Monthly hydrological report March 2012

Melting of snow increased water levels in southern and western regions; winter continued in Lapland

March was mild and precipitation was close to the seasonal mean. The snow burden varied; by the end of the month the snow had mainly melted in south-western Finland, whereas along the coast of the Bay of Bothnia deep snow was recorded. Watercourse water levels decreased in most areas, although in the coastal zone they increased as a result of snowmelt. Groundwater levels were above the seasonal mean and began to increase in the south. Ice and groundfrost depths were low. 

Precipitation

In most regions precipitation was close to the seasonal mean; in Pohjanmaa low precipitation was recorded in many areas. In Keski-Pohjanmaa the monthly precipitation was even below 20 mm, whereas in southern and central Finland 50 mm was observed in some places. Elsewhere the precipitation was generally 20–40 mm.

Snow cover

No great changes were recorded in the snow burden early in March. Some melting of snow occurred before the middle of the month in western and southern Finland and by the end of March melting was also under way in central regions. In eastern and northern Finland the snow burden continued to increase until the end of March. By the end of the month the heaviest snow cover was recorded in south-western Lapland, where water equivalent figures even exceeding 200 mm were recorded in many places. Elsewhere in northern Finland the water equivalent figures were generally 140–190 mm; in the county of Inari there was considerably less snow. In areas around Oulu, Kuopio and Savonlinna the water equivalent was generally 90–120 mm, decreasing to zero towards the south-western coastline. Along the coast of the Bay of Bothnia and in western Lapland the snow depths were high, whereas along the south-western coastline and near Inari, Etelä-Savo and Etelä-Karjala rather weak snow covers were recorded.

Water level and discharge

Melting of snow began to show in western watercourses towards the end of the month. Level increases in lakes were small, but in coastal rivers both levels and discharges increased rapidly. Along the south-western coast e.g. the levels of the rivers Uskelanjoki and Aurajoki began to decrease towards the end of the month after the peak flood, as most of the snow had already melted; in other rivers the increase in flood levels peaked during the last days of the month as the weather became colder. Water levels in eastern and northern Finland were still decreasing during March. In unregulated lakes the level changes were slow, but in many regulated lakes there was a rapid decrease as room was made for the developing spring floods. In eastern and northern Finland both water levels and discharges were generally close to their seasonal mean values at the end of March. In the west, particularly in the coastal zone, the levels and discharges were high because of the early onset of the spring flood. The mean monthly discharges of many of the major watercourses were above the mean for March, particularly in the river Kymijoki.

The deviation of the level of lake Pielinen from the seasonal mean at the end of March was +7 cm, and corresponding figures in some other lakes were: Kallavesi +17 cm, Saimaa 0, Keitele +21, Päijänne +34, Pyhäjärvi, Säkylä +13, Längelmävesi +20, Näsijärvi +38, Lappajärvi +19, Lammasjärvi +3, Oulujärvi +43, Lokka -33 and Inari +17 cm. The discharge of the river Pielisjoki was 93% of the mean March discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 97%, Kymijoki 135, Karjaanjoki 104, Kokemäenjoki 114, Kalajoki 90, Oulujoki 110, Iijoki 123, Kemijoki 114 and Tornionjoki 138%.

Groundwater level 

Groundwater levels at the end of March were 5–25 cm above the seasonal mean almost throughout the country. In the south, the levels were increasing as a result of the snowmelt.

Groundfrost

Groundfrost extended to 0–10 cm in southern and eastern Finland at the end of the month, to 10–20 cm in central Finland and to 25–80 cm in the north. Winter groundfrost depths were generally below the mean throughout the country, and in parts of southern Finland no groundfrost was recorded at all due to the thick snow cover and moist soil.

Ice cover

Lake ice thicknesses increased during the first half of March throughout the country, but began to weaken in southern and western districts towards the end of the month. In eastern and northern regions the ice thicknesses were unchanged or even increased slightly towards the end of the month. By the end of March, ice thicknesses in southern and south-western Finland were generally below 40 cm; in central and eastern districts and in Kainuu and Pohjois-Pohjanmaa they were 40–55 cm. In Lapland, ice depths of 50–60 cm were recorded and in lake Kilpisjärvi the depths were up to 85 cm. The recorded ice thicknesses were generally well below the seasonal mean in most parts of the country.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-04-25 at 10:35, updated 2013-07-01 at 12:32