Monthly hydrological report October 2011
Water levels high in Lapland and in central Finland
October was warmer than normally. In southern and central Finland the precipitation was below-average but in Lapland rather high precipitation was recorded. Water levels were high in central Finland and in Lapland, although in many areas they had decreased from even higher earlier levels. Groundwater levels increased as a result of autumnal precipitation. As a result of the mild weather during October, evaporation was still rather high. Surface water temperatures during October continued to be above the seasonal mean.
October precipitation was high in Lapland but below the seasonal mean in southern and central Finland. The monthly precipitation of 30–100 mm represented 70–200% of the seasonal mean. The highest precipitation occurred in the source regions of the river Ounasjoki and the lowest in inland areas of southern and central Finland. The first snowfalls were recorded in parts of Lapland on 11–12 October, but by the end of the month no snow persisted in any part of the country.
Water level and discharge
Autumn rainfall increased water levels to very high levels in central and western regions, although the levels began to decrease again in many lakes towards the end of the month. However, the levels recorded at the end of October were still considerably above the seasonal mean at the end of the month. In the larger lakes of eastern Finland water levels were generally decreasing. Lakes Saimaa and Pielinen were still below their mean levels but in several other lakes, e.g. Kallavesi and Kyyvesi, the levels were still clearly above the seasonal mean at the end of the month. In Lapland, the precipitation caused level increases early in the month in both lakes and rivers, and in western Lapland some long-term level and discharge records were almost reached. The increase in the level of lake Inari continued almost to the end of the month, when the recorded level was 35 cm above the seasonal mean. Both river water levels and discharges also increased in south-western Finland as a result of the high precipitation, although changes in lake water parameters in these areas were rather small and water levels at the end of the month were close to the seasonal mean. The discharges of the major river watercourses varied on both sides of the mean; in the rivers Vuoksi and Oulujoki the discharges were low but in the rivers Kymijoki, Kokemäenjoki and Kemijoki they were high. The discharge of the river Kemijoki was exceptionally high for the time of year and even higher than that recorded in the peak spring flood. The mean monthly discharges of the rivers Tornionjoki and Ounasjoki represented new all-time records for these watercourses in October.
The deviation of the level of lake Pielinen from the seasonal mean at the end of October was -27 cm, and corresponding figures in some other lakes were: Kallavesi +23 cm, Saimaa -33, Keitele +39, Päijänne +28, Pyhäjärvi, Säkylä +4, Längelmävesi -5, Näsijärvi -4, Lappajärvi +58, Lammasjärvi +12, Oulujärvi +40, Lokka -99 and Inari +37. The discharge of the river Pielisjoki was 66% of the mean October discharge during the reference period 1971-2000, and corresponding figures in some other river watercourses were: Vuoksi 76%, Kymijoki 141, Karjaanjoki 161, Kokemäenjoki 133, Kalajoki 164, Oulujoki 89, Iijoki 152, Kemijoki 154 and Tornionjoki 290%.
Groundwater levels increased during October as autumnal rainfall replenished the groundwater reserves. The recorded levels were generally 15–45 cm above the seasonal mean. Along the southern coastline, in the province of the Åland Islands and along the eastern boundary groundwaters were in some areas below their mean levels, but even here the levels were increasing.
As a result of the mild weather during October, evaporation was still rather high. Evaporation from the larger lakes of southern and central Finland was 40–60 cm and from soil areas 10–20 mm.
Surface water temperatures remained above the seasonal mean during most of October. The recorded temperatures decreased during the month by 3–6°C. By the end of October the surface temperatures were 5–8°C in southern and central Finland and below 5°C in the north. These figures were in many cases about 2°C above the seasonal mean. No ice layer had yet formed over the lakes of Lapland by the end of the month.
Tables and graphs
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