Monthly hydrological report November 2011

Onset of winter unusually late in Lapland; discharges high on the southern and south-western coastline

November was mild throughout the country. Monthly precipitation was below the seasonal mean in most regions. The onset of winter was delayed at all observation sites. In Lapland both the first snow and the freezing over of lakes were delayed in many cases; in the case of freezing of watercourses some new records were set. Water levels were generally decreasing in November, but heavy precipitation at the end of the month increased river levels and discharges considerably along the southern and south-western coastline. Groundwater levels were generally above the mean. A thin layer of groundfrost formed in northern and eastern Finland. Ice thicknesses were unusually low for the time of year in northern areas at the end of the month; in other regions the ice cover had not yet formed.

Precipitation

Precipitation in November was generally below the seasonal mean, 15–60 mm. The highest precipitation was in central and eastern Lapland, where above-average figures were recorded in some areas. In southern and central Finland, precipitation was clearly below the mean. The lowest precipitation was in northern parts of Keski-Suomi and Pohjois-Savo and in southern and central parts of Pohjanmaa.

Snow

At the beginning of the month a shallow covering of snow was present only in the tundra regions of northern Lapland; elsewhere in Lapland the first snow fell only very late after the middle of the month. During the second half of the month the edge of the snow-covered area was in central Finland, but by the very end of November snow was recorded only in Lapland and Koillismaa. The water equivalent of the snow cover varied from 2 mm near the coast of the Bay of Bothnia to about 40 mm near Utsjoki. In central Finland some patches of snow were recorded but the south was still completely free of snow.

Water level and discharge

Water levels decreased in southern and central Finland and in Pohjois-Pohjanmaa towards the end of the month as a result of the low precipitation. However, heavy rain at the end of November caused an increase in water levels, particularly in rivers of the southern and south-western coastal zone where discharges also increased rapidly. In northern Finland precipitation came more evenly during the month, and water levels fluctuated accordingly. Lake water levels were generally decreasing in Lapland and in Kainuu. By the end of the month water levels were high along the western coastline and in Lapland; in other areas they were close to the seasonal mean. Of the larger lakes, the levels of lakes Inarijärvi, Oulujärvi, Lappajärvi and Puulavesi were several tens of centimeters above their seasonal mean levels, whereas lakes Saimaa and Pielinen were still rather low. Discharges of the major river watercourses varied on both sides of the seasonal mean. The discharges of the rivers Kymijoki, Oulujoki and Kemijoki were above the mean, whereas those of the rivers Vuoksi and Koemäenjoki were rather low.

The deviation of the level of lake Pielinen from the long-term seasonal mean at the end of November was -25 cm, and corresponding figures in some other lakes were: Kallavesi +5 cm, Saimaa -33, Keitele +32, Päijänne +18, Pyhäjärvi, Säkylä +0, Längelmävesi -15, Näsijärvi -15, Lappajärvi +29, Lammasjärvi +1, Oulujärvi +19, Lokka -61 and Inari +38 cm. The mean monthly discharge of the river Pielisjoki was 72% of the mean for the reference period 1971–2000, and corresponding figures in some other rivers were: Vuoksi 82% Kymijoki 127, Karjaanjoki 52, Kokemäenjoki 81, Kalajoki 68, Oulujoki 114, Iijoki 118, Kemijoki 133 and Tornionjoki 213%.

Groundwater level and groundfrost

Groundwater levels were 5–30 cm above the seasonal mean in most regions, except in the coastal zone where they were 0–30 cm below the mean. The levels were increasing in all areas except northern Finland. The soil layers were very wet in many regions, and groundfrost generally did not develop. Groundfrost depths of 0–5 cm and 0–10 cm were recorded in eastern and northern Finland, respectively.

Water temperature and ice cover

Lake water temperatures were above the seasonal mean throughout the country. In Lapland an ice cover developed over the smaller lakes during the second half of the month, in many cases unusually or even unprecedentedly late. By the end of the month a generally weak ice cover persisted only in the smaller lakes of northern Finland. Lake Kilpisjärvi was still partly unfrozen at the end of November, which represented a new record for late freezing. Unprecedentedly late freezing was also recorded in some other lakes. Surface water temperatures in southern and central Finland at the end of November were below 5°C.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-01-17 at 15:30, updated 2013-07-01 at 12:28