Monthly hydrological report August 2009
Surface- and groundwater levels low in many areas
Mean temperatures in August were above the seasonal mean almost throughout the country, particularly in Lapland. Precipitation varied around the mean. Watercourse and groundwater levels decreased in typical summer fashion. Water levels in the watercourses of Lapland and eastern parts of the Vuoksi basin at the end of the month were at or above the seasonal mean; in most of southern and central Finland the levels were rather low. Discharges were generally below the seasonal mean. Groundwater levels were low in most parts of the country. Evaporation varied on both sides of the mean. Lake surface water temperatures were mainly above the mean.
Precipitation in August varied on both sides of the seasonal mean. Monthly precipitation was generally 40...110 mm, i.e. 50...150% of the mean. In many southern and central regions precipitation was low, in Lapland rather high. The highest precipitation was in Pohjois-Pohjanmaa and in Lapland and the lowest in eastern parts of Kainuu. Particularly during the last days of the month some heavy regional precipitation was recorded.
Water level and discharge
Water levels decreased during August in most areas, as is typical for the summer season. The levels turned to an increase in some areas at the very end of the month due to heavy precipitation. In southern and central Finland the levels were below the mean in many regions, particularly in parts of Pohjois-Savo, Keski-Suomi and Pirkanmaa. In Lapland and easternmost Finland the levels were closest to the seasonal mean. Discharges of the major watercourses were rather low, with the exception of the river Vuoksi. Very low discharges were also recorded in the smaller rivers of the coastal zone and in the county of Pohjanmaa. In Pohjanmaa the peak discharges during June-August were exceptionally low in many cases.
The deviation of the level of lake Pielinen from the seasonal mean at the end of August was -6 cm and corresponding figures in some other lakes were: Kallavesi -26 cm, Saimaa -4, Keitele -24, Päijänne -15, Pyhäjärvi, Säkylä -12, Längelmävesi -14, Näsijärvi -14, Lappajärvi -9, Lammasjärvi -23, Oulujärvi -2, Lokka -54 and Inari +1 cm. The discharge of the river Pielisjoki was 94% of the mean August discharge during the reference period 1971-2000 and corresponding figures in some other rivers were: Vuoksi 98%, Kymijoki 80, Karjaanjoki 43, Kokemäenjoki 61, Kalajoki 32, Oulujoki 78, Iijoki 71, Kemijoki 60 and Tornionjoki 50%.
Groundwaters continued to decrease in August as a result of the warm weather. In most parts of the country the levels were 10...30 cm below the seasonal mean. In coastal areas the deficit was 0...20 cm and in central and eastern Finland 15...40 cm.
Evaporation in August showed high regional variation, as in the case of precipitation. The recorded evaporation from Class A pans was 60...100 mm, on both sides of the mean. In the north, evaporation was high due to the warm weather. Lake evaporation in August as based on watercourse model calculations was about 80 mm from lake Päijänne, 60...70 mm from Saimaa and Pielinen, 90 mm from Oulujärvi and 30 mm from lake Inarijärvi. Soil evaporation according to the model was generally 40...80 mm in southern and central Finland and 30...50 mm in Lapland.
Surface water temperature
Surface water temperatures of internal waters were above the mean during most of August. Waters were very warm in Lapland at the beginning of the month. At this time the highest water temperatures of the summer were recorded in northern Finland, over 20°C in lakes even as far to the north as lake Inari. The waters began to cool starting in mid-August. By the end of the month, water temperatures in southern and central Finland and in the county of Oulu were 14...18°C and in Lapland 10...14°C. These figures were rather close to the seasonal mean.
Tables and graphs
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