Monthly hydrological report in January 2004
Heavy snow in late January increased the snow cover closer to the normal level
January temperatures were 1-2°C colder than normally in southern and central regions but several degrees above the mean in the north and along the western coast. The monthly precipitation was close to or somewhat above the mean in the north but
generally below the mean in southern and central regions. The water equivalent of the snow cover in January was mainly below the seasonal mean, but heavy snowfall at the end of the month increased the snow cover closer to the mean. Water levels decreased
in typical fashion for the winter season. Ice thicknesses at the end of the month were somewhat below the mean.
Precipitation during January was 20-50 mm as water. A major part of the overall monthly precipitation occurred during a heavy snow storm at the end of the month. In southern and central Finland the monthly precipitation remained somewhat below the
mean, whereas in Lapland, Kainuu and northern parts of eastern Finland average or slightly above-average precipitation was recorded.
The snow cover was below the seasonal mean until the very last days of the month, when a heavy snowfall deposited a thick blanket of snow particularly in the south and east. This precipitation increased the snow cover nearer to the normal level,
although in most areas the water equivalent at the end of the month was still below the seasonal mean. At the end of January the water equivalent of the snow cover was below 40 kg/m2 in western Finland and northern Ostrobothnia and 40-80
kg/m2 in most parts of southern and central Finland and in southern and northern Lapland. In Kainuu, Pohjois-Savo and Pohjois-Karjala and in north-western Lapland the snow cover was generally 0-130 kg/m2, in places even above 130
Water levels decreased during January in typical fashion for the winter season. At the end of the month some large lakes, e.g. Pielinen, Kallavesi, Päijänne, Vanajavesi, Näsijärvi and Lappajärvi, were above the mean whereas lakes Saimaa, Pyhäjärvi,
Säkylä, Oulujärvi, Lokka and Inari were below the mean. The exceptionally high river water levels recorded in the south and south-west at the beginning of the year decreased during January to below their mean levels.
Discharges in the rivers Pielisjoki, Kymijoki and Kemijoki were close to the seasonal mean, whereas in the rivers Kokemäenjoki and Oulujoki the measured discharges were 20-25% below the mean. In the small rivers of the southern and south-western
coastal zone, discharges decreased during January from their high levels in early January to typically low levels at the end of the month.
The deviation of the surface of lake Pielinen from the seasonal mean at the end of January was +15 cm and corresponding figures in some other Finnish lakes were: Kallavesi +17 cm, Saimaa –10, Keitele –9, Päijänne +6, Pyhäjärvi, Säkylä
–18, Längelmävesi –6, Näsijärvi +15, Lestijärvi +7, Lentua +4, Oulujärvi –58, Lokka –113 and Inari –10 cm. The discharge of the river Pielisjoki was 100% of the mean January discharge during the reference period 1971-2000 and
corresponding figures in some other rivers were: Vuoksi 96%, Kymijoki 107, Paimionjoki 82, Kokemäenjoki 80, Kalajoki 103, Oulujoki 75, Iijoki 84, Kemijoki 102 and Tornionjoki 98%.
Groundwater levels began to decrease for the duration of the winter. Groundwater reserves showed considerable regional variation. In some areas the levels increased during December and January close to the seasonal mean, but in large sand and gravel
formations and in other fine-grained soil types the groundwater levels were well below the mean.
Groundfrost depths in January were 20-40 cm in coastal areas with little snow and 50-80 cm in Lapland, i.e. 10-20 cm thicker than normal. In central and eastern regions with a heavier snow cover the groundfrost depths were generally 10-30 cm.
The last open reaches of the larger lakes, Tehinselkä in lake Päijänne and the outer reaches of lake Näsijärvi, froze over during the first days of January. Ice thicknesses at the end of the month were 25-45 cm in southern and central Finland and 40-60
cm in the north. These ice thicknesses were generally 5-10 cm weaker than normally.
Graphs and tables