Annual hydrological report 2012

A hydrological year characterised by floods

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The hydrological year 2012 will be remembered for its exceptionally heavy precipitation and flooding in many parts of the country, particularly in eastern Finland and in Kainuu. Annual precipitation was generally 10−35% above the mean. Very heavy snow covers were recorded at the end of winter particularly in south-western and western Lapland. Groundfrost and ice covers were mainly weaker than normally throughout the country. Many floods were registered during the year: spring floods in Lapland in May, summer flooding in Pohjois-Pohjanmaa and Kainuu in August and autumn floods in Pohjanmaa and Satakunta in October. During the summer, water temperatures were rather cool and evaporation was low. Groundwater levels were high almost throughout the country due to the heavy precipitation. The onset of winter occurred in November-December in the south, but earlier in Lapland. Slush ice dams were formed in November and December in many parts of the country in unfrozen rivers.

Precipitation

Overall precipitation in 2012 was above the mean throughout the country. Annual precipitation was 600−950 mm in southern and central Finland as far north as southern Lapland, and 450−750 mm in central and northern Lapland. In southern and central Finland the recorded precipitation was 25−35% above the mean and in Lapland 10−20%. Particularly high precipitation occurred in eastern Finland, and in the basin of the river Oulujoki the annual precipitation was a new high record. In the Vuoksi and Kokemäenjoki river basins the figures were the second highest ever recorded during the 100 year observation series. High precipitation was also recorded elsewhere in southern and western Finland; the wettest period was from June to October.

Water equivalent of snow cover

At the beginning of the year there was no snow cover in southern Finland, but heavy snowfall occurred in January throughout the country. Accumulation of snow was high in south-western and western Lapland during the winter, with a water equivalent even reaching 250 mm. By contrast, in the area around lake Inari and in parts of eastern Finland there was rather little snow, with water equivalent maxima generally below 130 mm. The first snow of the new winter came in October in most parts of the country, but persisted only in Lapland. In November the weather was noticeably mild, but during the last days of the month the winter set in even in the south. The snow burden grew considerably during December in most parts of the country. In southern and south-western Finland and parts of northern Lapland snow depths were clearly higher than normally, whereas in southern Lapland, Koillismaa, Kainuu and Pohjois-Karjala they were below the mean.

Water level and discharge

The hydrological year was characterised by many floods. Snowmelt caused serious flooding in the spring in Lapland, and heavy rainfall caused river floods in many regions during the summer and autumn. The most serious flooding was in Etelä-Pohjanmaa and Satakunta in early October. Many areas were isolated and damages amounted to millions of euros. High lake water levels also caused problems during the course of the year. The great lakes of eastern Finland were already high at the start of the year, and the exceptionally heavy precipitation only worsened the situation. The levels of some smaller lakes were also unusually high in many regions, and new records were set towards the end of the summer e.g. in Kainuu. The regulated discharge from lake Saimaa was kept high throughout the autumn in order to prevent or at least limit damage due to high water levels. Frosty weather towards the end of the year eventually caused water levels to decrease. Even then, slush ice dams caused problems in several rivers during December, due to below-zero temperatures during a period of exceptionally high discharge and therefore no ice cover. In some cases the slush ice dams had to be detonated on several days. The last slush dam problems of the year were in the river Kymijoki at the end of December. By the end of the year water levels in most parts of the country were close to their seasonal mean levels, although in the larger lakes of eastern and central Finland levels were still several tens of centimetres above the seasonal mean. For example, the level of lake Saimaa was 70 cm above the seasonal mean.

Except in Lapland, the annual discharges of the major watercourses were considerably above the mean. The discharges of the rivers Vuoksi and Kokemäenjoki were ca. 30% and those of the rivers Kymijoki and Oulujoki almost 50% above the mean.

Groundwater level

Groundwater levels were somewhat above the seasonal mean in early spring. In the south, the level increase began in late March but further north replenishment of the groundwater reserves only began in May. In June, groundwater levels decreased throughout the country, but were still generally somewhat above the seasonal mean. Due to heavy rainfall in July and August, the levels were generally 10−30 cm above the seasonal mean at the end of the summer season, and in Pohjanmaa and Kainuu as much as 30−80 cm. Continuing heavy rainfall during autumn maintained the groundwater levels in most parts of the country at 20−60 cm above the seasonal mean. With the onset of winter in December the water levels began to decrease slowly, but were still 10−30 cm above the mean.

Groundfrost

A heavy snow cover and high soil moisture slowed down the rate of formation of groundfrost early in 2012. In southern and eastern Finland groundfrost depths were generally 0−10 cm, although in some areas no freezing of soil layers occurred at all. In central Finland the groundfrost depths were generally 10−20 cm and in the north 20−80 cm. By the end of April the groundfrost had melted almost completely in southern and central Finland; in the north melting occurred in May and June. In autumn, high soil moisture delayed the formation of groundfrost. In November-December groundfrost was generally recorded only in the north, where the shallow snow cover and frosty weather favoured the freezing of soil layers. At the close of the year groundfrost depths of 10−20 cm were recorded in northern Finland.

Ice cover

Lake ice covers at the beginning of the year were clearly thinner than normally throughout the country, and the ice was generally structurally weak. The proportion of snow ice of the total ice thickness was high. The maximum ice thicknesses at the end of winter were 40−55 cm in southern and central Finland and 55−65 cm in the north, although the ice cover over lake Kilpisjärvi had a depth of almost 90 cm. Break-up of ice occurred in southern and central regions at approximately the normal time and in Lapland slightly ahead of schedule. In autumn, freezing over of watercourses generally occurred at the normal time. The smaller lakes of Lapland developed an ice cover at the end of October; the majority of lakes in southern and central Finland froze over in early December and the outer reaches of the larger southern lakes by Christmas. Ice thicknesses at the end of the year were generally close to the seasonal mean.

Surface water temperature

Lake surface water temperatures during the summer were generally close to or below the seasonal mean. In Lapland, the waters were exceptionally cool. A hot period early in July increased temperatures above 20°C for the first time in southern and central Finland. The highest temperatures of the summer were recorded in most parts of the country at the end of July and the beginning of August. The recorded temperatures were 19−23°C in southern and central Finland, 17−20°C in Kainuu and southern Lapland and only 10−14°C in northern Lapland. These figures were generally below or at best close to the mean annual maximum temperatures. In autumn, the waters cooled generally at a normal or somewhat slow rate.

Evaporation

Evaporation during the summer season was considerably below the mean, due to the cool and wet weather. The greatest deviation from the mean was recorded in Pohjanmaa, where monthly evaporation for June measured using Class A pans was only 50% of the normal level. The evaporation sum for May-September measured with Class A pans was 200−300 mm in northern Finland and 400−500 mm in southern and central regions. Lake evaporation during the unfrozen season was 500−600 mm in southern and central regions and 250−350 mm in the north.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899
Published 2013-04-11 at 8:00, updated 2013-06-27 at 9:48