Monthly hydrological report January 2011

Low water levels in the great lakes; heavy snow in southern and central Finland

Precipitation was high in January throughout the country. Accumulation of snow was also above-average in most areas, although record high snow depths were not recorded. Water levels were rather low in the larger lakes of eastern Finland; in lake Saimaa the deficit was about half a metre. Groundwater levels decreased in typical winter fashion in all regions. Groundfrost was rather shallow in the south, where the snow cover was deeper, and 60-80 cm in Lapland with only a light cover of snow. Ice thicknesses exceeded the long-term seasonal mean in southern and central Finland, whereas in northern Finland the ice covers were close to or below the mean.

Precipitation

Precipitation was above the seasonal mean almost throughout the country. In southern and central Finland and in the former county of Oulu the recorded precipitation was 50-70 mm; in Kaakkois-Suomi and Etelä-Pohjanmaa (southern Ostrobothnia) the figures of up to almost 100 mm were over twofold compared to the mean. In Lapland the January precipitation was generally 30-50 mm and in some eastern areas even less, clearly below the seasonal mean. With the exception of parts of southernmost Finland, all the precipitation came as snow.

Snow water equivalent

The snow cover in southern and central Finland at the end of January was clearly above the average for 1961-2010. In the west the snow burden was generally 70-110 kg/m2 and in the east 100-130 kg/m2. The difference in snow depth between the coastal zone of the Gulf of Finland and the inland region decreased, but the amount of snow was still about 20% higher along the coast than 50-100 km to the north. A heavier cover of snow was last recorded in most parts of southern and central Finland in 1984, in the east as recently as 2005. The amount of snow recorded in January was clearly higher than one year previously.

In Pohjois-Pohjanmaa the snow cover was 70-110 kg/m2 and in Kainuu it was 100-130 kg/m2. Apart from the Ahvenanmaa Archipelago, southern Lapland was the area of Finland with the least snow, with snow burdens of only 60-80 kg/m2. Such a low amount of snow is recorded in southern Lapland with a return frequency of 10-20 years. Elsewhere in Lapland accumulation of snow was close to the seasonal mean and snow burdens were 80-160 kg/m2.

The density of snow on roofs in many places exceeded 200 kg/m2, at which level a depth of 50 cm of snow weighs over 100 kg/m2 and a one-metre depth weighs over 200 kg/m2. In the southern and south-western coastal zone as well as along parts of the western coast and in the region around Kuopio, these values exceeded the building norms applied during the period 1955-1969 in some cases. Elsewhere, the mean snow burdens recorded in open areas were clearly below the maximum design loadings for building roofs. However, accumulation of snow on roofs differed from that in open areas, and drift accumulation resulted in up to twofold burdens on some roofs compared with the situation in open country. Building owners were therefore compelled to follow closely the development of the snow cover on roofs, and to organize removal of the snow as necessary.

Water level and discharge

Changes in water levels were moderate during January. In southern Finland a period of above-zero temperatures caused a transient increase in both water levels and discharges, but elsewhere the water levels generally continued to decrease slowly and discharges remained low. In southern and eastern Finland and in southern Lapland, water levels in January were generally below the seasonal mean. The greatest deviations from the seasonal mean water levels were recorded in the larger lakes of eastern Finland. For example the levels of lakes Saimaa and Pihlajavesi at the end of January were half a metre, and of Haukivesi and Pyhäselkä about 60 cm below the seasonal mean. Discharges of the major river watercourses were also below the seasonal mean almost throughout the country.

The devitation of the level of lake Pielinen from the seasonal mean at the end of January was -42 cm and corresponding figures in some other Finnish lakes were: Kallavesi -18 cm, Saimaa -50 cm, Keitele -8 cm, Päijänne -14 cm, Säkylän pyhäjärvi -11 cm, Längelmävesi  -19 cm, Näsijärvi -2 cm, Lappajärvi 0 cm, Lammasjärvi-9 cm, Oulujärvi -44 cm, Lokka -126 cm and Inarin 0 cm.   The discharge of the river Pielisjoki was 62 % of the mean January discharge for the reference periois 1971-2000, and corresponding figures in some other rivers were Vuoksi 72 %, Kymijoki 66 %, Karjaanjoki 71 %, Kokemäenjoki 50 %, Kalajoki 53 %, Oulujoki 82 %, Iijoki 75 %, Kemijoki 83 % ja Tornionjoki 83 %.

Groundwater level

Groundwater levels showed decreases typical for the winter season in almost all regions. Along the southern coast, the decrease in groundwater levels halted temporarily due to a period of mild weather. However, the levels were still 15-30 cm below the seasonal mean. In central and eastern Finland groundwater levels were 10-50 cm below the seasonal mean, in northern Finland only slightly below the mean.

Groundfrost

The exceptional distribution of snow over the country also affected the occurrence of groundfrost. In southern Finland, where the snow cover had been heavy throughout the winter season, the groundfrost depth was only 0-5 cm. In Lapland, however, where the snow cover was only shallow, groundfrost extended to 60-80 cm. In other regions groundfrost depths were 10-40 cm depending on local conditions.

Ice thickness

Ice thicknesses in southern and central Finland at the end of January were generally 35-55 cm, i.e. 5-15 cm greater than the seasonal mean. In northern Finland the recorded ice thicknesses were close to the mean at 40-55 cm. The thickest ice cover, 61 cm, was measured on January 30th over lake Lappajärvi.

In southern Finland a considerable proportion of the overall ice thickness was composed of snow ice, which was formed as a result of freezing of water and slush. Due to the thick cover of snow, the load-carrying properties of the ice were generally speaking unreliable. The heavy snow burden also caused rising of water over the ice on many lakes.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2011-03-31 at 12:30, updated 2013-07-01 at 12:45