Monthly hydrological report December 2011

Watercourses flooding in southern and south-western Finland; ice and groundfrost weaker than normally

December was exceptionally mild, and record-breaking precipitation occurred in some southern regions. Exceptionally high precipitation caused watercourse levels to increase to very high levels during the Christmas week, in some cases even to spring flood levels. Groundwater levels were generally above the seasonal mean. Snow, ice and groundfrost were present at low levels if at all.

Precipitation

Precipitation in December was exceptionally high along the southern and south-western coast, where the recorded values of 120–160 mm were over twofold the seasonal mean. High precipitation also occurred around the outlets of the rivers Kemijoki and Tornionjoki. Monthly precipitation was generally 60–120 mm, but below 50 mm in northern Lapland. New record high precipitation was recorded in the watercourses of the rivers Vantaanjoki, Porvoonjoki, Karjaanjoki, Kiskonjoki, Aurajoki and Eurajoki.

Snow

A snow cover existed at the beginning of December only in Lapland. Before the middle of the month a covering developed over the whole of the country, but in southern and western Finland it soon melted. Some snow also fell during the Christmas week almost throughout the country. By the end of December the snow line ran between Vaasa and Savonlinna. The water equivalent of the snow cover was 40–80 mm in Lapland, Koillismaa and Kainuu and elsewhere generally below 40 mm. Snow depths were low in northernmost Lapland and in southern and central Finland and about normal for the time of year in other regions.

Water level and discharge

Water levels and discharges varied widely in southern and central Finland due to the mild weather and high precipitation. River water levels were exceptionally high in southern and south-western coastal rivers, in some cases reaching or even exceeding the levels of the previous spring flood. Floodwater covered fields in many regions and caused blocking of some roads. The heavy precipitation was also reflected in lake waters, which were generally at flood levels after Christmas in southern Finland. Except for northern Finland, water levels were also high at the end of December in other parts of the country. The levels continued to increase in the south; elsewhere they began to decrease as the weather became drier and colder. In the north, temperatures were generally below zero and water levels were decreasing. By the end of the month the water levels in these areas were close to the seasonal mean. Monthly discharges were high in the rivers Kymijoki, Kokemäenjoki and Kemijoki, whereas that of the river Vuoksi was somewhat below the mean and of the river Oulu close to the mean. The mean discharges of the smaller rivers of the southern coastal zone were exceptionally high, but new records were set only in the rivers Kiskonjoki and Paimionjoki.

The deviation of the level of lake Pielinen from the seasonal mean at the end of December was -14 cm and corresponding figures in some other lakes were: Kallavesi +12 cm, Saimaa -22, Keitele +35, Päijänne +28, Pyhäjärvi, Säkylä +17, Längelmävesi +11, Näsijärvi +13, Lappajärvi +45, Lammasjärvi +12, Oulujärvi +20, Lokka -40 and Inari +43 cm. The mean monthly discharge of the river Pielisjoki was 77% of the mean December discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 94%, Kymijoki 139, Karjaanjoki 161, Kokemäenjoki 165, Kalajoki 158, Oulujoki 103, Iijoki 143, Kemijoki 126 and Tornionjoki 203%.

Groundwater level 

Groundwater reserves were high for the time of year, with levels generally 10–40 cm above the seasonal mean. The mild and wet weather favoured replenishment of the reserves. In the north, groundwater levels had begun to decrease in typical winter fashion but were still 10–20 cm above the seasonal mean.

Groundfrost

In southern and central regions the wet soil and mild weather prevented the formation of groundfrost, which hampered harvesting of wood from forests and exacerbated the damage caused by stormy weather around and just after Christmas. In most parts of the country no groundfrost was recorded; in the north the depths were 0–30 cm.

Ice cover

Ice was present at the beginning of the month only in Lapland, and even there was in many cases exceptionally thin. By the end of the month ice had formed throughout northern Finland but most of the lakes of central and southern regions were still without a complete ice cover. By the end of December ice thicknesses were 20–30 cm in Lapland, below 25 cm in Kainuu and Pohjois-Pohjanmaa and below 10 cm in central Finland. These ice thicknesses were clearly below the seasonal mean, in parts of Lapland even unprecedentedly low.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-01-27 at 15:32, updated 2013-07-01 at 12:30