Monthly hydrological report October 2013

Water reserves increased considerably at the end of October

Precipitation in October 2013

October was warmer than normally in southern and central Finland. Precipitation was low at first but increased towards the end of the month. During the last two weeks of October heavy rainfall in southern and central regions caused increases in both water levels and discharges after a dry period of several months. The October rainfall did not yet increase groundwater reserves, although surface water levels increased. Winter weather began in Lapland after the middle of the month; snowfalls were recorded, some smaller lakes froze over and groundfrost began to form. The larger lakes in Lapland were however still ice-free at the end of October, with ice covers only along the shoreline and in shallow bays. In southern and central Finland lake water temperatures were generally above the seasonal mean.

Precipitation

Precipitation was very low at the beginning of October in southern and central Finland but high towards the end of the month. Rainfall during the last two weeks of the month amounted to the normal amount for the whole month; in Lapland the accumulating precipitation was more even. Precipitation figures for October varied between below 50 mm in western Lapland and Pohjanmaa to over 180 mm in parts of southern and eastern Finland, and was above the seasonal mean in most areas.

Snow

Most of Finland received its first snow cover during October, but it did not persist except in parts of Lapland and Koillismaa, where water equivalents at the end of the month were 0−40 mm.

Water level and discharge

The beginning of October was similar to September with regard to discharges and water levels. The recorded figures were generally well below the seasonal mean and water levels were still decreasing. At the end of the month, however, high precipitation caused rapid increases in discharges in eastern and southern Finland and also in Pohjanmaa. By the end of the month discharges in these areas were clearly above the seasonal mean. In Lapland, the discharges remained low throughout the month. Lake water levels increased towards the end of the month almost throughout the country, although surfaces of the larger lakes particularly in the north were still below their seasonal mean at the end of October. Due to the dry start to the month, the mean discharge figures for the whole of October were clearly below normal in the larger watercourses.

The deviation of the level of lake Pielinen from the seasonal mean at the end of October was -16 cm, and corresponding figures in some other Finnish lakes were: Kallavesi -3 cm, Saimaa -14, Keitele -11, Päijänne +8, Pyhäjärvi, Säkylä -16, Längelmävesi -16, Näsijärvi -5, Lappajärvi +3, Lammasjärvi -25, Oulujärvi -26, Lokka -158 and Inari -35 cm. The mean monthly discharge of the river Pielisjoki was 68% of the mean October discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 99%, Kymijoki 71, Karjaanjoki 51, Kokemäenjoki 46, Siikajoki 61, Oulujoki 67, Iijoki 45, Kemijoki 49 and Tornionjoki 77%.

Groundwater level

Groundwater levels were generally 20−80 cm below the seasonal mean. Dry weather in the early autumn delayed the replenishment of groundwater reserves, but heavy rainfall at the end of October caused a turn towards an increasing trend in the water levels in southern and central Finland.

Groundfrost

Groundfrost was recorded to a depth of couple of centimeters in parts of northern Finland.

Evaporation

Due to rather high surface water temperatures, considerable evaporation was still recorded in October. According to watercourse model calculations, evaporation from the Great Lakes was 30−60 mm. Evaporation from soil layers was very low due to their low moisture content.

Surface water temperature and ice cover

Surface water temperatures varied during October on both sides of the seasonal mean according to the weather conditions. Water temperatures were generally decreasing, but warm weather at the end of the month still caused an increase. By the end of the month surface water temperatures in southern and central Finland were 3−7˚C, slightly above the seasonal mean. In Lapland, freezing over of smaller watercourses was completed during a cold period in mid-October. The larger lakes were still without an ice cover, with some ice only along the shorelines and in some bay areas.

Maps, graphs and tables

Published 2014-01-14 at 15:58, updated 2014-01-14 at 15:58