Monthly hydrological report February 2011
Lake- and groundwater surfaces low; a large percentage of the ice cover present as snow ice
High precipitation occurred in early February, and snow depths increased throughout the country. Later in the month the weather was colder and precipitation was lower. Snow covers were clearly above the seasonal mean in southern and central Finland, but unusually low in southern Lapland. Water levels continued to decrease during February, and were still exceptionally low in the great lakes of eastern Finland. Groundwater levels were also low throughout the country, as were groundfrost depths in all areas except southern Lapland with its light snow cover. Lake ice thicknesses exceeded the seasonal mean in many regions, but especially in the south a large proportion of the overall ice thickness was composed of weak snow ice.
Almost all the precipitation during February came before the 10th of the month, after which very low precipitation was recorded especially in southern and central Finland. The monthly precipitation was below 30 mm in all regions, i.e. 60-85% of the seasonal mean.
Snow water equivalent
The water equivalent of the snow increased considerably during the first 10 days of the month. The highest precipitation occurred in the basin of the river Vuoksi, where the snow burden increased by over 20 mm (20 kg/m2). Thereafter the snow cover remained almost constant during a period of clear, frosty weather until more snow fell mainly in Lapland during the last days of the month. The water equivalent of the snow cover at the end of February was 90-130 mm almost throughout the country. Along the southern coast the figures were 130-160 mm and in Savo, Pohjois-Karjala and northernmost Lapland even higher. In parts of Pohjois-Savo the recorded values of the water equivalent even exceeded 200 mm. The least snow was still in southern Lapland, where the water equivalent was only 70-90 mm. In southern and central Finland the snow burden was clearly above the seasonal mean, whereas in Lapland the figures were well below normal. The snow burden in the southern and western coastal zone and in Etelä-Pohjanmaa and Pohjois-Savo was greater than the norm in the case of buildings constructed.
Water level and discharge
Lake water levels in February were generally decreasing in typical winter fashion. In unregulated lakes the recorded changes in water level were rather small, but in many regulated lakes the levels decreased considerably due to the requirements of both power generation and flood protection. By the end of the month water levels in southern and central Finland and in southern Lapland were generally below the seasonal mean. Particularly high deviation was observed in the great lakes of the river Vuoksi watercourse. In addition to lakes Saimaa, Haukivesi and Pihlajavesi the level of lake Oulujärvi was also about half a metre below the seasonal mean. Discharges were low, as expected during February, and were in many cases below the seasonal mean.
The devitation of the level of lake Pielinen from the seasonal mean at the end of February was -30 cm and corresponding figures in some other Finnish lakes were: Kallavesi -10 cm, Saimaa -48 cm, Keitele -6 cm, Päijänne -14 cm, Säkylän pyhäjärvi -11 cm, Längelmävesi -21 cm, Näsijärvi +4 cm, Lappajärvi +1 cm, Lammasjärvi -9 cm, Oulujärvi -46 cm, Lokka -122 cm and Inarin -6 cm. The discharge of the river Pielisjoki was 68 % of the mean February discharge for the reference periois 1971-2000, and corresponding figures in some other rivers were Vuoksi 72%, Kymijoki 69 %, Karjaanjoki 69 %, Kokemäenjoki 68 %, Kalajoki 62 %, Oulujoki 82 %, Iijoki 80 %, Kemijoki 82 % ja Tornionjoki 93 %.
The winter decrease in groundwater levels continued during February. In most parts of southern and central Finland the recorded levels were 15-35 cm below the seasonal mean. Even greater deficiencies of 30-60 cm were recorded in southern Häme and in parts of Etelä-Savo and Pohjois-Karjala. In northern Finland the deficits were generally 5-20 cm.
Groundfrost during February extended to 10-30 cm in southern and central Finland. In those parts of the southern coastal zone with a heavy snow cover the figures were 0-10 cm, i.e. 10-20 cm below the mean. In western coastal regions groundfrost depths were 20-25 cm below the seasonal mean. In central and eastern Finland the groundfrost depths were 5-20 cm, i.e. 5-10 cm above the mean. In these areas the level of groundwater, which normally releases heat to the soil, had been lower than normal since the onset of autumn, and so groundfrost could extend further than normally. In Kainuu and Koillismaa a large amount of the snow cover was trapped in tree foliage and less had fallen to the ground, and therefore groundfrost extended in some cases to greater depths. However, even in these areas the groundfrost depths were 5-10 cm below the seasonal mean. In the areas of Lapland with a light snow cover the recorded groundfrost depths were 40-140 cm, about 20-35 cm above the mean.
Despite the heavy frosts in February, ice thicknesses increased only by 2-10 cm because of the insulating effect of the snow cover. Black ice was no longer formed, but the ice thicknesses increased due to the formation of snow ice as a result of the freezing of water and slush. On the basis of measurements made on February 28th, ice thicknesses were 50-70 cm throughout the country. In southern and central Finland these figures were 5-15 cm above the seasonal mean, whereas in northern Finland they were generally close to the mean. Particularly in southern and central Finland the proportion of weak snow ice of the total thickness was considerable, even well over 50%.
Tables and graphs
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