Monthly hydrological report August 2014

Warm weather ended with heavy rainfall, but with only slight effects on watercourses

Surface water temperature in August 2014

The parameters of weather and water reserves in August were divided. Temperatures were unusually high, particularly early in the month. The heat waves of the first week gave way to heavy showers lasting throughout the remainder of the month. Heavy rainfall caused flooding in some built-up areas. Because the water deficiency in soil layers was so great after the long period of hot, dry weather, the precipitation did not result in increased water reserves until the very end of the month. Groundwater levels were also very low after the warm period, but in areas with particularly heavy rainfall some replenishment occurred at the end of August. Lake water temperatures were exceptionally high during the first half of the month but decreased thereafter towards the seasonal mean.

Precipitation

Precipitation in August came mainly in the form of showers, with considerable local variation. Monthly rainfall figures in southern Finland were generally 80–150 mm, although some parts of the south-west received up to 200 mm, over twofold the mean for August. Elsewhere the precipitation figures were 50–100 mm. In central Finland the precipitation was slightly below the seasonal mean, in the north the recorded figures were close to the mean.

Water level and discharge

The hot weather which had started at the beginning of July continued until the second week of August, and during the first half of the month both water levels and discharges continued to decrease. When the period of heavy precipitation began the soil layers absorbed moisture for some time before changes were observed in discharges and water levels at the end of the month. As a result of the rainfall the summer decrease in discharges and water levels ended, and rainfall-induced peak discharges were observed particularly in some smaller rivers of south-western and western regions. In the river Uskelanjoki the recorded peak discharge due to heavy rainfall was the highest of the year so far.

Lake water levels at the end of August varied on both sides of the seasonal mean throughout the country. Of the larger lakes the levels of lakes Pielinen and Kallavesi, as well as many lakes of western Finland, were below the mean. Mean monthly discharges in the major watercourses were generally at or below the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of August was -31 cm, and corresponding figures in some other Finnish lakes were: Kallavesi -15 cm, Saimaa +14, Keitele -18, Päijänne -7, Pyhäjärvi, Säkylä -10, Längelmävesi -17, Näsijärvi +0, Lappajärvi +1, Lammasjärvi -21, Oulujärvi -10, Lokka -56 and Inari -12 cm. The mean monthly discharge of the river Pielisjoki was 75% of the mean for August during the reference period 1971-2000, and corresponding figures in some other major rivers were: Vuoksi 105%, Kymijoki 81, Karjaanjoki 102, Kokemäenjoki 61, Siikajoki 62, Oulujoki 56, Iijoki 82, Kemijoki 89 and Tornionjoki 103%.

Groundwater level

Groundwater levels after the dry summer were generally 20-40 cm below the seasonal mean. In areas with heavy precipitation during August, a small level increase was observed towards the end of the month.

Surface water temperature

Lake surface water temperatures at the beginning of August were exceptionally high. In southern and central Finland the temperatures were 22–24°C, and even in the north figures of 18–20°C were recorded in most areas except north-western Lapland. In Lauritsala, lake Saimaa, the highest ever August temperature was recorded. Cooling of the water layers generally began after the tenth of the month, with decreases of as much as 6–9°C by the end of August. By this time the water temperatures were 14–18°C in southern and central Finland and 10–13°C in Lapland. These figures were close to the seasonal mean.  

Evaporation

As a result of the hot weather, evaporation from soil layers was high early in the month. Evaporation from the great lakes on the basis of watercourse model calculations was 100–170 mm in southern and central Finland and below 100 mm in the north; evaporation from soil layers was 40–70 mm.

Maps, graphs and tables

Published 2014-12-29 at 12:50, updated 2014-12-29 at 12:47