Monthly hydrological report September 2009
Water levels low in western and central Finland
September was considerably warmer than normally. Precipitation was below the seasonal mean in western and central regions but above the mean in the north. Lake water levels decreased except in areas with the highest precipitation. Surface- and groundwater levels were very low at the end of the month in western and central Finland; in other areas near-normal levels were recorded. River discharges in Ostrobothnia were very low, as they were throughout the summer. Surface water temperatures of lakes were above the seasonal mean. Evaporation was above the seasonal mean, particularly in the west, due to the warm weather and low precipitation.
Precipitation in September was on average close to the seasonal mean. In western and central regions the rainfall was clearly above the mean, whereas in northern Finland the precipitation figures were high. Monthly precipitation was generally 20...90 mm, i.e. 40...160% of the mean. The highest rainfall was recorded in southern Lapland and the lowest in southern Ostrobothnia. The first falls of snow were recorded in the last days of the month in parts of Lapland, Koillismaa and Kainuu.
Water level and discharge
The rainfall of early September caused increases in lake water levels. In southern and central Finland the increase was only a few centimetres, whereas in Lapland level increases of up to 20 cm were recorded. Water levels decreased towards the end of the month almost throughout the country. In central and western Finland the levels were still below the seasonal mean at the end of the month. In the county of Pirkanmaa, the surfaces of e.g. lakes Palovesi and Tarjanne were exceptionally low and the low level of lake Toisvesi set a new record. Lake Näsijärvi was also regulated to a very low level due to dam construction works in the Tammerkoski rapids. In other parts of the country water levels were closer to the seasonal mean, in southern Lapland even above the mean due to heavy precipitation. Discharges of the major watercourses were below the mean in southern and central Finland but close to or above the mean in Lapland. The discharges of rivers in Lapland reached a small flood peak level as a result of the heavy precipitation. By contrast the discharges of the rivers of Ostrobothnia were low, as they were throughout the summer season. Peak discharges in June-September were exceptionally low in Ostrobothnia, and discharges in rivers of the southern coastal zone were also low.
The deviation of the level of lake Pielinen from the mean for September was -8 cm and corresponding figures in some other lakes were: Kallavesi -23 cm, Saimaa -15, Keitele -32, Päijänne -21, Pyhäjärvi, Säkylä -15, Längelmävesi -13, Näsijärvi -21, Lappajärvi -19, Lammasjärvi -25, Oulujärvi +7, Lokka -43 and Inari +15 cm. The discharge of the river Pielisjoki was 92% of the mean September discharge during the reference period 1971-2000 and corresponding figures in some other rivers were: Vuoksi 101%, Kymijoki 65, Karjaanjoki 61, Kokemäenjoki 39, Kalajoki 32, Oulujoki 67, Iijoki 100, Kemijoki 113 and Tornionjoki 83%.
Low precipitation delayed replenishment of groundwater reserves. Groundwater levels were low in September in most parts of the country. In western and central Finland the levels were 20...40 cm below the seasonal mean; elsewhere near-normal levels were recorded. High evaporation due to the warm weather in September increased the soil moisture deficit in western and central regions.
Evaporation was high in western Finland due to the warm and dry weather in September. In eastern and northern regions evaporation was lower due to the higher precipitation. Evaporation from Class A pans was 25...60 mm, i.e. generally above the seasonal mean. Lake water evaporation on the basis of watercourse model calculations was 35 mm from lake Päijänne, 20...30 mm from Saimaa and Pielinen, 40 mm from Oulujärvi and 10 mm from lake Inari. The soil evaporation sum for the whole country was generally 20...40 mm on the basis of watercourse model calculations.
Surface water temperature
Due to the warm weather in September surface water temperatures were above the seasonal mean. In parts of southern and central Finland the waters were even exceptionally warm. The temperatures decreased during September generally by 4...8°C, particularly during a cool period during the last days of the month. By the end of the month surface water temperatures were 6...13°C in southern and central Finland and 2...8°C in Lapland. These figures varied on both sides of the seasonal mean.
Tables and graphs
Top of the page