Monthly hydrological report February 2012

Heavy snow cover in south-western Lapland; groundfrost depths below the mean

February was colder than normally throughout the country. Precipitation varied on both sides of the seasonal mean. Heavy snowfalls during the second half of the month increased the snow burden, which was particularly high in south-western Lapland at the end of February. Both lake- and groundwater levels decreased in typical winter fashion. Due to the heavy snow cover and moist soil layers, groundfrost depths were low. Lake ice was weak throughout the country. 

Precipitation

Precipitation during February varied on both sides of the mean; monthly precipitation figures were 20-60 mm. The highest precipitation was in Länsi-Uusimaa: 60 mm i.e. over 150% of the February mean. Low precipitation, below 20 mm, was recorded in northern Lapland and in eastern Finland.

Snow cover

Increase in the water equivalent of the snow cover was slow during a cold period early in the month, but heavy snowfalls after mid-February increased the water equivalent considerably. By the end of the month the heaviest snow covers were still measured in a zone extending from western and south-western Lapland to Koillismaa, Kainuu and northern and eastern parts of Pohjois-Karjala. In this area the snow burden was generally 130–160 mm and in many places even higher. Elsewhere in northern Finland the snow cover was mainly 90–130 mm. In southern and central regions the areas with the most snow were, in addition to Pohjois-Karjala, the southern coastal zone and Keski-Suomi; in these areas the snow burden was 110–130 mm. In some parts of the south-western and western coastal zone with low snowfall, the water equivalents were below 60 mm. Along the coast of the Bay of Bothnia, the southern coast and in parts of western Lapland the snow cover was clearly heavier than normally, whereas in Etelä-Karjala, Etelä-Savo and parts of eastern Lapland the covering was rather light.

Water level and discharge

Lake water levels decreased during February in typical winter fashion almost throughout the country. Both river water levels and discharges generally remained low, although at the beginning of the month hanging ice dams caused level increases e.g. in lower reaches of the river Kymijoki watercourse. By the end of the month water levels were generally close to the seasonal mean, although the surfaces of some of the larger lakes in central Finland were still high as a result of heavy precipitation at the end of the previous year. For example the levels of lakes Päijänne, Puulavesi, Oulujärvi and Inari were clearly above the seasonal mean. The mean monthly discharges of the major watercourses were normal or above-normal; in the smaller coastal rivers typically low winter discharges were recorded.

The deviation of the level of lake Pielinen from the seasonal mean at the end of February was +3 cm, and corresponding figures in some other Finnish lakes were: Kallavesi +15 cm, Saimaa -3, Keitele +28, Päijänne +45, Pyhäjärvi, Säkylä +7, Längelmävesi +17, Näsijärvi +21, Lappajärvi +37, Lammasjärvi +5, Oulujärvi +53, Lokka -25 and Inari +30 cm. The monthly discharge of the river Pielisjoki was 100% of the mean monthly discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 101%, Kymijoki 125, Karjaanjoki 96, Kokemäenjoki 116, Kalajoki 111, Oulujoki 103, Iijoki 114, Kemijoki 118 and Tornionjoki 157%.

Groundwater level 

Groundwater levels decreased throughout the country in typical winter fashion and were 0–25 cm above the seasonal mean.

Groundfrost

Heavy snow covers and moist soil layers slowed the rate of development of groundfrost despite a period of heavy frost at the end of January and the beginning of February. Groundfrost depths in southern Finland were 0–10 cm, i.e. 5–20 cm below the seasonal mean. Along the western coast the groundfrost depths of 10–30 cm were 10–30 cm below the mean, and in Lapland the depths of 30–80 cm were 10–40 cm less than normally. In other regions groundfrost extended generally to 10–20 cm, which was 5–20 cm below the seasonal mean. Winter maximum depths were attained earlier than normally during February, except in Lapland.

Ice cover

Ice thicknesses increased slightly during heavy frost at the beginning of February, although the insulating snow cover checked thickening of the ice layer considerably. Towards the end of the month there was very little increase in ice thickness; in some cases even decreasing ice depths were recorded. By the end of the month ice thicknesses were generally 20–45 cm in southern and central Finland and 40–60 cm in the north, except for lake Kilpisjärvi where depths of up to 75 cm were recorded. Lake ice depths were 10–20 cm below the seasonal mean almost throughout the country, and in many cases over half of the total ice thickness was composed of weak snow ice.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-04-20 at 11:08, updated 2013-07-01 at 12:31