Monthly hydrological report March 2013

Lake ice covers increased in frosty weather but groundfrost depths unusually low

Ice-thickness-march2013

March was cold and precipitation was low. Changes in snow water equivalent were consequently rather small. Both watercourse and groundwater levels decreased in typical winter fashion, although water reserves were still high after the previous wet autumn. Heavy frosts increased watercourse ice thicknesses towards normal levels throughout the country, although groundfrost was still weaker than normally in most areas.

Precipitation

Precipitation during March was generally only 5−30 mm, clearly below the seasonal mean. In south-eastern Finland the accumulation was only a few millimetres; the highest precipitation was in Kainuu.

Snow cover

The water equivalent of the snow cover increased only slightly during the first half of the month, except in Etelä-Pohjanmaa and Kainuu, where increases of >40 mm were recorded. Later in the month there was almost no precipitation, and thawing did not begin except in some south-facing slopes in southern Finland. By the end of the month the water equivalent was below 100 mm in south-western Finland, along the Ostrobothnian coast and in Etelä-Savo and 100−130 mm elsewhere in southern and central Finland. Northwards from Oulu the corresponding figures were generally 140−180 mm, although lower values were recorded in south-eastern Lapland and higher values in the area around Utsjoki and in north-western Lapland. The snow burden was clearly above the seasonal mean in parts of southern and western Finland and northern Lapland, whereas unusually low figures were recorded in Kainuu, Pohjois-Karjala and southern Lapland.

Water level and discharge

Lake water levels continued to decrease during March in normal winter fashion. The level of lake Saimaa decreased by over 10 cm, but by the end of the month was still half a metre above the seasonal mean. Many other of the larger lakes, e.g. Päijänne, Oulujärvi and Inari, were also still above their seasonal mean levels as a result of the wet autumn season of 2012. An exception to this was e.g. lake Lohjanjärvi, the level of which was below the seasonal mean at the end of March. Mean monthly discharges were above the seasonal mean in the rivers Vuoksi, Kymijoki, Kemijoki and Tornionjoki, close to the mean in the river Oulujoki and below the mean in Kokemäenjoki. The discharges of the small coastal rivers were at a typically low winter level.

The deviation of the level of lake Pielinen from the seasonal mean at the end of March was -11 cm, and corresponding figures in some other lakes were: Kallavesi +1 cm, Saimaa +52, Keitele +16, Päijänne +15, Pyhäjärvi, Säkylä -6, Längelmävesi +4, Näsijärvi -11, Lappajärvi +10, Lammasjärvi -5, Oulujärvi +30, Lokka -22 and Inari +17 cm. The mean monthly discharge of the river Pielisjoki was 119% of the mean March discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 132%, Kymijoki 116, Karjaanjoki 61, Kokemäenjoki 76, Siikajoki 75, Oulujoki 103, Iijoki 106, Kemijoki 124 and Tornionjoki 118%.

Groundwater level

Groundwater levels decreased in typical winter fashion, although in most areas the recorded levels were still 0−20 cm above the seasonal mean. In some western regions the groundwater levels were below the mean.

Groundfrost

In southern and central Finland groundfrost layers became thinner, whereas in the north they remained stable or even increased. In southern and central regions the recorded depths of 0−10 cm were 10−35 cm below the seasonal mean, and in western Finland the depths of 15−30 cm were only about half the mean. In the Åland Islands groundfrost depths were 5−15 cm, slightly above the mean. In the north the measured groundfrost depths of 30−70 cm were close to the mean in some areas but e.g. in most of Lapland 10−20 cm below the mean.

Ice thickness

Heavy frost during March resulted in increasing lake ice covers throughout the country. Ice thicknesses increased at most observation sites by 5−15 cm. By the end of the month thicknesses of 45−65 cm were generally recorded in southern and central Finland and 55−70 cm in the north. The cover over lake Kilpisjärvi exceeded 90 cm. In most areas the overall ice depths were approaching their seasonal mean levels, but the proportion of snow ice was high. In northern Lapland the ice covers were thinner than normally, whereas in some southern and south-western regions they were thicker.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2013-06-14 at 12:00, updated 2013-06-27 at 13:28