Monthly hydrological report September 2012

Lake surfaces high in the great lakes of Eastern Finland; high discharges in the southern coastal zone

September was warmer and wetter than normally. Particularly high precipitation occurred in eastern and southern Finland and lake surfaces clearly increased in the east. Rainfall at the end of the month increased river discharges and lake levels to exceptionally high levels in the southern coastal zone. Groundwater levels were very high particularly in Kainuu and Pohjois-Pohjanmaa, partly due to high precipitation in earlier seasons. Evaporation was generally rather low. Water temperatures decreased during September in normal fashion.


Precipitation was high in September, particularly in the south and east and in parts of Lapland, where monthly precipitation figures were 80–160 mm. In some areas the precipitation was even twofold the seasonal mean. In the river Porvoonjoki watercourse the precipitation was the highest for September since 1965, and in the river Vantaa it was the second highest over an observation period of 100 years. The lowest rainfall was in Pohjanmaa, where the monthly precipitation of 50–70 mm was close to the seasonal mean. The first snow fell in Lapland, but did not persist.

Water level and discharge

At the beginning of the month lake water levels were generally decreasing, but the heavy rain towards the end of the month caused renewed increases in many parts of the country. Because the soil layers were already soaked, the precipitation caused rapid watercourse level increases. In most regions, the September discharges were above the seasonal mean. In Pohjanmaa water levels were high e.g. in the rivers Lapväärtinjoki and Teuvanjoki, and in the coastal zone of the Bay of Bothnia in the rivers Siuruanjoki, Kuivajoki and Simojoki. Heavy precipitation at the end of the month brought rivers up to flood levels in Uusimaa, Etelä-Häme and Varsinais-Suomi. Particularly along the southern coastline discharges were exceptionally high for September. Rapid level increases were also recorded in some lakes: e.g. the level of lake Pyhäjärvi, Artjärvi increased almost half a metre in one week. By the end of the month lake water levels were generally above the seasonal mean, the greatest deviation being in the great lakes of eastern Finland. The decreasing level of lake Saimaa turned to an increase as a result of the heavy rainfall in late September despite the fact that regulatory flow from the lake was increased to 1000 m3/s from the 10th of the month. The discharges of the main channels of the rivers Vuoksi, Kymijoki and Oulujoki were clearly above the seasonal mean, whereas those of the rivers Kokemäenjoki and Kemijoki were typical for September or even slightly below the mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of September was +68 cm, and corrseponding figures in some other lakes were: Kallavesi +37 cm, Saimaa +65, Keitele +31, Päijänne +20, Pyhäjärvi, Säkylä +1, Längelmävesi +3, Näsijärvi +7, Lappajärvi +25, Lammasjärvi +33, Oulujärvi +21, Lokka -52 and Inari +13 cm. The monthly discharge of the river Pielisjoki was 161% of the mean for September during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 163%, Kymijoki 140, Karjaanjoki 85, Kokemäenjoki 99, Siikajoki 104, Oulujoki 171, Iijoki 130, Kemijoki 93 and Tornionjoki 147%.

Groundwater level

As a result of the high precipitation, groundwater levels were 5–15 cm above the seasonal mean in most regions; in Kainuu and Pohjois-Pohjanmaa the deviation was even 40–60 cm. Uneven distribution of the precipitation was reflected in rather varying groundwater levels in the south.


Due to the high rainfall in September, evaporation was generally low despite the rather warm weather. Monthly evaporation measured with Class A pans varied between slightly over 10 mm in the north to as much as 45 mm near Ylistaro. Evaporation from soil layers on the basis of model calculations was generally 20–30 mm, with slightly higher results in some southern regions. Evaporation from larger lakes was clearly higher due to their considerable heat content, 50–90 mm on the basis of model calculations.

Surface water temperature

Watercourse temperatures decreased during September in typical fashion for the season. Decreases were generally 4–7˚C, but the measured temperatures were generally somewhat above the seasonal mean during most of the month. By the end of September surface water temperatures were 9–12˚C in the lakes of southern and central Finland and 4–9˚C in the north. These figures were generally close to the seasonal mean.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-11-15 at 12:00, updated 2013-06-24 at 15:39