Monthly hydrological report April 2012

Heavy spring floods in Pohjois-Pohjanmaa

April was cooler than normally and precipitation was high. At the beginning of the month the snow burden was still increasing, but during the last week of April melting began even in Lapland. Warming of the weather, melting of snow and precipitation combined to increase water levels and discharges in many regions from the middle of the month. At the end of April both water levels and discharges increased considerably above the seasonal mean in Pohjois-Pohjanmaa, with flooding of wide areas. Groundwater levels were above the mean in southern and western Finland but rather low in the east and north. By the end of the month groundfrost was recorded only in the north. Lake ice broke up at the end of April in the lakes of southern Finland.

Precipitation

Precipitation was high during April throughout the country; the recorded figures of 30–60 mm were 120–190% of the seasonal mean. The heaviest precipitation was in Kainuu and south-western Lapland and the lowest, below 30 mm, was in north-western Lapland.

Snow cover

A pause in the melting of snow occurred at the beginning of April and the snow burden increased almost throughout the country. However, before the middle of the month the weather warmed again and melting continued even as far north as Oulu. By the last week of the month melting also began in Lapland. In the middle of April only Varsinais-Suomi and parts of Satakunta were snow-free, but by the beginning of May the snow-free area extended throughout most of western and southern Finland and Etelä-Savo. Near Ranua and in parts of northern Lapland the water equivalent of the snow cover still exceeded 160 mm at the end of April; elsewhere in Lapland and in Koillismaa the figures were generally 80–140 mm and in Kainuu 20–100 mm. By the beginning of May the snow cover was above the seasonal mean in south-western and western Lapland; near Inari and in parts of eastern Finland low snow covers were recorded.

Water level and discharge

Water levels decreased at the beginning of the month throughout the country. Warm weather and rainfall in mid-April began to increase the levels rapidly in western Finland almost as far north as Oulu. In some southern lakes the water levels began to decrease before the end of the month, but elsewhere the increase continued into the beginning of May. In many rivers the flood discharge was divided into two peaks due to the varying weather. Flood peaks along the south-western coastline and in Etelä-Pohjanmaa were lower than in March. In southern parts of Pohjois-Pohjanmaa the flood level at the end of the month increased well above the normal spring flood level, and flood waters covered wide areas. In eastern Finland and Kainuu the slow decrease in water levels continued to the end of the month, until rapid increases were recorded as a result of melting of snow in many places. In Lapland the effects of meltwaters were evident only in some watercourses. Water levels at the end of April were close to their seasonal mean levels in Lapland; elsewhere the levels were generally above the mean. The discharges of the major watercourses in April were in most cases above the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of April was +10 cm, and corresponding figures in some other lakes were: Kallavesi +21 cm, Saimaa +8, Keitele +31, Päijänne +32, Pyhäjärvi, Säkylä +5, Längelmävesi +34, Näsijärvi +37, Lappajärvi +42, Lammasjärvi +24, Oulujärvi +27, Lokka -56 and Inari +9 cm. The discharge of the river Pielisjoki was 90% of the mean April discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 87%, Kymijoki 134, Karjaanjoki 165, Kokemäenjoki 131, Kalajoki 148, Oulujärvi 135, Iijoki 78, Kemijoki 102 and Tornionjoki 91%.

Groundwater level

Groundwater levels in southern and western Finland were 5–25 cm above the seasonal mean. In eastern Finland and in Lapland the levels were 0–20 cm below the mean, as meltwaters had not yet had time to replenish the water reserves.

Groundfrost

Groundfrost had melted almost completely in southern and central Finland by the end of April and was recorded only in isolated areas. In the north, the groundfrost depths were still 10–80 cm.

Ice cover and surface water temperature

Melting of lake ice started at the beginning of the month in the south, whereas in Lapland the ice thicknesses increased to the end of the month. Break-up of ice occurred in southern and south-western Finland during the last week of the month at approximately the normal time. In Lapland and Koillismaa the ice thicknesses were still 50–60 cm at the beginning of May, except over Kilpisjärvi where a depth of 84 cm was recorded. In central regions the ice covers were already very weak. In southern lakes, surface water temperatures increased at the end of the month even to 8°C.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-05-31 at 9:54, updated 2013-07-01 at 12:32