Annual hydrological report 2013

A hydrologically varied year, including many floods and decreased water reserves in Lapland  

Precipitation in 2013

Hydrological conditions in 2013 were very variable and overall the year was very warm. Precipitation was high in central and eastern areas, but low in Lapland. Heavy snow covers were recorded in spring in southern and western Finland and in parts of northern Lapland. Spring floods were high in many coastal rivers and ice dams also caused problems. Groundwater levels were high at the beginning of the year, but decreased to below-average levels during the summer in many areas. A drought was experienced in some places in late summer, until heavy rain later in the autumn caused replenishment of the water reserves. Wintry weather began almost throughout the country in November-December, but mild weather caused melting of snow and ice in southern and central Finland during December. Melting of snow and heavy rainfall caused record-breaking floods in coastal rivers around Christmas.

Precipitation

Annual precipitation was at or above the mean in an area extending from eastern Finland to Pohjois-Pohjanmaa, but in southern and western Finland and in Lapland the precipitation was low. Annual precipitation varied between below 400 mm in northern Lapland and over 700 mm in eastern Finland. In northern Lapland the annual precipitation was even 20% below the mean; elsewhere the deviations from the mean were smaller, although 20% higher than the average precipitation was recorded in parts of Kainuu. The driest month of the year was March and the wettest was generally June, except in the river Oulujoki basin where December had the highest precipitation.

Snow cover

During the first months of the year snow accumulation was above the mean in parts of southern and western Finland and in northern Lapland, whereas in parts of Kainuu, Pohjois-Karjala and southern Lapland the snow burden was considerably below the mean. The duration of the winter was longer than normally and melting began in mid-April almost throughout the country. In southern Finland the snowmelt was rapid and continuous, but in the north the melting season was divided into two parts. In autumn the first snowfalls were recorded in most areas in October, but did not persist except in Lapland and Koillismaa. Most of the country had a snow cover at the end of November, but this snow melted almost completely by the end of the year below a line from Oulu to Joensuu. In the north more precipitation of snow was recorded, and snow levels were even above the seasonal mean. Melting of snow during December extended as far north as southern Lapland.

Water level and discharge

Winter-season decreases in water levels continued normally at the beginning of the year. The onset of spring was delayed in southern and central Finland and increases in water levels occurred almost at the same time in April from the southern coast as far north as Pohjois-Pohjanmaa. The floods increased very rapidly and were clearly above the mean in almost all coastal rivers. Ice dams aggravated the situation in many rivers. Water levels decreased during the summer, and particularly in the north a period of drought was experienced. The autumn precipitation was rather late to arrive, but in southern and central Finland eventually replenished the water reserves and increased lake levels to their seasonal mean levels or higher. In many coastal rivers autumn discharges reached levels typical of the spring flood season. In Lapland the dry weather continued until the onset of winter and lake water levels were in many cases below the seasonal mean during the end of the year. Although the water levels continued to decrease in Lapland, in other parts of the country rainfall and melting of snow increased both water levels and discharges. Between Satakunta and Pohjois-Pohjanmaa water levels were exceptionally high in December, with new records being set in many places. Major damage was avoided, although many fields, forests and some roads were inundated. In some areas the highest levels of the year were recorded in December. By the end of the year lake surfaces were above the seasonal mean in all areas except Lapland.

Mean annual discharges of the main river watercourses varied on both sides of the mean. The discharges of the rivers Kymijoki and Tornionjoki were close to the mean, but in the rivers Kokemäenjoki, Oulujoki and Kemijoki below-average discharges were recorded. The annual discharge of the river Vuoksi was above the mean.

Groundwater level

After a rather wet autumn in 2012, groundwater levels at the beginning of the year 2013 were 10−20 cm above the seasonal mean. The slow onset of spring delayed the replenishment of groundwater reserves throughout the country. By the beginning of summer the groundwaters were somewhat below the seasonal mean in southern and central Finland and close to the mean in the north. The dry summer decreased groundwater levels, which were in most areas 20−40 cm and in the north even 30−70 cm below the seasonal mean by late summer and well into the autumn. Towards the end of the year the water levels increased as a result of the mild weather in southern and central regions, but in the north they remained 5−60 m below the seasonal mean.

Groundfrost

As a result of the heavy snow cover and precipitation early in the year, groundfrost was clearly weaker than normally in southern and central Finland, extending only to 0−30 cm. In the north the groundfrost depths of 20−100 cm were 10−20 cm below the seasonal mean. The maximum groundfrost depths were generally recorded in March-April, and due to the late onset of spring the groundfrost only melted at the end of April and in early May. In autumn, groundfrost formation began in some northern areas starting in October; by the end of the year groundfrost depths could still be measured only in the north, where they were generally 0−10 cm.

Ice cover

Lake ice thicknesses in January and February were close to or below the seasonal mean. The quality of ice varied considerably, and the proportion of blue ice was in some cases very low. By the end of the winter season ice thicknesses in northern Lapland were below the seasonal mean but in parts of southern and south-western regions they were above the mean. Maximum ice thicknesses were recorded almost throughout the country at the beginning of April. Break-up of ice occurred approximately according to the normal timetable in most regions except in northern Lapland, where the ice left earlier than normally. In autumn the smaller lakes began to freeze over at the end of October, but most of the northern lakes froze during November. Frosty weather at the beginning of December caused freezing of smaller lakes even to southernmost Finland, but milder weather thereafter caused weakening or melting of the ice. By the end of December lake ice overs were recorded generally only in Lapland and Kainuu, whereas most of the southern lakes were ice-free.

Surface water temperature

Summer lake water temperatures showed considerable variation. Hot weather in late May and early June increased lake water temperatures to exceptionally high levels. Later in June the water temperatures were close to the seasonal mean, only to increase again in June-July. Northerly winds caused considerable cooling of surface waters after the middle of July, and even exceptionally low temperatures were recorded in southern and central Finland. The highest temperatures of the summer were measured in June-July in southern and central Finland, but in early August in Lapland. In southern and central Finland the maximum temperatures were 22−25˚C, whereas in Lapland they were generally below 20˚C. During late summer and autumn, water temperatures were generally above or close to the seasonal mean.

Evaporation

Evaporation varied considerably during the summer of 2013 according to the prevailing weather. The summed evaporation from Class A pans during May-September was 350−500 mm in southern and central Finland. During much of the summer the evaporation in these areas was generally close to or below the seasonal mean. In Lapland the evaporation was generally clearly above the mean in early and late summer, but somewhat below the mean in the middle of the summer season. The seasonal evaporation sum measured with Class A pans in Lapland was 200−370 mm. Lake evaporation during the whole open-water season on the basis of watercourse model calculations was 450−700 mm in southern and central Finland and 250−450 mm in the north. Evaporation from soil layers on the basis of models varied between 100 mm in northern Lapland and 400 mm in southern Finland.

Maps, graphs and tables

Published 2014-06-24 at 18:11, updated 2014-06-24 at 18:09