Monthly hydrological report March 2011

Snow cover still heavy; lake ice thicknesses almost unchanging

Melting of snow began after the middle of March in southern and western Finland but more snow fell during the last week of the month even in the south. In southern and central Finland the snow cover was still heavier than normally, whereas in Lapland there was rather little snow. Due to cold weather the winter decrease in water levels continued, and discharges remained low. Watercourse levels were low particularly in southern Finland and in the basin of the river Vuoksi. Groundwater levels were also low almost throughout the country. Groundfrost was weak in southern and central Finland but heavy in the north. Lake ice thicknesses were almost constant during March, although some decrease in the proportion of black ice occurred in some areas.

Precipitation

Precipitation during March varied generally on both sides of an average of 30 mm except in northern Lapland, where the precipitation was only about 10 mm in some areas. The monthly precipitation was close to or slightly below the seasonal mean. Most of the precipitation came as snow.

Water equivalent of snow

The water equivalent of the snow (mm), i.e. the snow burden (kg/m2) increased during early March in all regions, particularly in southern Lapland and Pohjois-Pohjanmaa. Melting of snow began after the middle of the month in southern and western Finland, although the water equivalent did not decrease appreciably. During the last week of March falls of snow were again recorded even in the south. By the end of the month the water equivalent of the snow cover was generally 130–180 mm. In south-western Finland, Häme, Etelä- and Pohjois-Pohjanmaa and southern Lapland the water equivalents were lower, 100–130 mm. The highest levels, even exceeding 200 mm, were recorded in easternmost Finland. In southern and central Finland the amount of snow was well above the seasonal mean, in some coastal regions even double the mean. By contrast, the amount of snow in most parts of Lapland was exceptionally low

Water level and discharge

Due to the cold weather in March, very little meltwater entered the watercourses and therefore water levels continued to decrease and discharges remained low. In unregulated lakes the changes in water levels were mainly rather small, but particularly in areas with high snowfall the levels of many regulated lakes were decreased considerably in order to provide sufficient room when the snowmelt began. In southern and central Finland and in southern Lapland water levels at the end of the month were generally below the seasonal mean. Measured deviations from the seasonal mean were greatest, even half a metre, in larger lakes of the river Vuoksi watercourse. In other parts of the country water levels were closer to the mean. Discharges of the major watercourses were below the mean, particularly in southern and eastern Finland.

The deviation of the level of lake Pielinen from the seasonal mean at the end of March was -27 cm and corresponding figures in some other lakes were: Kallavesi -8 cm, Saimaa -7 cm, Keitele -11 cm, Päijänne -11 cm, Pyhäjärvi, Säkylä -48 cm, Längelmävesi -24 cm, Näsijärvi 0 cm, Lappajärvi 0 cm, Lammasjärvi   -10 cm, Oulujärvi -6 cm, Lokka -110 cm and Inari +4 cm. The monthly discharge of the river Pielisjoki was 65% of the mean during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 71%, Kymijoki 66 %, Karjaanjoki 48 %, Kokemäenjoki 60 %, Kalajoki 60 %, Oulujoki 46 %, Iijoki 85 %, Kemijoki 79 % and Tornionjoki 90%.

Groundwater level

Groundwater levels in March were 20–40 cm below the seasonal mean in most areas except Lapland, where the deficit was 5–20 cm. Because temperatures remained rather close to zero, the effects of meltwaters were not yet seen in groundwater levels.

Groundfrost

Groundfrost depths at the end of March were generally 0–30 cm, i.e. 5–20 cm below the seasonal mean. The maximum depth of groundfrost was reached in eastern Finland at the middle of the month, where the measured depths of 10–40 cm were 0–10 cm greater than normally. In northern Finland the depths at the end of the month were 40–160 cm, which was 10–40 cm greater than the seasonal mean.

Ice thickness

Overall ice thicknesses hardly changed during March, although in parts of southern and central Finland the depth of black ice decreased by 2–7 cm. Ice thicknesses at the end of the month were 50–70 cm almost throughout the country. In southern and central regions these figures were still 5–10 cm above the seasonal mean; in northern Finland they were close to or slightly below the seasonal mean. In southern and central Finland the proportion of snow ice of the overall thickness was high, even above 50%. In the north the depth of snow ice was 5–10 cm.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2011-08-15 at 12:58, updated 2013-07-01 at 12:21