Monthly hydrological report May 2012

Heavy flooding in Lapland and exceptionally high water levels in the eastern Lake District

May was slightly warmer than normally. Monthly precipitation was generally above the seasonal mean. Precipitation and snowmelt increased discharges in the rivers of northern Finland and water levels in the eastern Lake District. Spring floods in the rivers of Lapland were rather heavy and in some areas approached the damage level. Of the larger lakes in eastern Finland the levels of Kallavesi, Pielinen, Keitele and Puula were particularly high. Groundwater reserves were replenished by snowmelt water. By the end of the month almost the whole country was free of ice, and groundfrost was recorded only in some northern regions. Watercourse temperatures were close to their seasonal mean values.

Precipitation

Precipitation in May was generally 20–90 mm, generally above the seasonal mean. The highest figures were recorded in Kainuu and Pohjanmaa and the lowest in northern Lapland. Precipitation had been high throughout the winter and spring. In the river Kymijoki and Kokemäenjoki watercourses the precipitation sum for December-May was unprecedentedly high, and in the river Vuoksi and Oulujoki basins the second highest during the past 100 years.

Snow cover

Snow still covered the ground to a considerable depth at the beginning of May in Lapland, Koillismaa and Pohjois-Kainuu, as well as to some extent in central Finland. However, warming of the weather combined with falls of rain melted the snow rather rapidly. At best, the decrease in the water equivalent of the snow cover even exceeded 40 mm per day. By the 10th of the month the land to the south of a line from Oulu to Sotkamo was completely snow-free and by the 20th the snow had melted even throughout most of Lapland. By the end of May a snow cover persisted only in parts of the tundra region of northern Lapland. Melting of snow began somewhat later than normally in the north but proceeded rather rapidly.

Water level and discharge

In most lakes water levels reached their spring maxima during May. In the smaller lakes of southern Finland the peak had been passed by the beginning of May and water levels decreased throughout the month. In some large lakes, e.g. Saimaa and Pielinen, as well as in some regulated lakes the level increases continued. Particularly in the eastern Lake District spring flood levels were high, and remained well above the seasonal mean to the end of the month. For example the levels of lakes Kallavesi and Keitele were higher than for 20 years and Puulavesi for 30 years. Lake Pielinen, which been at an unusually low level during the previous autumn, was now 40 cm above the seasonal mean.

River flooding occurred in northern Finland during May, when snowmelt and simultaneous rainfall caused rapid level increases. Floodwaters blocked roads, inundated fields over wide areas, dislodged boat jetties and other structures and even caused some flooding of buildings, although serious damage was generally avoided. For example in the municipality of Kittilä in the river Ounasjoki basin the flood peak exceeded the damage level by a few centimetres, and heavy discharges were recorded in the rivers Kemijoki, Tornionjoki and Ivalojoki. Discharges of all the major watercourses were above the long-term mean for May; in the smaller rivers of the coastal zone the discharges were generally close to the mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of May was +41 cm, and corresponding figures in some other lakes were: Kallavesi +26 cm, Saimaa +26, Keitele +30, Päijänne +38, Pyhäjärvi, Säkylä -3, Längelmävesi +15, Näsijärvi +3, Lappajärvi +32, Lammasjärvi +10, Oulujärvi +55, Lokka -24 and Inari +12 cm. The discharge of the river Pielisjoki was 127% of the mean May discharge during the reference period 1971-2000, and corresponding figures in some other river watercourses were: Vuoksi 100%, Kymijoki 143, Karjaanjoki 65, Kokemäenjoki 119, Kalajoki 143, Oulujoki 218, Iijoki 144, Kemijoki 132 and Tornionjoki 135%.

Groundwater level

Groundwater reserves were replenished by melting of snow and the levels were generally 10–35 cm above the seasonal mean. In the south the levels began to decrease during May, but in other regions the levels were still increasing. 

Groundfrost

Even in northern Finland the groundfrost had melted almost completely, with groundfrost depths of 0–15 cm being recorded only in scattered areas.

Ice cover and surface water temperature

Lake ice broke up in the lakes of central Finland during the first days of the month and in Kainuu by mid-May. In Lapland the lake ice covers broke up during the second half of the month, somewhat ahead of schedule. After break-up of ice surface water temperatures increased approximately in accordance with the seasonal mean. Water layers were exceptionally warm on the 27th and 28th of the month, but with subsequent cooler weather they again approached the seasonal mean by the very end of May. Surface water temperatures at the end of May were 8–16˚C in southern and central Finland and below 9˚C in Lapland; lake Kilpisjärvi was still covered with a 75 cm ice layer.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-06-27 at 12:57, updated 2013-07-01 at 12:10