Monthly hydrological report May 2014

Heavy spring floods in parts of Lapland; dry conditions in south-western Finland

Precipitation in May 2014

Temperatures and precipitation varied considerably in May, and wide deviation in water reserves was recorded in different parts of the country. Snow and ice melted almost completely throughout northern Finland. In Lapland the thaw caused flooding. In eastern and central Finland heavy rainfall increased water levels and discharges, whereas western and south-western regions were unusually dry. Groundwater reserves varied considerably. Groundfrost was still recorded in northern Finland. Water temperatures increased even to seasonal record high levels in some areas.

Precipitation

Precipitation during May was uneven. In eastern and central Finland rainfall amounting to 120 mm was even above twofold the seasonal mean, and in some areas in the east as much as 50 mm was recorded in a single day. In south-western and western Finland and in the north, the precipitation of 20−80 mm was clearly below the seasonal mean. In north-western Lapland the precipitation was even below 20 mm.

Snow cover

In Koillismaa and most of Lapland, abundant reserves of snow were still recorded at the beginning of May; the highest water equivalence figures in northern Lapland exceeded 150 mm. Cool weather at the beginning of the month halted the melting of snow for many days, but after warming of the weather before mid-April the thaw progressed rapidly. By the 20th of the month a coherent snow cover was recorded only in north-western Lapland and around Saariselkä, and by the end of May only the north-west retained its snow cover.

Water level and discharge

Rainfall increased lake levels in many parts of southern and central Finland in the middle and end of May. In eastern Finland the water reserves were replenished significantly, but elsewhere the increases were rather modest. In the north, water levels increased considerably due to melting of snow. In lake Inarijärvi and in north-western Finland the water levels continued to increase at the end of the month, whereas elsewhere they had begun to decrease. Lake water levels at the end of the month were generally close to the seasonal mean throughout the country, with however some exceptions on both sides of the mean.

Peak floods due to the spring thaw occurred in Lapland towards the end of the month and exceeded the mean e.g. in the source waters of the river Ounasjoki. In eastern and central Finland and in Pohjois-Pohjanmaa, heavy precipitation increased discharges after the middle of the month. Mean monthly discharges in the south-west and west were generally below the mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of May was -6 cm, and corresponding fifures in some other lakes were: Kallavesi -5 cm, Saimaa +38, Keitele -16, Päijänne +22, Pyhäjärvi, Säkylä -9, Längelmävesi -34, Näsijärvi -5, Lappajärvi -3, Lammasjärvi -14, Oulujärvi +30, Lokka -39 and Inari -19 cm. The discharge of the river Pielisjoki in May was 94% of the mean during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 110%, Kymijoki 83, Karjaanjoki 24, Kokemäenjoki 32, Siikajoki 28, Oulujoki 137, Iijoki 58, Kemijoki 77 and Tornionjoki 90%.

 

Groundwater level and groundfrost

After a winter with little snow and the early onset of spring, groundwater levels in southern and central Finland in May were 10−30 cm below the seasonal mean. In southern Lapland the corresponding figures were 10−30 cm above the mean, but in central and northern Lapland 20−40 cm below the mean. In northern Finland groundfrost layers still extended down to 5−20 cm.

Ice cover

Lake ice layers broke up almost throughout northern Finland during May. The ice layers melted in Kainuu early in the month and in northern Lapland towards the end of May, with the exception of north-western Lapland. The dates of ice melting were early in Kainuu but close to the seasonal mean in Lapland. The ice thickness over lake Kilpisjärvi was still 79 cm at the end of May.

Surface water temperature

Water temperatures early in May were close to or above the seasonal mean. Hot weather during the third week of the month increased surface water temperatures to very high figures in southern and central Finland, with new record high temperatures for May being recorded in some cases. At their highest, the temperatures were generally at least 20ºC at many observation stations. Thereafter, rapid cooling of the weather and high winds caused considerable decreases in water temperatures. By the end of the month the temperatures were generally close to the seasonal mean, in most regions 10−15 and in Lapland still below 10ºC.

Maps, graphs and tables

Published 2014-08-27 at 13:05, updated 2014-08-27 at 13:05