Monthly hydrological report November 2012

High water levels in the Great Lakes of eastern and central Finland; winter started in the south in late November

Water-equivalent-of-snow-november2012

November was rather mild throughout the country. Precipitation was above-average in Lapland but low in the west. Lake water levels were high in eastern and central Finland, particularly in the larger lakes. Groundwater levels were still high in all regions. Winter came to southern Finland at the very end of the month; heavy falls of snow occurred in the southern coastal zone and freezing of watercourses began even in the south. Groundfrost was recorded at the end of November only in northern Finland and lake ice thickness could be measured only in Lapland.

Precipitation

Precipitation in November was generally 30−90 mm, the highest figures being recorded in the southern coastal zone and in southern and western Lapland and the lowest in Pohjanmaa. Compared with the long-term November mean, precipitation was low in the west but rather high (120−170%) in the north.

Snow cover

At the beginning of the month most of the country had a light snow cover, but mild weather soon pushed the snow boundary as far north as Koillismaa and Kainuu. A new snow cover developed in central Finland in the middle of the month, before a renewed mild period caused melting up to southern Lapland. Only in northern Lapland did the water equivalent of the snow cover increase almost continuously throughout the month. Frosty weather extended throughout the country during the last days of the month, with heavy snowfalls in the south. By the very end of November snow covered almost the whole country with the exception of Pohjanmaa and parts of central Finland. In the south the highest water equivalents (20−40 cm) were recorded in the coastal zone between Hanko and Helsinki. In Lapland the corresponding figures were below 10 mm in the south and almost 100 mm in the north. 

Water level and discharge

Water levels were generally increasing in early November because of the mild and wet weather. Water levels then decreased during a cold spell and increased again during warmer weather in mid-November. At the end of the month wintry weather throughout the country caused the levels to decrease in all but some of the largest lakes. In northern Lapland wintry weather continued throughout November and water levels decreased steadily. Lake water levels at the end of the month were still very high in most of the Lake District, e.g. the level of lake Saimaa was 80 cm above the seasonal mean and almost 30 cm above the so-called normal range. The discharges of the major watercourses were clearly above the seasonal mean. The discharge of the river Kymijoki corresponded to flood flow (> 500 m3/s) and the regulatory discharge from lake Saimaa was 1000 m3/s. The discharges of the smaller coastal rivers were also high. Most discharges turned to a decrease at the end of the month due to the onset of wintry weather.

The deviation of the level of lake Pielinen from the mean level at the end of November was +64 cm, and corresponding figures in some other lakes were: Kallavesi +41 cm, Saimaa +84, Keitele +46, Päijänne +30, Pyhäjärvi, Säkylä +5, Längelmävesi +28, Näsijärvi +1, Lappajärvi +40, Lammasjärvi +29, Oulujärvi +15, Lokka -28 and Inari +33 cm. The mean monthly discharge of the river Pielisjoki was 161% of the mean November discharge during the reference period 1970-2000, and corresponding figures in some other rivers were: Vuoksi 171%, Kymijoki 183, Karjaanjoki 166, Kokemäenjoki 197, Siikajoki 202, Oulujoki 145, Iijoki 204, Kemijoki 121 and Tornionjoki 149%.

Groundwater level

Groundwater levels were in most regions 20−40 cm above the seasonal mean, in central Finland even 40−60 cm above the mean. The recorded levels at some groundwater measurement stations even reached those normally observed during the spring snowmelt.

Groundfrost

Groundfrost in November extended to 0−5 cm in northern Finland.

Surface water temperature and ice cover

Water temperatures were above the seasonal mean in southern and central Finland. Colder weather during the last week of the month caused cooling of the water layers and freezing over of lakes began at the change of the month even in southern Finland. Ice thicknesses at the end of November were up to 30 cm in Lapland; in Kainuu and in southern and central Finland the ice was still very thin. The larger lakes of southern Finland were still unfrozen at the end of the month.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2013-01-04 at 12:00, updated 2013-06-27 at 12:37