Monthly hydrological report January 2013

Frosty periods strengthened groundfrost and lake ice

Ice-thickness-january2013

Mean temperatures in January were somewhat below the mean in the south but above the mean further north. Precipitation was close to normal. The snow cover did not grow appreciably, but both groundfrost and lake ice increased in thickness during periods of heavy frost. Despite this, groundfrost depths were still below the mean. Ice thicknesses were near-normal, although the structure was rather weak because of the high proportion of snow ice. Watercourse and groundwater levels generally decreased in typical winter fashion.

Precipitation

Precipitation in January was close to the seasonal mean in most regions. Precipitation figures in the north were slightly above and those in southern and central regions slightly below the mean. Monthly precipitation was >60 mm in south-western Lapland, 40−60 mm in other parts of Kainuu and in southern Lapland and generally 20−40 mm elsewhere in the country.

Snow cover

A mild period at the beginning of the year melted some snow in southern and western Finland, after which increase in the water equivalent of snow was slow. Changes in snow amounts during January were also rather low in other parts of the country. In Lapland the water equivalent of the snow cover increased most significantly during the last week of the month, and during the last days snowfalls also brought new snow to other areas. Along the south-western coastline the precipitation came as rain and the weather was mild, which caused some melting. By the end of the month the water equivalent of snow in the western coastal zone was 30−60 mm and elsewhere in southern and central Finland 60−90 mm. From the latitude of Oulu to central Lapland snow water equivalents were 80−130 mm and further north 130−180 mm. In northern Lapland the snow cover was in some areas above the seasonal mean; elsewhere the recorded figures were close to the mean.

Water level and discharge

Mild weather around the beginning of the year caused melting of snow, with the result that water levels and discharges actually increased during the first half of January in southern and central Finland. Elsewhere decreases were observed, which is more normal for the time of year. By the end of the month water levels in the larger lakes of eastern and central Finland were still well above the seasonal mean; elsewhere they were close to the mean.

The discharges of the major river watercourses during January were generally above the seasonal mean. Slush ice dams caused blockages of flow during periods of heavy frost in some unfrozen rivers. In the lower reaches of the river Kymijoki, slush dams increased the water level almost throughout the month.

The deviation of the level of lake Pielinen from the seasonal mean at the end of January was +25 cm, and corresponding figures in some other lakes were: Kallavesi +8 cm, Saimaa +67, Keitele +26, Päijänne +30, Pyhäjärvi, Säkylä -3, Längelmävesi +16, Näsijärvi +2, Lappajärvi +30, Lammasjärvi +1, Oulujärvi +25, Lokka -21 and Inari +31 cm. The mean monthly discharge of the river Pielisjoki was 132% of the January mean during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 141%, Kymijoki 136, Karjaanjoki 114, Kokemäenjoki 111, Siikajoki 73, Oulujoki 104, Iijoki 119, Kemijoki 107 and Tornionjoki 113%.

Groundwater level

In southern Finland groundwater levels increased slightly at the beginning of January; elsewhere the levels continued to decrease in typical winter fashion throughout the month. The recorded levels were generally 10−25 cm above the seasonal mean. In the largest formations the levels were even 20−40 cm above the seasonal mean as a result of high precipitation during the previous year.

Groundfrost

A heavy snow cover and high soil moisture slowed down the rate of groundfrost formation in early winter, but heavy frosts during the first half of January promoted the development of groundfrost. In southern and central Finland groundfrost depths were 0−15 cm; in the north they were 20−60 cm.

Ice cover

Occasional above-zero temperatures at the beginning of January did not affect lake ice. During the periods of heavy frost the ice thicknesses increased, although often by formation of weak snow ice rather than stronger black ice. The overall increase in ice thickness during January was in many lakes 10−15 cm. By the end of the month ice thicknesses were generally 40−60 cm in Lapland and 30−40 cm further south. These figures were rather close to the seasonal mean. The structural quality of ice varied considerably between different lakes, and in some cases the proportion of black ice was rather low.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899
Published 2013-05-08 at 12:00, updated 2013-07-01 at 13:01