Monthly hydrological report July 2012

Lake surfaces high in central and eastern Finland; water temperatures increased during warm weather

July was rather cool in Lapland but temperatures were close to the mean elsewhere. Precipitation was high in many regions, and exceptionally high rainfall caused flooding in Etelä-Pohjanmaa. In the eastern Lake District and in the basin of the river Ounasjoki the surfaces of larger lakes were unusually high. Extra water release was started in lake Saimaa and was increased towards the end of the month. Groundwater levels were also high. Lake water temperatures were slightly below the seasonal mean but increased at the end of the month.

Precipitation

Precipitation in July was high particularly in central Finland. The highest figures were recorded in Etelä-Pohjanmaa, Keski-Suomi, Itä-Suomi and Kainuu, where the monthly precipitation of 100–170 mm was even twofold the seasonal mean in some places. In Uuusimaa, Häme and Lapland the precipitation was slightly below the seasonal mean, generally 50–80 mm. The precipitation in July came largely in the form of discrete showers. Heavy thunderstorms caused floods in several cities throughout the country.

Water level and discharge

Heavy rainfall at the beginning of July increased water levels and discharges particularly in Pohjanmaa and Keski-Suomi. Exceptionally high discharges were recorded in Etelä-Pohjanmaa, in some cases even reaching the level of a heavy spring flood. The floods covered hundreds of hectares of fields, blocked local roads and dislocated jetties and other shoreline possessions. After mid-July the heaviest rainfall occurred in eastern Finland and in Kainuu, where the levels of even the larger lakes increased rapidly. Particularly high levels were recorded in the unregulated lakes Pielinen, Konnevesi, Keitele and Saimaa, but some regulated lakes e.g. Päijänne and Oulujärvi also had very high levels despite considerable water release. The level of lake Saimaa increased by 10 cm in only five days. Extra water release was initiated in lake Saimaa at the beginning of July and was increased towards the end of the month in order to check the level increase. Precipitation in mid-July also increased discharges in the rivers Tornionjoki and Simojoki. Watercourse discharges were above the seasonal mean almost throughout the country. Only in the smaller rivers of the southern coastal zone and in northern Lapland were below-average or average discharges recorded.

The deviation of the level of lake Pielinen from the seasonal mean at the end of July was +60 cm, and corresponding figures in some other rivers were: Kallavesi +41 cm, Saimaa +57, Keitele +38, Päijänne +26, Pyhäjärvi, Säkylä -1, Längelmävesi +7, Näsijärvi +7, Lappajärvi +25, Lammasjärvi +42, Oulujärvi +39, Lokka -12 and Inari +17 cm. The mean monthly discharge of the river Pielisjoki was 131% of the mean July discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 124%, Kymijoki 153, Karjaanjoki 105, Kokemäenjoki 106, Kalajoki 248, Oulujoki 200, Iijoki 154, Kemijoki 135 and Tornionjoki 137%.

Groundwater level

Groundwater reserves were exceptionally high during July in most areas, with water levels generally 20–30 cm above the seasonal mean; in parts of central and eastern Finland and in southern Lapland the deviation reached as much as 30–50 cm. The abundance of groundwater was due to the wet summer weather following a winter with high snowfall and a wet autumn in 2011.

Evaporation

Evaporation was clearly below the seasonal mean due to the rainy weather and rather cool temperatures. In southern and central Finland the highest evaporation figures were recorded at the beginning and the end of the month, whereas in mid-July the daily precipitation figures were generally low. Further north the evaporation was more even throughout the month. The evaporation measured using Class A pans varied from 70–80 mm in Pohjois-Pohjanmaa and Kainuu to around 110 mm in southern and central Finland. The evaporation from the larger lakes on the basis of watercourse model calculations was generally 100–150 mm and that from soil layers was 60–100 mm.

Surface water temperature

Lake waters were generally cooler than normally at the beginning of July, but a period of hot weather from the 6th to the 10th of the month increased the temperatures to 20–22°C. Thereafter the waters cooled as a result of cooler weather. Surface water temperatures turned again to an increase around the 25th of the month, when it is generally expected that cooling begins, and the highest temperatures of the summer season were measured in southern and central Finland at the end of July and the beginning of August. Water temperatures at the end of July were 19–23°C in southern and central Finland, 17–20°C in Kainuu and southern Lapland and 10–14°C in northern Lapland. These figures were generally 0–2°C above the seasonal mean; in northern Lapland the waters were however even exceptionally cool.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2012-09-10 at 13:07, updated 2013-07-01 at 12:27