Monthly hydrological report in August 2001
High water levels in northern Lapland; water deficits in the south
Precipitation in northern and western Lapland was as high as 250 mm, and the drought of spring and early summer ended. In the south of the country, water reserves were rather low by the end of August and the waters were unusually warm. Particularly in
eastern Finland the surfaces of larger lakes and groundwater formations decreased rapidly.
Precipitation during August had a very uneven geographical distribution. In the Åland Archipelago and the south-western islands, the coastal zone of the Bothnian Bay and in parts of northernmost Lapland the recorded levels exceeded 100 mm. In most
other parts of the country the precipitation varied between 30 and 80 mm. Particularly low precipitation figures were recorded in Kaakkois-Suomi, Häme, Etelä-Savo, central Lapland and the area around lake Oulujärvi.
Evaporation from watercourses was very high, due to the warm water and strong winds.
Water level and discharge
Water levels and discharges. In northern Lapland and parts of western Lapland, water reserves were exceptionally high due to the period of heavy precipitation which had continued for about two months. The discharge of the river Tenojoki around 20
August was even greater than during the snowmelt period in May. The surfaces of lakes Lokka, Porttipahta and Inari increased considerably. Particularly unusual was the increase in the surface of lake Inari from its low level in early summer almost to the
regulatory maximum: the increase during the period May-August was as much as 150 cm. Further south, water levels generally decreased. The most significant decrease was observed in lake Pielinen, even to 30 cm below the seasonal mean. The level of lake
Saimaa also decreased below the seasonal mean. In many smaller rivers, with the exception of those in Lapland, discharges were low by the end of the month.
The deviation of the level of lake Pielinen from the seasonal mean at the end of August was -30 cm and corresponding figures in some other Finnish lakes were: Kallavesi -17, Saimaa -7, Keitele -6, Päijänne-10, Pyhäjärvi, Säkylä -5, Langelmävesi 0,
Näsijärvi -4, Lappajärvi +9, Lentua -6, Oulujärvi -5, Lokka +47 and Inari +32 cm. The discharge of the river Pielisjoki was 76% of the mean August discharge during the reference period 1961-1990 and corresponding figures in some other rivers were: Vuoksi
105%, Kymijoki 116, Paimionjoki 33, Kokemäenjoki 69, Kalajoki 39, Oulujoki 38, Iijoki 60, Kemijoki 111 and Tornionjoki 140%.
level generally decreased in normal fashion for the time of year, except in parts of Lapland where the levels increased above the seasonal mean. Further south the groundwater levels at the end of August varied on both sides of the seasonal mean.
Particularly in south-western Finland groundwater reserves were in some areas rather low. Soil moisture was very high in northern Lapland.
A change in the type of weather at the beginning of August, with increasing winds, caused mixing of lake waters and surface water temperature decreased from the exceptionally high values of July to somewhat below-average levels in southern and central
Finland. After the middle of the month the water temperatures again in-creased above the seasonal mean, and by the end of August and the beginning of September the deviation from the seasonal mean in southern and central Finland was as much as +1-+3°C.
Particularly warm waters, around 20°C, were recorded in the south-east. In Lapland, water temperatures remained close to their seasonal mean levels throughout the summer.
Tables and graphs