Monthly hydrological report December 2012

Wintry weather throughout the country, hanging ice dams caused problems in many rivers

Water-equivalent-of-snow-december2012

December was colder than normally throughout the country. Precipitation varied on both sides of the seasonal mean; early in the month heavy falls of snow occurred even in southern Finland. Water levels were generally decreasing in keeping with the wintry weather, although hanging ice dams developed in some open rivers with rather high early winter water levels. Groundwater levels also began to decrease during December, although due to the wet autumn they were above the seasonal mean. Groundfrost was recorded only in northern Finland. Freezing over of lakes occurred at the beginning of December in southern and central regions, and by the end of the month ice thicknesses were close to the seasonal mean.

Precipitation

Precipitation during December was generally 20−80 mm, on both sides of the seasonal mean. The highest precipitation, over 80 mm, was recorded in Kymenlaakso and the lowest, even below 20 mm, in northern Lapland. Precipitation during the month came mainly as snow, although some rain fell towards the end of the month.

Snow cover

Winter came rapidly even to southern Finland at the end of November and the beginning of December, and the snow cover grew considerably in most parts of the country. Along the southern coastline and in south-western Finland and in parts of northern Lapland the snow cover was clearly above the mean for December, whereas in southern Lapland, Koillismaa, Kaunuu and Pohjois-Karjala the snow cover was rather light. Mild weather and rainfall during the last days of the month melted some snow in southern and central Finland. Snow burdens at the end of the month were 60−90 kg/m2 in the southern coastal zone, 30−60 kg/m2 elsewhere in southern and central Finland as far north as southern Lapland and 60−130 kg/m2 in central and northern Lapland. In north-western Lapland snow burdens even exceeding 130 kg/m2 were recorded.

Water level and discharge

Wintry weather during December meant that water levels were generally decreasing. However, obstruction due to hanging ice dams caused problems in several high-flowing rivers during periods of heavy frost before the formation of an ice cover. Ice dams developed e.g. in Pohjois-Pohjanmaa, southern Lapland, Pohjois-Savo and Etelä-Savo and in the river Kymijoki. In the most difficult cases the ice dams had to be detonated over periods of several days. By the end of the month water levels were generally close to their seasonal means, although in the great lakes of eastern and central Finland the levels were still several tens of centimetres above the mean. River discharges generally exceeded the seasonal mean. In many of the larger regulated rivers the flow rates were decreased in order to prevent the formation of hanging ice dams.

The deviation of the level of lake Pielinen from the seasonal mean level at the end of December was +45 cm, and corresponding figures in some other lakes were: Kallavesi +19 cm, Saimaa +70, Keitele +38, Päijänne +29, Pyhäjärvi, Säkylä -3, Längelmävesi +17, Näsijärvi +11, Lappajärvi +31, Lammasjärvi +7, Oulujärvi +23, Lokka -19 and Inari +31 cm. The discharge of the river Pielisjoki was 149% of the mean December discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 164%, Kymijoki 155, Karjaanjoki 87, Kokemäenjoki 94, Siikajoki 64, Oulujoki 114, Iijoki 117, Kemijoki 114 and Tornionjoki 111%.

Groundwater level

As a result of the wet autumn season, groundwater levels were above the seasonal mean in many regions. When the winter arrived in December the levels began to decrease slightly, but were still 10−30 cm above the seasonal mean.

Groundfrost

The very wet soil slowed down the rate of formation of groundfrost, which was observed only in the north. The rather thin snow cover in northern regions, coupled with heavy frosts, resulted in the formation of a substantial layer of groundfrost extending to 10−20 cm by the end of the month.

Ice cover

The majority of lakes in southern and central Finland froze over at the beginning of December and even the largest lakes of southern Finland had developed an ice cover by Christmas. In many lakes the ice was formed at approximately the normal time. By the end of the month lake ice thickness was 35−50 cm in Lapland and 10−35 cm in southern and central Finland, rather close to the seasonal mean.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2013-03-14 at 12:00, updated 2013-06-27 at 12:33