Monthly hydrological report April 2013

Ice dams caused major damage in many watercourses

Water-equivalent-of-snow-april2013

Precipitation in April was generally above the seasonal mean. Sudden increases in discharge due to rapid melting of snow and heavy precipitation, combined with unusually thick river ice covers, resulted in the formation of many ice dams which caused serious flooding in the coastal rivers of Pohjanmaa. In many parts of both Etelä-Pohjanmaa and Pohjanmaa the situation was even more severe than during the previous autumn. Groundwater levels were generally rather low. Groundfrost was weak, but ice thicknesses were still increasing during the first half of the month.

Precipitation

Precipitation was above the mean for April in most parts of the country, with monthly precipitation sums of 25−60 mm. The heaviest precipitation was along the western coastline, with even twofold the seasonal mean in places. The lowest precipitation, below 20 mm, was in north-western Lapland.

Snow cover

The water equivalent of the snow cover hardly changed during the first half of April. Thereafter there was a mild period, as a result of which rapid melting of snow was recorded throughout the country. In southern and central Finland the snowmelt occurred clearly later than normally, although very rapidly. In most of Lapland melting began at approximately the normal time but was unusually rapid, until cooling of the weather towards the end of the month caused a decrease in the rate of melting. By the end of April the area to the south of a line from Raahe to Joensuu was almost free of snow, but in Lapland there was still snow. In southern Lapland the snow cover was interspersed with open patches and the main cover was in some areas below 20 kg/m2, but in the tundra regions of northern Lapland the snow water equivalent was still about 200 kg/m2.

Water level and discharge

Water levels at the beginning of April were typically low for the winter season. However, rapid snowmelt and heavy precipitation in the middle of the month caused an unusually rapid increase in water levels. The first peak floods were recorded in the coastal rivers of southern and south-western Finland, followed by the western coastal area and further north as far as the river Simojoki. Discharges were considerably above the seasonal mean in almost all coastal rivers. Many ice dams increased the water levels even further, causing exacerbation of the flooding particularly in coastal rivers emptying into the Gulf of Bothnia. The greatest problems were registered in the rivers Loimijoki, Lapväärtinjoki, Kyrönjoki, Pyhäjoki and Kiiminkijoki, but flood prevention measures were also needed in many other rivers. In Etelä-Pohjanmaa and Pohjanmaa the flood waters rose to exceptionally high levels, in many cases reaching the highest levels since spring 1984. Flood damage was considerable. In Lapland water levels began to increase during the latter half of April, but a spell of cool weather at the very end of the month checked the increase in discharges. In the south, water levels of smaller lakes reached their spring maxima at the end of April, but in most regions lake levels were still increasing at the end of the month.

The deviation of the level of lake Pielinen from the seasonal mean at the end of April was +5 cm, and corresponding figures in some other lakes were: Kallavesi +9 cm, Saimaa +41, Keitele +17, Päijänne +15, Pyhäjärvi, Säkylä +5, Längelmävesi +7, Näsijärvi +2, Lappajärvi +27, Lammasjärvi +38, Oulujärvi +47, Lokka -42 and Inari +13 cm. The discharge of the river Pielisjoki was 85% of the mean April discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 125%, Kymijoki 103, Karjaanjoki 74, Kokemäenjoki 92, Siikajoki 105, Oulujoki 85, Iijoki 187, Kemijoki 152 and Tornionjoki 148%.

Groundwater level

The slow onset of spring was also seen in delayed replenishment of groundwater reserves. In general, level increases were not recorded during April and the recorded levels were 10−50 cm below the seasonal mean.

Ground frost

Due to the heavy snow cover, groundfrost depths were only 5−20 cm in southern and central Finland, i.e. 10−30 cm below the seasonal mean. Maximum groundfrost depths were generally reached at the end of March and the beginning of April. In western Finland night frosts still caused increases in groundfrost depth as the snow cover melted. In the north the recorded groundfrost depths of 20−110 mm were 10−20 mm less than normally. In these areas the groundfrost maxima were reached during the first half of April.

Ice thickness

Maximum ice thicknesses were recorded around the 10th of the month almost throughout the country. In southern and central regions they were 50−65 cm and in the north 55−70 cm, in lake Kilpisjärvi even >90 cm. Warming of the weather during the middle of the month caused rapid weakening and melting of ice in southern and central Finland. By the end of the month break-up of ice was taking place in southern and central regions and the ice covers were very weak as far north as Kainuu and southern Lapland. In northern Lapland the lake ice covers were still 50−90 cm.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2013-07-04 at 12:00, updated 2013-07-08 at 13:18