Monthly hydrological report October 2012

Serious flooding in Pohjanmaa and Satakunta

Precipitation-map-october2012

Very heavy precipitation was recorded at the beginning of October in western Finland. Severe flooding occurred in rivers of the northern parts of Pohjanmaa and Satakunta as a result of record-breaking high water levels and discharges. After the wet autumn season lake water levels were also high, particularly in eastern and central Finland. Groundwater levels were high throughout the country. Evaporation was low. At the beginning of the month water temperatures were above the seasonal mean, but later small watercourses froze over in the north.

Precipitation

Precipitation during October was exceptionally high along the western and south-western coastline. Accumulating precipitation of over 100 mm in two days set new records in some places. The monthly precipitation exceeded 140 mm in wide areas, representing 2.5-fold the seasonal mean. Precipitation was above the mean almost throughout the country. The lowest precipitation, 50−80 mm, was recorded in eastern Finland and in Lapland. Elsewhere the figures were generally 80−140 mm; in the north some of the precipitation came as snow.

Snow

The first snow of the winter fell in central and southern Lapland and in parts of Pohjois-Pohjanmaa and Kainuu after the middle of the month. During the last week of October almost the whole country was covered with a coat of snow, which was rather thick in western coastal areas for a few days. On the last day of the month the water equivalent of the snow cover in Lapland varied between <10 mm in the south-east to >30 mm in the north. Further south the snow cover was insignificant or non-existent.

Water level and discharge

An exceptionally severe autumn flood occurred at the beginning of the month in Pohjanmaa and northern parts of Satakunta, when many river water levels and discharges increased to new seasonal high records as a result of the heavy rainfall. The increase was very rapid because the soil was already soaked and did not absorb rainwater. The flood damage was considerable; over 100 buildings and 10 000 field hectares were inundated and some roads were blocked for several days. Damages were estimated to amount to at least 6 million euros. The precipitation also caused lake level increases, particularly in western districts. In the north, the onset of wintry weather at the end of the month caused lake level decreases. In the south the increase slowed in small lakes, although in most of the great lakes the water level increase continued until the very end of the month. In southern and central Finland lake water levels were considerably above the seasonal mean. In Lapland the corresponding figures were close to their seasonal means, with the exception of lake Inari which was rather high. Mean watercourse discharges in October were above the seasonal mean throughout the country, and the discharge of the main course of the river Kymijoki was close to the flood level at the end of the month.

The deviation of the level of lake Pielinen from the seasonal mean at the end of October was +70 cm, and corresponding figures in some other lakes were: Kallavesi +49 cm, Saimaa +71, Keitele +42, Päijänne +36, Pyhäjärvi, Säkylä +16, Längelmävesi +27, Näsijärvi +11, Lappajärvi +51, Lammasjärvi +31, Oulujärvi +18, Lokka -32 and Inari +26 cm. The monthly discharge of the river Pielisjoki was 165% of the mean for October during the reference period 1971-2000, and corresponding figures in some other river watercourses were: Vuoksi 167%, Kymijoki 164, Karjaanjoki 254, Kokemäenjoki 215, Siikajoki 195, Oulujoki 160, Iijoki 193, Kemijoki 142 and Tornionjoki 195%.

Groundwater level

Groundwater levels were 10−50 cm above the seasonal mean throughout the country. As a result of the heavy precipitation, water reserves were also fully replenished in the largest groundwater formations.

Evaporation

Due to the wet weather, evaporation was low during October. Maximum evaporation from the larger lakes in southern and central Finland on the basis of watercourse model calculations was 40 mm. Evaporation from soil layers was very low.

Surface water temperature and ice cover

Due to warm weather in early October the water layers cooled rather slowly for the time of year and temperatures were above the mean. Later in the month, cooler weather caused decreases in water temperatures. Small lakes in northern Lapland froze over as a result of frosty weather after the middle of the month, and some small river watercourses in Pohjois-Pohjanmaa and Kainuu also froze. By the end of the month surface temperatures in the larger lakes of southern and central Finland were 3−6°C and in smaller lakes <4°C.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-12-12 at 12:00, updated 2013-06-27 at 10:42