Monthly hydrological report July 2013

Surface and groundwater levels low in areas with low precipitation

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July temperatures were close to but slightly below the seasonal mean. Precipitation was low in most regions. Water levels were generally below the seasonal mean, although heavy showers increased discharges in some small watercourses. Lake water levels were generally decreasing. Groundwater levels decreased considerably in some areas due to evaporation and low precipitation. Surface water temperatures of lakes were above the seasonal mean at the beginning of July but clearly below the mean at the end of the month.

Precipitation

High variation in local precipitation figures was recorded in July due to the patchy distribution of heavy showers. In most regions the monthly precipitation of 40−80 mm was below the seasonal mean. Rainfall in the southern and south-western coastal zone was particularly low, below 40 mm. In parts of Pohjois-Savo and north-western Lapland the precipitation was above the mean, in some areas above 80 mm.

Water level and discharge

In Pohjanmaa, rainfall at the beginning of the month temporarily increased water levels and discharges. Heavy showers e.g. in the county of Uusimaa were so local that they did not cause significant level or discharge increases in the larger watercourses. Overall, the water level and discharge figures for July varied in different parts of the country. In parts of Pohjois-Pohjanmaa the seasonal mean figures were exceeded, whereas in southern and south-western Lapland both water levels and discharges were low. The surface of lake Pielinen continued to decrease in July and by the end of the month the water level was over 40 cm below the seasonal mean. The level of lake Päijänne, which was still increasing in June, began to decrease at the beginning of July and regulatory discharge was decreased. The level of lake Saimaa decreased steadily and by the end of the month was only slightly above the seasonal mean. In Lapland, water levels were close to or below the seasonal mean. Discharges of the major watercourses were also close to or below the seasonal mean.

The deviation of the level of lake Pielinen from the seasonal mean at the end of July was -44 cm, and corresponding figures in some other lakes were: Kallavesi -12 cm, Saimaa +9, Keitele -6, Päijänne +2, Pyhäjärvi, Säkylä -8, Längelmävesi -12, Näsijärvi +3, Lappajärvi +21, Lammasjärvi -29, Oulujärvi -13, Lokka -72 and Inari -16 cm. The mean monthly discharge of the river Pielisjoki was 65% of the mean July discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 98%, Kymijoki 87, Karjaanjoki 54, Kokemäenjoki 68, Siikajoki 174, Oulujoki 70, Iijoki 123, Kemijoki 102 and Tornionjoki 97%.

Groundwater level

Groundwater levels were decreasing, and in areas of southern and central Finland receiving low precipitation were generally 10−40 cm below the seasonal mean. In some cases the deficit even reached 50 cm. Elsewhere, the groundwater deficit was less pronounced and water levels were close to the seasonal mean. High local variation in precipitation did however cause regional differences in groundwater levels.

Evaporation

The evaporation sum in July was below the seasonal mean in many areas. The highest evaporation was recorded at the end of the month in central Finland. Evaporation from Class A pans varied between ca. 50 mm and somewhat below 120 mm. On the basis of watercourse model calculations evaporation in July was generally 50−90 mm from soil layers and 100−180 mm from the larger lakes.

Surface water temperature

Surface water temperatures were clearly above the mean at the beginning of July throughout the country. During the first half of the month the temperatures were rather stable in southern regions but were decreasing in the north. A northern wind cooled the lake waters after the middle of July in all regions. Around the 20th of the month even exceptionally low temperatures for July were recorded in parts of southern and central Finland, but From July 25th the waters started to warm again. By the end of July, surface water temperatures were 18−21˚C in southern and central Finland and 14−18˚C in Lapland. These figures were on both sides of the seasonal mean in the south but around 2˚C above the mean in northern Lapland.

Maps, graphs and tables

Published 2013-11-11 at 9:54, updated 2013-11-11 at 9:51