Monthly hydrological report September 2011

Water levels unprecedentedly high in Pohjanmaa and central Finland

September was warm and wet. Precipitation in southern and central Finland was clearly above the seasonal mean, whereas in the north below-average figures were generally recorded, except in western Lapland. Exceptionally heavy rainfall increased watercourse water levels and discharges particularly in Pohjanmaa and central Finland, where water levels set new seasonal high records in some cases. Groundwater levels also increased as a result of the precipitation. Surface water temperatures were exceptionally high due to the warm weather.

Precipitation

Precipitation in September was clearly above the seasonal mean in southern and central Finland, whereas in the north the precipitation figures were generally below the mean except in western Lapland. Monthly precipitation was generally 20–150 mm. Precipitation was exceptionally high in Pohjanmaa, in the river Kymijoki and Kokemäenjoki watercourses and in the Kymenlaakso valley; in some cases the rainfall was even twofold the seasonal mean. The regional September precipitation record was broken in the river Karvianjoki and in northern reaches of the river Kokemäenjoki watercourse. The lowest precipitation was in eastern Lapland.

Water level and discharge

The September precipitation increased water levels almost throughout the country, particularly in western regions. In Pohjanmaa and central Finland water level records were exceeded in many lakes. For example the level of lake Pääjärvi in Karstula exceeded the seasonal mean at the end of September by 110 cm and represented the highest ever recorded level in September during 100 years of continuous observations. The level of lake Lappajärvi was also 75 cm above the seasonal mean. In Pohjanmaa, river water levels and discharges at the end of the month were very high. In many places the mean spring high flood level was clearly exceeded. In eastern Finland water levels at the end of September were closer to the seasonal mean, and in eastern Lapland the levels were even below the mean. The levels of lakes Saimaa and Pielinen were still about 40 cm below the seasonal mean. The discharges of major watercourses were below the seasonal mean in the watercourses of the rivers Vuoksi, Oulujoki and Kemijoki. In the river Kokemäenjoki the discharge was close to the seasonal mean and in the river Kymijoki above-average discharge was recorded. The discharges of the smaller rivers of the southern coastal zone increased towards the end of the month.

The deviation of the level of lake Pielinen from the seasonal mean at the end of September was -42 cmm and corresponding figures in some other Finnish lakes were: Kallavesi +7 cm, Saimaa -38, Keitele +35, Päijänne +14, Pyhäjärvi, Säkylä +7, Längelmävesi 0, Näsijärvi +5, Lappajärvi +75, Lammasjärvi +14, Oulujärvi +19, Lokka -150 and Inari +6 cm. The discharge of the river Pielisjoki was 63% of the mean September discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 74%, Kymijoki 115, Karjaanjoki 97, Kokemäenjoki 101, Kalajoki 296, Oulujoki 91, Iijoki 83, Kemijoki 83 and Tornionjoki 151%.

Groundwater level

The high September precipitation caused increasing groundwater levels throughout the country except in the largest formations. Groundwater levels were 5–50 cm above the seasonal mean in the coastal zone and in central Finland. Around Kuusamo and in eastern Lapland the levels were 10–40 cm below the seasonal mean; elsewhere they were generally 0–30 cm below the mean.

Evaporation

Evaporation during September was low due to the autumnal weather and decreasing day length. The monthly evaporation measured using Class A pans was generally close to the seasonal mean, with a variation of 20–45 mm. The evaporation sum for May-September was generally 400–500 mm in southern and central Finland and 300–400 mm in the north. These figures were higher than normally in the north, but close to the mean elsewhere. On the basis of watercourse model calculations evaporation during September was generally 20–40 mm from soil areas and 20-50 mm from the larger lakes.

Water temperature

Lake surface water temperatures in September were generally above the seasonal mean almost throughout the country. The recorded temperatures were exceptionally high, with new records being set in some places. During the month the waters cooled by 3–7°C. Water temperatures at the end of September were 11–13°C in southern and central Finland, 8–10°C in Kainuu and Pohjois-Pohjanmaa and 7–9°C in Lapland. These figures were generally about 2°C above the seasonal mean.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2011-10-31 at 9:29, updated 2013-07-01 at 12:27