Monthly hydrological report in December 2004
Heavy precipitation increased water levels and discharges in coastal areas
December was warmer than normally throughout the country, with differences from the seasonal mean as high as 2-6°C in Lapland. Monthly precipitation was also above the seasonal mean, the heaviest rainfall being recorded in south-western Finland. Water
levels and discharges were exceptionally high for the time of year in the south and south-western coastal zone and in parts of Ostrobothnia. Both surface and groundwater levels increased during December in southern Finland, whereas in the north they were
already decreasing. Ice covers at the end of the year were generally thinner than normal, as were the depths of groundfrost and the snow cover.
Precipitation in December was 110-150% of the seasonal mean in most parts of the country. The monthly precipitation was generally 50-100 mm in southern and central regions and 40-80 mm in the north. The heaviest precipitation, over 100 mm, was recorded
in the south-western coastal zone. Precipitation came both as rain and as snow.
The water equivalent of the snow cover increased particularly in eastern and northern Finland. By the end of the month the water equivalent was below 40 mm in southern and western Finland, 40-80 mm in eastern and central regions and in southern Lapland
and over 80 mm in other parts of Lapland. The amount of snow was below the seasonal mean in most areas.
Water level and discharge
At the end of the year water volumes in lakes and rivers were generally above the seasonal mean. Along the southern and western coast and in Ostrobothnia water levels and discharges increased to exceptionally high levels around Christmas as a result of
heavy precipitation. In the western coastal zone the highest water levels and discharges of the year were recorded in December. In the river Kokemäenjoki the development of hanging ice dams caused local flooding in some areas. Water levels decreased
during December in Lapland, whereas in eastern Finland they were still clearly above the seasonal mean. Mean discharges of the largest rivers were above or close to the seasonal mean in December.
The deviation of the level of lake Pielinen from the seasonal mean at the end of the month was +49 cm and corresponding figures in some other lakes were: Kallavesi +18 cm, Saimaa +51, Keitele +13, Päijänne +16, Pyhäjärvi, Säkylä +21, Längelmävesi +18,
Näsijärvi -1, Lappajärvi +27, Lammasjärvi +10, Oulujärvi +11, Lokka -19 and Inari +31 cm. The mean discharge of the river Pielisjoki was 146% of the mean December discharge during the reference period 1971-2000 and corresponding figures in some other
Finnish rivers were: Vuoksi 140%, Kymijoki 121, Paimionjoki 172, Kokemäenjoki 124, Kalajoki 123, Oulujoki 124, Iijoki 114, Kemijoki 135 and Tornionjoki 94%.
As a result of the mild and wet weather groundwater levels continued to increase during December in southern Finland. In Lapland, the groundwater table decreased in typical winter fashion. By the end of the month water levels were generally above the
seasonal mean almost throughout the country. Only in large sand and gravel formations of southern and western Finland were groundwater levels clearly below the seasonal mean.
Except in Lapland, groundfrost did not develop to a great extent before the end of the year. At the end of December the depth of groundfrost was 10-50 cm in Lapland and below 10 cm elsewhere, i.e. below the seasonal mean throughout the country.
At the end of the year many of the larger lakes, e.g. Päijänne, Näsijärvi, Saimaa, Pyhäjärvi at Säkylä and Lohjanjärvi were still unfrozen. Ice thicknesses at the end of December were 30-50 cm in Lapland and below 30 cm in southern and central Finland,
generally below the seasonal mean in all areas.
Tables and graphs