Monthly hydrological report May 2013

Record high water temperatures; normal spring floods in Lapland except in the river Tornionjoki


May was warm and precipitation was generally low. Snowmelt in Lapland was rapid and peak flooding occurred after the middle of the month. In the rivers Tornionjoki and Muonionjoki the peak discharges were higher than normally; elsewhere they were close to the mean. Groundwater levels were already decreasing in southern and western regions, whereas in the north they were above the seasonal mean. Break-up of ice occurred earlier than normally in northern Lapland and lake water temperatures at the end of the month were unprecedentedly high in some cases.


Precipitation in May varied on both sides of the seasonal mean, more generally below the mean. The highest precipitation, 40-60 mm, was recorded in Kainuu and eastern Finland. In many parts of Lapland as well as of southern and western Finland the precipitation was below 20 mm, only half the seasonal mean. The end of the month was very dry.

Snow cover

Small patches of snow still persisted in forest areas at the beginning of May at the latitude of Oulu and even in places further south. In northern Lapland the snow burden was still high, in open tundra areas over 150 kg/m2 and over 60 kg/m2 elsewhere. Melting was then rapid, and after mid-May snow persisted only in northern Lapland and in the most shaded forest areas. By the end of the month the snow had melted throughout the country.

Water level and discharge

Of the larger lakes the levels of lakes Päijänne, Inarijärvi and Oulujärvi were still increasing by the end of the month, but water levels were otherwise decreasing throughout the country. The measured levels were in most cases close to the seasonal mean or slightly below it. The level of lake Saimaa was still above the seasonal mean, but extra regulatory discharges were discontinued. The level of lake Inarijärvi was also still clearly above the seasonal mean.

Discharges in southern and central Finland were decreasing, and draining of the areas in Pohjanmaa which were flooded in April continued until the middle of May. Due to a cold period at the beginning of the month, the peak flood was not reached in the rivers of Lapland until the second half of the month. Ice dams did not cause significant problems in Lapland, in contrast to the situation in Pohjanmaa the previous month. The discharges of the river Ounasjoki and its tributaries were approximately normal, whereas the flooding of the rivers Tornionjoki and particularly Muonionjoki was unusually high, causing blocking of roads in some places at the end of the month. With the exception of the rivers Vuoksi and Tornionjoki, mean discharges of the major river watercourses were close to or below the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of May was -15 cm, and corresponding figures in some other lakes were: Kallavesi +0 cm, Saimaa +32, Keitele +3, Päijänne +10, Pyhäjärvi, Säkylä -2, Längelmävesi -9, Näsijärvi -4, Lappajärvi -2, Lammasjärvi -37, Oulujärvi +15, Lokka -34 and Inari +32 cm. The discharge of the river Pielisjoki was 82% of the mean monthly discharge for May during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 118%, Kymijoki 100, Karjaanjoki 68, Kokemäenjoki 75, Siikajoki 77, Oulujoki 97, Iijoki 79, Kemijoki 94 and Tornionjoki 136%.

Groundwater level

Groundwater levels turned to a decrease in southern and western regions, but were still 0-15 cm above the seasonal mean. In northern and eastern Finland they were at their highest or still slightly increasing in mid-May. In these regions groundwaters were in many cases 10-50 cm above the seasonal mean.


Some groundfrost still persisted in eastern and northern regions, but was already melting.

Surface water temperature

Exceptionally warm weather at the end of May caused warming of lake waters. Surface water temperatures increased well above the seasonal mean, often exceptionally so and even in some cases setting new seasonal high records. The warmest water temperatures were recorded in western regions. Surface water temperatures the end of the month were 14-20˚C in southern and central Finland and 12-17˚C in the north with the exception of north-western Lapland. The recorded temperatures were as much as 10˚C above the seasonal mean in Lapland and several degrees high in other regions.

Ice thickness

Break-up of ice occurred at the very beginning of the month in central Finland, by mid-May in Kainuu and by the end of May in Lapland, with the exception of lake Kilpisjärvi. In most areas the dates were close to the seasonal mean, but in Lapland they were clearly earlier than normally.

Maps, graphs and tables

Published 2013-08-23 at 12:00, updated 2013-08-26 at 9:20