Monthly hydrological report March 2010
The snow burden increased almost throughout the country; discharges began to increase at the end of the month in small coastal watercourses
March was colder than normally and precipitation was generally high. The water equivalent of the snow cover increased in most areas to the end of the month and was in many cases above the seasonal mean. In southern and western regions the snow began to melt during the last week of March, with the result that both water levels and discharges began to increase in small coastal watercourses. Lake water levels remained unchanged or were still decreasing in some regulated watercourses. Groundwater levels were in most regions below the seasonal mean. In central and eastern Finland groundfrost was rather heavy. Ice thicknesses increased during March mainly as a result of the formation of snow ice.
Precipitation during March was above the seasonal mean in many areas. The recorded monthly precipitation figures of 20...70 mm were 80...230% of the seasonal mean. The lowest precipitation was in northern Lapland and the highest was in southern Lapland and in an area extending from Pohjois-Pohjanmaa to Pohjois-Karjala.
Water equivalent of snow cover
The water equivalent of the snow cover increased almost throughout the month. The most snow fell in eastern Finland and in Lapland. In southern and western Finland the snow began to melt during the last week of the month. The water equivalent or snow burden at the end of March was 80...130 mm (kg/m2) in central and western Finland. Along the southern coastline and in eastern Finland, around Oulu and in northern Lapland the recorded values were generally 130...180 mm. Elsewhere in Lapland and in the Kuusamo region the corresponding figures varied around 200 mm. In most parts of the country the water equivalent at the end of the month exceeded the long-term seasonal mean, although in eastern Finland and some parts of northern Lapland below-average figures were recorded.
Water level and discharge
The surfaces of unregulated lakes hardly altered during March. In many regulated lakes the levels decreased markedly due to release of water to prevent spring flooding. In southern and south-western Finland water levels and discharges began to decrease during the last days of the month as a result of warming weather and melting of snow. By the end of March water levels were close to their long-term seasonal means in most parts of the country, except in many of the larger lakes of central and eastern Finland where below-average water levels were recorded. Discharges of the major watercourses were generally below the mean for March. Coastal rivers also had typically low winter discharges until the beginning of snowmelt.
The deviation of the surface of lake Pielinen from the seasonal mean at the end of March was +7 cm and corresponding figures in some other lakes were: Kallavesi +6 cm, Saimaa -9, Keitele -27, Päijänne -13, Pyhäjärvi, Säkylä -16, Längelmävesi -14, Näsijärvi 0, Lappajärvi -1, Lammasjärvi -6, Oulujärvi -8, Lokka -46 and Inari -2 cm. The monthly discharge of the river Pielisjoki was 90% of the mean for the reference period 1971-2000, and corresponding figures in some other Finnish rivers were: Vuoksi 87%, Kymijoki 71, Karjaanjoki 70, Kokemäenjoki 59, Kalajoki 76, Oulujoki 67, Iijoki 94, Kemijoki 89 and Tornionjoki 89%.
Groundwater levels continued to decrease during March. At around the middle of the month the recorded levels were 0...70 cm below the seasonal mean in southern and central Finland and in Pohjanmaa. In coastal regions, the levels began to increase at the end of the month. In northern Finland groundwater levels were close to the seasonal mean.
Warming of the weather at the end of March halted the development of groundfrost in coastal regions. The groundfrost began gradually to melt from below and the thickness was generally 5...25 cm. In central and eastern Finland the groundfrost depth was 40...70 cm, i.e. 20...30 cm thicker than normally. In northern Finland, groundfrost depths were close to the seasonal mean throughout the winter. In the counties of Kainuu and Koillismaa groundfrost depths around 20 cm were recorded, and in central and northern Lapland 50...100 cm.
Lake ice thicknesses increased to the end of March in most regions, mainly due to the formation of snow ice. In southern Finland the ice cover began to weaken at the end of the month. By the end of March ice thicknesses were 50...70 cm in southern and central Finland and 55...90 cm in the north. These figures were generally above the seasonal mean in southern and central regions but below the mean in Lapland. The proportion of snow ice in total ice thickness was in many areas as much as about one half.
Tables and graphs
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