Monthly hydrological report in October 2007
Warm weather in October; water levels still high in eastern and northern regions
October was warmer than normally, particularly in the north. Monthly precipitation was close to or below the seasonal mean. Most of the precipitation came as rain. Water levels at the end of the month were above the seasonal mean in eastern and northern regions. However, lake water levels in eastern areas with low precipitation began to decrease during the month. Groundwater levels were also above the mean in most areas. Surface water temperatures at the end of the month were above the mean and all lakes were still ice-free throughout the country.
Precipitation during October was at or below the seasonal mean, generally in the range 30-70 mm. The lowest rainfall fell in the river Vuoksi watercourse and in northern Lapland, where the recorded precipitation of 40 mm was only about half the seasonal mean. The highest precipitation occurred in a belt extending from the south-western coast to Kainuu and south-western Lapland, where figures even exceeding 70 mm were recorded in some places. The precipitation fell mainly as rain throughout the country. Some snow fell during the period 9-15 October in scattered areas even in the south of the country. By the beginning of November a thin layer of snow covered parts of Lapland.
Discharge and water level
In most parts of the country water levels hardly changed during October. Water levels and discharges did increase locally in some areas with high precipitation. In eastern Finland, surfaces of unregulated lakes decreased as a result of the low rainfall. The level of lake Saimaa began to decrease during the first half of October and the decrease continued by a further 10 cm during the second half of the month. However, by the end of October water levels were still above the seasonal mean in eastern and northern regions. The levels of natural lakes in the Kokemäenjoki and Kymijoki river watercourses were in places below the seasonal mean. Heavy rain at the end of the month increased discharges in many rivers of Pohjanmaa. In the smaller rivers of the southern coastal zone discharges were close to the seasonal mean. Of the larger watercourses, the discharges of the rivers Vuoksi and Tornionjoki were above the seasonal mean and those of the rivers Kokemäenjoki and Oulujoki were below the mean. The discharges of the rivers Kymijoki and Kemijoki were close to the mean.
The deviation of the level of lake Pielinen from the seasonal mean at the end of October was +18 cm and corresponding figures in some other lakes were: Kallavesi +14 cm, Saimaa +26, Keitele -6, Päijänne -11, Pyhäjärvi, Säkylä +1, Längelmävesi -15, Näsijärvi +2, Lappajärvi +18, Lentua +5, Oulujärvi +36, Lokka +2 and Inari +16 cm. The discharge of the river Pielisjoki was 119% of the mean October discharge during the reference period 1971-2000 and corresponding figures in some other Finnish river watercourses were: Vuoksi 122%, Kymijoki 106, Karjaanjoki 107, Kokemäenjoki 70, Kalajoki 124, Oulujoki 64, Iijoki 95, Kemijoki 109 and Tornionjoki 142%.
Groundwater levels changed very little during October in most areas. The levels were generally 10-30 cm above the seasonal mean in most parts of the country. In parts of northern Finland groundwater levels slightly below the seasonal mean were recorded.
Groundfrost was not recorded during October anywhere in Finland.
On the basis of watercourse model calculations soil evaporation sum during October was below 10 mm throughout the country. Evaporation from lakes was also low, <20 mm.
Surface water temperature
Surface water layers at the beginning of October were rather warm in many areas. During the course of the month, temperatures decreased in typical fashion for the time of year. A period of night frosts before mid-October increased the rate of decrease in temperature, but warmer weather at the end of the month slowed down the cooling rate and caused even slight increases in surface water temperature. By the end of the month, surface water temperatures were 5-8ºC in southern and central regions and 2-4ºC in Lapland. These figures were generally about 2ºC above the seasonal mean. Lakes even in Lapland were still without an ice cover at the end of the month, although the small lakes in northern Lapland and in the tundra regions normally freeze over in late October.
Tables and graphs
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