Monthly hydrological report September 2013

Watercourse and groundwater levels low, especially in the north

Precipitation in September 2013

September was warm and precipitation was generally below the mean. As a result of the low rainfall, both watercourse and groundwater levels were below the seasonal mean in many areas, and decreasing. Lapland was exceptionally warm and dry, and both surface and groundwater levels were very low. Mean monthly discharges were only half the mean for September in many rivers.  Due to the warm weather, both surface water temperatures and evaporation were high in many regions. In the middle of the month even record-breaking high temperatures were recorded in some watercourses.

Precipitation

Precipitation in September was generally very low. The recorded monthly precipitation figures of 10−40 mm in the west and north of the country were even below half the seasonal mean; 50−80 mm in eastern regions varied on both sides of the mean. The first snow was recorded at the end of September in parts of northern and eastern Finland.

Water level and discharge

Water levels in September were generally decreasing and discharges were low, although rainfall at the end of the month did cause slight increases both in river flows and in the water levels of some small lakes. In northern Finland lake water levels at the end of September were clearly below the seasonal mean; in lakes Inarijärvi and Oulujärvi the deviations from the mean were several tens of centimetres. Elsewhere in the country water levels were close to or slightly below the long term seasonal mean. The discharges of the major watercourses were well below the seasonal mean throughout the country, with the exception of the river Vuoksi; in many of the larger rivers the monthly discharges were only half the mean for September.

The deviation of the level of lake Pielinen from the long term seasonal mean at the end of September was -27 cm, and corresponding figures in some other Finnish lakes were: Kallavesi -11 cm, Saimaa +0, Keitele -12, Päijänne +5, Pyhäjärvi, Säkylä -15, Längelmävesi -17, Näsijärvi -1, Lappajärvi +9, Lammasjärvi -36, Oulujärvi -21, Lokka -153 and Inari -35 cm. The mean monthly discharge of the river Pielisjoki was 73% of the mean September discharge during the reference period 1971-2000, and corresponding figures in some other rivers were: Vuoksi 99%, Kymijoki 79, Karjanjoki 50, Kokemäenjoki 48, Siikajoki 37, Oulujoki 76, Iijoki 38, Kemijoki 53 and Tornionjoki 54%.

Groundwater level

The dry weather in September caused a delay in the autumn replenishment of groundwater reserves. Groundwater levels were generally 10−40 cm below the seasonal mean. Groundwater levels in southern and central regions were 0−30 cm below the seasonal mean, but in the north they were even unprecedentedly low in some areas; in eastern and northern Lapland the recorded levels were 30−70 cm below the mean.

Evaporation

Evaporation in September varied on both sides of the seasonal mean. Above-average figures were recorded e.g. in Maaninka and Rovaniemi, whereas in Jyväskylä and Ylistaro evaporation was below the mean. Evaporation was 15−40 mm from Class A pans, and according to calculations based on watercourse models generally 20−40 mm from land areas and 60−100 mm from the larger lakes. Total evaporation from Class A pans during the summer season (May-September) was 300−500 mm in southern and central Finland and 200−400 mm in Lapland. According to model calculations the figures for evaporation in May-September were 150−350 mm from soil layers and 350−550 mm from the Great Lakes.

Surface water temperature

The warm weather kept water temperatures clearly above the seasonal mean until the last week of September. Many lake water temperatures were unprecedentedly high for the time of year in the middle of the month. During September, surface temperatures decreased by 5−9˚C; this decrease occurred mainly during the last week of the month. Surface water temperatures at the end of September were 7−12˚C in southern Finland and 5−8˚C in Lapland. These figures varied on both sides of the seasonal mean.

Maps, graphs and tables

Published 2014-01-14 at 15:00, updated 2014-01-14 at 14:56