhere, the snowfall was more evenly distributed during the month. The greatest snow depths were in southern and south-western Lapland and in Koillismaa, Kainuu and parts of Pohjois-Karjala; in these areas the water equivalent at the end of January was generally 100–120 mm (kg/m2) and in some places even higher. In other parts of northern Finland the corresponding figures were generally 70–100 mm. In southern and central Finland the water equivalents varied between about 20 mm in the south-western coastal zone to a maximum of 80 mm in the counties of Kymenlaakso and Keski-Suomi. The snow covers in eastern Finland and in northern Lapland were in many areas below the seasonal mean; elsewhere the recorded figures were close to the long-term seasonal mean.

Water level and discharge

During frosty weather early in January water levels began to decrease. Of the larger lakes the surfaces of lakes Saimaa and Päijänne continued to increase even to the end of the month; in the north, water levels decreased throughout January. By the end of the month water levels in southern and central Finland were clearly above the seasonal mean and the surfaces of lakes Saimaa and Pielinen, which had long been below the mean, had reached their normal winter levels. In many rivers without an ice cover, rapid freezing caused the development of slush ice dams and level increases; severe flooding was not however reported. The mean monthly discharges of the major river watercourses varied on both sides of the seasonal mean. In the rivers Kymijoki and Kokemäenjoki discharges were still high after the heavy precipitation at the end of the previous year, whereas below-average discharges were recorded in the rivers Vuoksi and Oulujoki. The discharge of the river Kemijoki and Tornionjoki in the north were above the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of January was +2 cm, and corresponding figures in some other Finnish lakes were: Kallavesi +18 cm, Saimaa -3, Keitele +35, Päijänne +41, Pyhäjärvi, Säkylä +11, Längelmävesi +19, Näsijärvi +18, Lappajärvi +47, Lammasjärvi +10, Oulujärvi +39, Lokka -32 and Inari +45 cm. The mean monthly discharge of the river Pielisjoki was 94% of the mean January discharge during the reference period 1971-2000, and corresponding figures in some other Finnish rivers were: Vuoksi 90 , Kymijoki 148, Karjaanjoki 214, Kokemäenjoki 153, Kalajoki 158, Oulujoki 89, Iijoki 123, Kemijoki 108 and Tornionjoki 176%.

Groundwater level 

Groundwater levels were 10–40 cm above the seasonal mean in most parts of the country except in the east, where the levels were close to the mean. With the onset of wintry weather the levels began to decrease throughout the country.

Groundfrost

Groundfrost depths in southern and central Finland were only 0–10 cm; in the north the depths were 5–20 cm. These groundfrost depths were below the seasonal mean almost throughout the country.

Ice cover

The larger lakes of southern and central Finland froze over during frosty weather early in January, although ice layers were weaker than normally in all parts of the country. In Lapland, unprecedentedly thin ice covers were recorded e.g. over lakes Inari and Kevojärvi. Heavy frost towards the end of the month did not markedly increase ice thicknesses. By the end of the month the recorded ice layers were generally less than 20 cm in the south, 15–35 cm in central Finland, Kainuu and Pohjois-Pohjanmaa and 30–40 cm in Lapland. These thicknesses were generally 5–30 cm below the seasonal mean. Near-normal ice thickness, 58 cm, was measured only over lake Kilpisjärvi.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-04-19 at 12:10, updated 2013-07-01 at 12:30