Annual hydrological report 2011

Heavy winter snowfall in the south; flooding in coastal watercourses in spring and at the close of the year

The year 2011 was generally rather warm, with high local precipitation. Heavy winter snowfall occurred in the south, and flooding in coastal watercourses was recorded in the spring and towards the end of the year. At the beginning of the year, both surface and groundwaters were below their mean levels in most parts of the country. Water equivalents of snow covers were exceptionally high in southern and central Finland but rather low in Lapland. Groundfrost was deep in Lapland but rather shallow further south because of the heavy snow cover. Ice covers were thick in southern and central Finland. Spring floods were exceptionally mild in Lapland but rather heavy in coastal rivers due to the heavy snow cover. Lake water temperatures were high during most of the summer season in southern and central Finland and even exceptionally high in July. Precipitation was heavy towards the end of the year. The onset of winter was very late in the north, and had not even started in the south by the end of the year. Both surface and groundwater temperatures were above the seasonal mean at the close of the year, and water surfaces in coastal watercourses reached flood levels in some places.

Precipitation

Precipitation during 2011 was generally at or above the mean, with annual precipitation figures of 500–800 mm. The end of the year was characterized by heavy precipitation in many regions, particularly in southern and central Finland during December. High rainfall also occurred during July, August and September. The lowest precipitation was during midwinter and in November.

Water equivalent of snow cover

The distribution of snow was unusual early in the year. In southern and central Finland snow covers were exceptionally heavy, towards the end of the winter even twofold the seasonal mean in coastal regions. In the north however, particularly in southern Lapland, water equivalents of snow were considerably lower than normally. The maximum value of the water equivalent of the snow cover was recorded early in April almost throughout the country, after which melting occurred rapidly in southern and central Finland. In Lapland the melting occurred during several different periods, although most of the snow cover had disappeared by the end of April. Melting was generally earlier than normally, particularly in northern Finland. Towards the end of the year, the onset of winter was delayed throughout the country. In most parts of Lapland the first snow fell very late during the second half of November. By the end of November the snow boundary was in central Finland, and during December a snow cover was recorded for a short period almost everywhere. However, a lasting snow cover was formed before Christmas only to the north of a line from Pietarsaari to Joensuuu, and even in these areas the snow cover was lighter than normally. In the southernmost coastal zone the first snow was recorded just before Christmas and soon melted again.

Water level and discharge

At the beginning of the year water levels were low in most areas. In southern and central Finland mild weather during spring melted the snow rapidly, and particularly in coastal rivers the spring floods in April were exceptionally heavy. In Lapland, where the snow cover was rather light, the spring flood was distributed over several rather low peaks. In southern Lapland the floods were clearly below the mean and in some areas were even exceptionally low. After the spring floods the water levels were rather close to their seasonal mean except in the great lakes of eastern and central Finland, where snowmelt was not sufficient to increase water levels to their normal summer height. During the summer, periods of heavy rainfall caused increases to exceptionally high water levels particularly in Pohjois-Pohjanmaa. The levels of lakes Saimaa and Pielinen remained low throughout the summer and to the end of the year. Autumn precipitation was highest in Pohjanmaa and central Finland, where water levels were exceptionally high in many areas. Towards the end of the year, water levels decreased towards their seasonal mean levels. Heavy precipitation during autumn also increased water levels in Lapland, and e.g. the level of lake Inarinjärvi was clearly above the seasonal mean at the onset of winter. In southern and central Finland water levels began to increase after a dry period during November, and during the Christmas week the water levels of southern, south-western and western coastal rivers increased even higher than during the spring. The ensuing flood caused waterlogging of fields, blocking of roads and in parts of Varsinais-Suomi flooding of summer residences.

Groundwater level 

Groundwater levels at the beginning of the year were generally 15–40 cm below the seasonal mean, in Lapland slightly below the mean. The groundwaters decreased until the end of March, after which they began to increase in the south and west as the snow cover melted. In central and eastern Finland the water levels began to increase only at the end of April, and in the north during May. Snowmelt waters did not significantly replenish the groundwater reserves, which were 0–30 cm below the seasonal mean throughout the country. During hot weather in the summer, high evaporation caused decreases in groundwater layers to 10–40 cm below the seasonal mean. Rainfall during August replenished soil moisture reserves, but only continuing rain during September caused increases in water surfaces. At this point the groundwater levels were 5–50 cm above the mean in coastal regions and in central Finland but 0–40 cm below the mean elsewhere. Replenishment of the groundwater reserves continued in October and November and water surfaces were 5–40 cm above the seasonal mean in most parts of the country. Mild and wet weather during December caused the groundwater levels to remain high.

Groundfrost

Groundfrost extended only to a few centimeters in the southern coastal zone with a heavy snow cover. In Lapland, where there was less snow, the groundfrost depths were 60–80 cm. In other regions the groundfrost depths were 10–40 cm depending on the depth of snow. In southern and central Finland the maximum groundfrost depth was reached earlier than normally, in January-February. The depths in the southern and western coastal zones were only 10–20 cm, about 20–25 cm less than the normal maximum. In the east, a somewhat greater maximum than normal, 10–40 cm, was reached in mid-March. In northern Finland the recorded groundfrost depths of 40–160 cm were 10–40 cm greater than normally. In southern and central regions the groundfrost melted mainly during April; in parts of Lapland rather deep groundfrost persisted during May, but by June groundfrost was confined to the extreme north. In November a groundfrost layer formed in Lapland and in parts of eastern Finland, but as a result of mild weather in December it melted in all areas except Lapland.

Ice cover

Lake ice was generally thicker than normally at the beginning of the year in southern and central Finland. However, a large part of the ice layer was in fact weak snow ice which had formed from slush and water rising over the ice surface. In the north, ice thicknesses were close to the seasonal mean. Later in the winter the ice generally increased in thickness, although the regional differences remained. The maximum thicknesses were recorded in southern and central regions in March-April and in the north during the first half of April. The ice thickness was generally 55–75 cm; deeper ice was recorded over lake Kilpisjärvi. These thicknesses were above the seasonal mean in southern and central Finland and close to or slightly below the mean in Lapland. Freezing over of watercourses in the autumn occurred exceptionally late in Lapland. Ice thickness measurements were possible only in December, and were in many places even unprecedentedly low. By the end of the year an ice cover had formed only over the northern lakes; throughout most of central and southern Finland the lakes were still without a complete ice cover.

Surface water temperature

Warm weather at the beginning of June increased surface water temperatures clearly above the seasonal mean. Later, windy and wet conditions around Midsummer caused mixing and cooling of the water. The weather warmed again at the end of June and the beginning of July, and water temperatures were above the seasonal mean almost throughout July. At some observation stations new high record temperatures were recorded. The highest water temperatures of the summer in southern and central Finland were 23–26°C, clearly above the mean maxima. In Lapland, water temperatures generally remained below 20°C. Water temperatures were above the seasonal mean throughout the country at the beginning of August, although cooling had already begun. The temperatures remained above the seasonal mean during the autumn.

Evaporation

Evaporation during the summer season was below the mean in Pohjanmaa as a result of the rather wet weather. In Lapland, evaporation was rather high due to the early onset of spring and the delayed start to the winter. Elsewhere, evaporation measured from Class A pans between May and September was close to the seasonal mean. The greatest evaporation occurred as normally during hot weather in the middle of the summer season. Because of the long day length, evaporation during June and July was clearly higher than during the other months. Due to the mild weather during autumn, the evaporation season continued for longer than normally throughout the country.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-04-12 at 12:59, updated 2013-07-01 at 12:18