Monthly hydrological report April 2014

Early onset of spring in southern and central Finland

Snow water equivalent in April 2014

April was rather mild and precipitation was low. After a winter with little snow and a dry spring season, increases in both water levels and discharges in the lakes of southern and central Finland occurred rather early and the peaks were in many areas unusually low. By the end of the month the water levels in these regions were generally decreasing. Groundwater levels also began to decrease as a result of the early onset of spring, although the level deviations from the seasonal mean were generally insignificant. Melting of ice occurred clearly earlier than normally in southern and central Finland.  

Precipitation

Precipitation in April was low in most regions. Monthly precipitation was 5−20 mm in southern and central Finland and generally 10−40 cm in the north. Heavy precipitation was recorded in Lapland during the last days of the month, and in these areas the monthly precipitation exceeded 40 mm.

Snow cover

At the beginning of April the snow cover persisted only in the north, where it was in fact rather heavy. By the middle of the month, melting of snow had begun even in most parts of Lapland. Cooler weather towards the end of the month checked the rate of thawing in the north, and new falls of snow occurred in Lapland just before the beginning of May, by which time the snow-free area extended as far north as south-western Lapland and the northern parts of Kainuu. In Koillismaa and most parts of Lapland the water equivalent of the snow cover was still as high as 40−130 mm and in north-western Lapland the corresponding figures were even higher. With the exception of this region, the snow burden was clearly lower than normally.

Water level and discharge

Changes in lake water levels were generally rather small. In the snow-free southern and central regions the spring level increase was either below the mean or totally absent. The surfaces of unregulated lakes were generally decreasing. In many larger lakes the water levels at the end of the month were still high after the heavy rainfall of the previous autumn and early winter seasons, but in smaller lakes they were generally close to or below the seasonal mean. Discharges in the rivers of the southern and western coastal zone were very low. In parts of Lapland the typical winter level decreases continued throughout April, but in most parts of northern Finland melting of snow began to increase lake water levels and river discharges during the middle and end of the month. In many areas the level increases began earlier than normally and therefore the recorded water levels were higher than normally. Dislodged ice floes resulted in damming of rivers in some regions and caused a danger of flooding e.g. in the municipality of Simo.

The deviation of the level of lake Pielinen from the seasonal mean level at the end of April was +31 cm, and corresponding figures in some other lakes were: Kallavesi -3 cm, Saimaa +39, Keitele +5, Päijänne +53, Pyhäjärvi, Säkylä -9, Längelmävesi -16, Näsijärvi +31, Lappajärvi +15, Lammasjärvi +40, Oulujärvi +85, Lokka -25 and Inari -4 cm. The discharge of the river Pielisjoki was 133% of the mean monthly discharge during the reference period 1971-200, and corresponding figures in some other rivers were: Vuoksi 116%, Kymijoki 80, Karjaanjoki 30, Kokemäenjoki 31, Siikajoki 39, Oulujoki 129, Iijoki 176, Kemijoki 127 and Tornionjoki 183%.

Groundwater level

Groundwater levels in most regions were 0−20 cm above the seasonal mean. In many cases the levels had begun to decrease one month earlier than normally as a result of the early onset of spring. In southern Finland the groundwater levels were close to the seasonal mean, whereas in the north they were still below the mean after the dry weather during autumn 2013.

Groundfrost

Despite the early onset of spring, groundfrost persisted in most parts of the country. In southern and central Finland the surface layer had thawed although frozen soil layers persisted at lower levels. Groundfrost depths were 10−20 cm in many areas and 20−40 cm in the north.

Ice cover

Break-up of ice occurred in southern lakes at the beginning of April and in central Finland after the middle of the month, about 2−4 weeks ahead of the normal schedule. In many places the ice cleared exceptionally early, with new records being set in lakes Näsijärvi and Kallavesi. Towards the end of the month the ice covers also began to weaken in the north, with the exception of northernmost Lapland. Ice thicknesses in northern Lapland at the end of the month were generally 60−70 cm and around I metre in lake Kilpisjärvi. Surface water temperatures at the end of April were 8−10ºC in southern and western regions and 5−9ºC in central Finland. These figures were above the seasonal mean.

Maps, graphs and tables

Published 2014-08-27 at 13:02, updated 2014-08-27 at 13:02