Monthly hydrological report August 2013

Water levels low in Lapland; elsewhere precipitation increased the levels close to the seasonal mean

Surface water temperature in August 2013, Surface water temperature in August 2013

August was warmer than average throughout the country. Precipitation was variable both temporally and regionally, generally high in the east and the south but low in the north and west. Heavy rainfall at the beginning of the month caused transient increases in discharges and water levels in southern and central Finland and even halted the decrease in groundwater levels. In some small southern rivers both water surfaces and discharges increased as a result of heavy precipitation almost to typical spring flood levels, but the end of the month was dry and the discharges decreased rapidly. Water levels in many lakes of Lapland were clearly below the seasonal mean as a result of dry weather. Surface water temperatures in August were generally above the seasonal mean.

Precipitation

Precipitation showed considerable regional and temporal variation. At the beginning of August very heavy precipitation was recorded in some areas, whereas the last third of the month was generally almost without rainfall. Monthly precipitation figures of 20−50 mm in Lapland were only about half the seasonal mean. Parts of eastern and southern Finland received above-average precipitation, in many cases exceeding 100 mm. Elsewhere the precipitation was close to or slightly below the seasonal mean.

Water level and discharge

With the exception of Lapland, the heavy precipitation early in the month was reflected in increased water levels and discharges in many lakes. The greatest effects were observed in southern and eastern Finland. By the end of August both levels and discharges had turned to a decrease as a result of the period with no or very low precipitation. In Lapland water levels decreased throughout the month and discharges were low. By the end of August both water levels and discharges were low in northern Finland but close to the seasonal mean elsewhere. The surface of lake Saimaa, which had been above its normal level since the previous year, decreased to its seasonal mean level early in August. Mean monthly discharges of the main watercourses in August were low almost throughout the country.

The deviation of the level of lake Pielinen from the long-term mean August level was -19 cm, and corresponding figures in some other lakes were: Kallavesi -7 cm, Saimaa +2, Keitele -6, Päijänne +5, Pyhäjärvi, Säkylä -8, Längelmävesi -11, Näsijärvi -5, Lappajärvi +23, Lammasjärvi -12, Oulujärvi -12, Lokka -113 and Inari -29 cm. The mean monthly discharge of the river Pielisjoki was 62% of the mean for August during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 101%, Kymijoki 94, Karjaanjoki 42, Kokemäenjoki 79, Siikajoki 82, Oulujoki 67, Iijoki 70, Kemijoki 70 and Tornionjoki 65%.

Groundwater level

Groundwater level decreases halted during August in many areas, particularly in parts of southern and eastern Finland receiving high precipitation. Deviations from the seasonal mean levels were generally between +10 and -20 cm, although in some areas the deficits were as much as 40−50 cm.

Evaporation

Evaporation sums in August were close to the seasonal mean in all regions. Evaporation from Class A pans was generally 55−80 mm in southern and central Finland and 30−60 mm in the north. On the basis of watercourse model calculations evaporation in August was 30−70 mm from soil layers and 80−120 mm from lakes in most parts of the country.

Surface water temperature

Lake surface water temperatures in August were generally above or close to the seasonal mean. During the first week of the month, temperatures of 20−23˚C were still recorded in southern Finland as far north as Pohjois-Pohjanmaa. After the tenth of the month the waters started to cool when the warm period ended, but renewed summery weather at the end of August caused a halt in the cooling. By the end of the month surface water temperatures were 16−19˚C in southern Finland, 13−16˚C in central Finland and Kainuu and 11−15˚C in Lapland. These figures were 0−3˚C above the seasonal mean throughout the country.

Maps, graphs and tables

Published 2013-11-22 at 13:07, updated 2013-11-22 at 12:59