Monthly hydrological report April 2011

Rapid melting of snow caused heavy spring floods in coastal rivers

April was dry and warm throughout the country. Melting of snow was rapid and began earlier than normally, particularly in Pohjois-Pohjanmaa. By the end of the month, appreciable depths of snow persisted only northern Lapland. Water levels increased rapidly with the input of meltwater, and spring floods were clearly heavier than normally in coastal regions. In southern and central Finland, meltwaters increased water levels in many areas from their rather low levels close to the seasonal mean. Groundwater levels at the end of the month were also at or even above the seasonal mean in southern and western Finland. Elsewhere, groundwater levels were still generally below the mean. Groundfrost melted in April in southern and central Finland almost completely. Break-up of ice from the lakes of southern and western Finland occurred at the end of the month. In all other areas except northern Lapland, ice thicknesses weakened so that measurement was no longer possible.

Precipitation

April was rather dry throughout the country. Precipitation figures were clearly below the seasonal mean, in western Finland even below 50% of the mean. In Pohjanmaa, north-western Lapland and south-eastern Finland the monthly precipitation was as low as about 10 mm, and the highest figures recorded in south-western and eastern Finland were only ca. 30 mm. Almost all the precipitation during April came as rain.

Water equivalent of snow

The warm weather began to melt snow rapidly from the beginning of the month almost throughout the country; only in northernmost Lapland did the snow burden increase slightly. Even there, melting of snow began at the end of the first week of the month. By April 20th some completely snow-free areas were recorded south-western Finland, and by the end of the month all the snow had melted to the south of a line from Oulu to Ilomantsi and in wide areas of southern Lapland. Elsewhere, with the exception of northern Lapland, only patches of snow were recorded in the forest. In northernmost Lapland the water equivalent of the snow cover still exceeded 100 mm. Particularly in the north of the country, melting of snow occurred ahead of the normal schedule, and amounts of snow in April were exceptionally low.

Water level and discharge

Both water levels and discharges increased sharply in southern and western Finland at the beginning of April with warming of the weather. In eastern Finland the increase in water levels began somewhat later and was slower. River peak spring floods in the coastal region were considerably higher than normally, and in many areas occurred rather ahead of the normal schedule. Ice dams further increased water levels in many rivers, although major flood damage was avoided. In southern and central Finland maximum water levels in smaller lakes of the source regions of drainage basins were reached during April and water levels turned to a decrease towards the end of the month. In larger lakes, receiving water from wide areas, the increase in water levels was slower and continued towards the end of April. In Lapland the early melting of snow caused water level increases ahead of the normal schedule; however, because of the small amount of snow the levels did not increase exceptionally. In the course of the river Simojoki the below-average spring flood was experienced during April, but elsewhere in the country the water levels continued to increase at the end of the month. Due to the early onset of the spring flood, water levels at the end of April were above the long-term seasonal mean throughout most of Lapland. In other parts of the country meltwaters increased water levels close to the long-term seasonal mean.

The deviation of the surface of lake Pielinen from the long-term seasonal mean at the end of April was +15 cm and corresponding figures in some other major lakes were: Kallavesi +17 cm, Saimaa -27, Keitele +11, Päijänne +7 cm, Pyhäjärvi, Säkylä +14, Langelmävesi -9, Näsijärvi +2, Lappajärvi +30, Lammasjärvi +51, Oulujärvi +47, Lokka -113, and Inari +11 cm. The discharge of the river Pielisjoki during April was 77% of the mean for the reference period 1971-2000, and corresponding figures in some other lakes were: Vuoksi 71%, Kymijoki 81 %, Karjaanjoki 115 %, Kokemäenjoki 107 %, Kalajoki 155 %, Oulujoki 57 %, Iijoki 158 %, Kemijoki 156 % and Tornionjoki 179%.

Groundwater level

Groundwater levels increased in early April in southern and western Finland, and were 0-30 cm above the long-term seasonal mean by the end of the month in many areas. In central and eastern Finland groundwater levels increased towards the end of the month but were still generally 20-40 cm below the seasonal mean. In some localities the levels were above the seasonal mean due to differences between the formations. In northern Finland the groundwater levels were 5-30 cm below the seasonal mean.

Groundfrost

In southern and central Finland the groundfrost layer had melted almost completely by the end of April. Mild night-time temperatures and rainfall both contributed to the melting. The maximum groundfrost thickness was recorded in northern Finland at the beginning of April. By the end of the month, groundfrost persisted in the north to depths of 60-140 cm.

Ice thicknesses and surface water temperatures

Lake ice covers began to weaken in southern Finland at the beginning of April and in central Finland and the county of Oulu after the middle of the month. Break-up of ice occurred in southern and western Finland at the end of the month at approximately the normal time. In central regions and in most of Lapland the ice layers were so weak at the end of April that measurements of ice thickness could not be made. Ice thickness data were obtained only from northernmost Lapland, where thicknesses of 55-60 cm were recorded; in lake Kilpisjärvi ice layers up to 82 cm were measured. These readings were 5-20 cm below the long-term seasonal mean.

After the break-up of ice the surface water temperatures in smaller lakes of southern Finland increased rather rapidly as a result of the warm weather. By the end of the month surface water temperatures in southern and western regions were 4-8°C, in the river Oulankajoki 2°C. These figures were close to the long-term seasonal mean.

Maps, graphs and tables

ISSN-L 0358-6367

ISSN 1799-6899

Published 2011-08-15 at 12:59, updated 2013-07-01 at 12:23