Monthly hydrological report August 2011

Heavy discharges in Keski-Pohjanmaa, groundwater levels low in many areas

August was warmer than normally throughout the country and monthly precipitation varied on both sides of the seasonal mean. Precipitation was very heavy in some areas, particularly in Pohjanmaa and in central and eastern Finland, where rainfall increased both watercourse water levels and discharges. High discharges were recorded particularly in Keski-Pohjanmaa; by contrast in the larger lakes of Eastern Finland the water levels were still low. Groundwater levels in August were below the seasonal mean in many areas, although in regions with high precipitation the groundwater reserves were replenished. Variation in evaporation was high; surface water temperatures were above the seasonal mean.

Precipitation

Precipitation during August was on both sides of the seasonal mean. Monthly precipitation figures showed wide regional variation; in some areas below 40 mm and in others over 150 mm. In some local heavy showers, e.g. in Helsinki, considerable masses of water accumulated in only a few hours. The highest precipitation was recorded in Pohjanmaa, the source waters of the river Kymijoki, easternmost Finland, Kymenlaakso and in the area of Greater Helsinki. Heavy rainfall also fell in northern Lapland towards the end of the month.

Water level and discharge

The levels of the larger lakes continued their slow decrease during August, but in some smaller lakes and particularly in rivers occasionally heavy rain caused the levels to vary. Particularly in Pohjanmaa the discharges of many rivers were very high for the time of year. In Etelä-Pohjanmaa the discharge peaks occurred close to the 10th of August, further north towards the end of the month. By the end of August water levels in Pohjanmaa were still in many cases above the mean. In eastern Finland, the levels of many of the larger lakes were still below the seasonal mean at the end of the month, particularly in lakes Saimaa and Pielinen, where the difference from the seasonal mean was almost half a metre. Elsewhere in the country, water levels were generally close to the long-term seasonal mean levels. Discharges from the major watercourses were variable. The discharge of the river Vuoksi was clearly below the seasonal mean due to regulated level reductions which were started in June, and the discharges of the rivers Kokemäenjoki and Kemijoki were also low. The discharge of the river Kymijoki was typical for August and that of the river Oulujoki was slightly above the mean. In the smaller rivers of the southern coastal zone, discharges were generally rather low and close to the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of August was -47 cm, and corresponding figures in some other lakes were: Kallavesi -5 cm, Saimaa -47, Keitele +8, Päijänne -4, Pyhäjärvi, Säkylä +2, Längelmävesi -14, Näsijärvi -14, Lappajärvi +39, Lammasjärvi +6, Oulujärvi +16, Lokka -156 and Inari +0 cm. The discharge of the river Pielisjoki was 64% of the mean August discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 63%, Kymijoki 99, Karjaanjoki 83, Kokemäenjoki 56, Kalajoki 280, Oulujoki 113, Iijoki 134, Kemijoki 75% and Tornionjoki 68%.

Groundwater level

Groundwater levels in August were generally 20–40 cm below the seasonal mean, in southern and eastern Finland 30–40 cm below. Precipitation during the month replenished the soil moisture reserves particularly in Pohjanmaa and Keski-Suomi, where groundwaters were 10–30 cm above the seasonal mean.

Evaporation

Evaporation during August showed great regional and temporal variation. During warm weather early in the month evaporation grew to typical summer levels in many areas, but during rainy weather and generally towards the end of the month the daily precipitation figures were rather low. The monthly evaporation sum measured using Class A pans was 50–95 mm. In the north the evaporation was close to the seasonal mean; elsewhere it was below the mean. On the basis of watercourse model calculations the precipitation during August was generally 30–70 mm from soil areas and 50–110 mm from the larger lakes.

Water temperature

Surface water temperatures were above the seasonal mean at the beginning of August throughout the country, although they had already started to cool. By the 10th of the month the temperatures had decreased approximately to the seasonal mean. Towards the end of August warm weather caused a renewed increase in water temperatures. The recorded temperatures at the end of August were 16–19°C in southern and central Finland, 15–17°C in Kainuu and Pohjois-Pohjanmaa and 11–15°C in Lapland. These figures were generally around 2°C above the seasonal mean, i.e. they were rather high.

Maps, graphs and tables

ISSN-L 0358-6367
ISSN 1799-6899

Published 2011-10-11 at 10:02, updated 2013-07-01 at 12:26