Monthly hydrological report August 2012

Floods in Pohjois-Pohjanmaa; high water levels also in eastern Finland and Kainuu

The mean August temperature was close to the seasonal mean in most parts of the country. Precipitation was very high in a belt extending from Pohjanmaa to Kainuu. Exceptionally high rainfall caused flooding in the rivers of Pohjois-Pohjanmaa, and very high lake water levels were also recorded in Kainuu. Groundwater levels were above the seasonal mean, and in the wettest areas even reached levels typical of the spring melt season. Evaporation was below the seasonal mean in most regions. Water temperatures decreased in typical fashion for the time of year and the recorded temperatures were generally close to the seasonal mean.

Precipitation

Precipitation in August was very high in Keski- and Pohjois-Pohjanmaa and in Kainuu. In the basins of the rivers Kalajoki, Siikajoki and Oulujoki the recorded precipitation of well over 100 mm during the first 10 days of the month set new high records. The monthly precipitation in the wettest areas was 100–150 mm. Elsewhere, the August precipitation of 20–100 mm was in many areas below the seasonal mean. The lowest rainfall was in Lapland, in places even below 20 mm or only half the seasonal mean.

Water level and discharge

In southern Finland and in Lapland the water levels generally decreased during August, although towards the end of the month the levels increased considerably in rivers of the southern coastal zone as a result of heavy rainfall. In Pohjanmaa, Kainuu and the north-western parts of the river Vuoksi and Kymijoki watercourses heavy rain during the first half of the month increased the water surfaces to high levels. In Keski-Pohjanmaa and Pohjois-Pohjanmaa the August floods were exceptionally heavy, causing inundation of crops in hundreds of hectares of arable land and blocking some roads. The discharges of the rivers Kalajoki, Pyhäjoki and Siikajoki were unprecedentedly high for August, even reaching figures normally associated with severe spring flooding. Water release in the regulated river Oulujoki was higher than ever before in August, and the level of lake Oulujärvi increased above the regulation maximum. Of the unregulated lakes, the levels of lakes Lentua and Lammasjärvi in Kainuu increased to new record high levels for August throughout the long history of regular observations. Water levels were also very high after the wet summer in the lakes of eastern and central Finland. In the south and in Lapland, the recorded water levels were mainly close to or even below the mean for August. The mean discharges of the main watercourses were above the seasonal mean in the rivers Vuoksi, Kymijoki and Oulujoki. The mean discharge of the river Oulujoki at the Merikoski rapids was the highest ever recorded during the period of regulation. In the river Kemijoki the mean discharge was below the seasonal mean and in the river Kokemäenjoki it was close to the mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of August was +75 cm, and corresponding figures in some other Finnish lakes were: Kallavesi +43, Saimaa +63, Keitele +41, Päijänne +14, Pyhäjärvi, Säkylä -1, Längelmävesi +1, Näsijärvi +5, Lappajärvi +34, Lammasjärvi +58, Oulujärvi +40, Lokka -45 and Inari -5 cm. The mean monthly discharge of the river Pielisjoki was 158% of the mean for August during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 142%, Kymijoki 156, Karjaanjoki 57, Kokemäenjoki 99, Siikajoki 645, Oulujoki 282, Iijoki 172, Kemijoki 80 and Inarinjoki 84%.

Groundwater level

Groundwater levels in most parts of the country were 0–30 cm above the seasonal mean, except in Pohjanmaa and Kainuu, where they were as much as 30–100 cm above the mean. In areas with very high precipitation the groundwater reserves were as high as normally during the spring flood. In southern Finland groundwater levels were somewhat below the seasonal mean.

Evaporation

Evaporation was below the seasonal mean in wide areas as a result of the high precipitation. The deviation from the seasonal mean was highest in Pohjanmaa; in the north, near-normal evaporation figures were recorded. The monthly evaporation sum measured using Class A pans varied between about 50 mm in the areas of Ruukki and Suomussalmi and over 80 mm in e.g. Maaninka and Jokioinen. Evaporation from the larger lakes on the basis of watercourse model calculations was generally 100–150 mm, and from soil layers the evaporation was 40–80 mm.

Surface water temperature

Surface water temperatures varied on both sides of the seasonal mean. The water layers generally cooled during the month, although in mid-August they increased transiently above the seasonal mean as a result of a period of warm weather. By the end of the month surface water temperatures were 15–18°C in southern and central Finland and 9–14°C in the north. These figures were close to the seasonal mean.

Tables and graphs

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-10-18 at 10:40, updated 2013-06-24 at 15:28