Monthly hydrological report July 2011
Water levels high in Pohjois-Pohjanmaa, lake waters very warm
July was considerably warmer than normally. Precipitation was high in most parts of the country and exceptionally high in Pohjois-Pohjanmaa and eastern Lapland. Water levels were very high in some regions, and discharges were high in those areas with high precipitation. In eastern Finland water levels were still rather low in many areas. Surface water temperatures were above the seasonal mean almost throughout the month, in some cases setting new records. Groundwater levels remained low in eastern and central Finland.
Precipitation in July was high in most parts of the country. Monthly precipitation varied generally between 20 and 140 mm. In parts of Pohjois-Pohjanmaa and eastern Lapland with the heaviest rainfall the monthly precipitation even reached 150 mm, double the normal for July. The heaviest precipitation occurred during the middle and the end of the month. The driest areas were easternmost Finland and along the southern coastline, where the monthly precipitation was close to or even below the seasonal mean. The very lowest rainfall, below 20 mm, was recorded in the municipality of Ilomantsi.
Water level and discharge
Occasional heavy rainfall caused major oscillations of both water levels and discharges in many rivers. The greatest variation was in Pohjois-Pohjanmaa, where the levels of some rivers increased by as much as several tens of centimeters per day towards the end of the month and discharges were exceptionally high for the time of year. Water levels also increased in many lakes in those areas with the heaviest precipitation. For example water levels in the river Iijoki watercourse were exceptionally high during the last week of the month. In other parts of the country the rainfall at least slowed the normal decrease in lake levels or caused a slight increase. By the end of the month water levels in most parts of the country were close to the seasonal mean. In Pohjois-Pohjanmaa and eastern Lapland, as well as in parts of Keski-Suomi, Kainuu, Etelä-Pohjanmaa and the western coastal zone, water levels were frequently above the seasonal mean, but in eastern Finland with low precipitation they were below the mean. The surface of lake Pielinen was still half a metre and that of lake Saimaa 40 cm below the mean for July. Despite the increases in lakes Lokka and Porttipahta, the levels of these lakes were still exceptionally low.
The deviation of the level of lake Pielinen from the seasonal mean at the end of July was -49 cm, and corresponding figures in some other lakes were: Kallavesi -10 cm, Saimaa -39, Keitele -8, Päijänne +3, Pyhäjärvi, Säkylä +4, Längelmävesi -14, Näsijärvi -24, Lappajärvi +27, Lammasjärvi -8, Oulujärvi +15, Lokka -147 and Inari +10 cm. The monthly discharge of the river Pielisjoki was 71% of the mean during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 66%, Kymijoki 79, Karjaanjoki 74, Kokemäenjoki 78, Kalajoki 188, Oulujoki 72, Iijoki 123, Kemijoki 96 and Tornionjoki 92%.
Groundwater levels during July were generally 0…30 cm below the seasonal mean. In central and eastern Finland the levels were even lower than in other areas, around 10…40 cm below the seasonal mean. Regional high precipitation did cause level increases in some areas, but generally the groundwaters continued to decline in typical style for the time of year.
Evaporation during the warm month of July was high in eastern Finland but lower than average in areas with the highest precipitation. The monthly evaporation sum measured using Class A pans was 90…135 mm. At observation stations in Pohjanmaa and the Åland Islands the recorded figures were clearly below the mean due to the rainy weather; elsewhere the precipitation sum was generally close to or above the seasonal mean. Evaporation during July on the basis of model calculations was generally 50…100 mm from soil layers and 80…150 cm from the larger lakes.
Surface water temperatures were above the seasonal mean almost throughout the month, in some cases setting new records. Temperatures began to increase early in the month and generally reached their peak values in mid-July and again at the end of the month. Mean temperatures for July were in many places exceptionally high, i.e. similar high mean temperatures are recorded only about once in 10 years. Surface water temperatures at the end of the month were 20…24°C in southern and central Finland and in Pohjois-Pohjanmaa and Kainuu, and 13…19°C in Lapland. In southern and central Finland these figures were generally several degrees above the mean, whereas in Lapland the recorded temperatures were close to the seasonal mean.
Tables and graphs
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