Monthly hydrological report September 2014

Surface- and groundwater levels low in many areas

Precipitation in September 2014

The beginning of September was dry and warm for the time of year, whereas the end of the month was in many areas cool and wet. Water levels and discharges were initially low in many areas, but heavy rainfall at the end of September caused clear level increases. Generally, both surface and groundwater levels were below the seasonal mean throughout the month. Surface water temperatures decreased during September in normal fashion, although the recorded temperatures were above the seasonal mean almost throughout the month.

Precipitation

The beginning of the month was very dry, but particularly in eastern Finland the accumulated precipitation towards the end of September corresponded almost to the normal amount for the whole of the month. In the west, precipitation was generally very low. Monthly precipitation figures varied between below 20 mm in the west and over 70 mm in eastern Finland. In most parts of the country the monthly precipitation was clearly below the seasonal mean, in the west only about one third of the mean. In northern and eastern Finland some of the precipitation came as snow towards the end of the month, but soon melted.

Water level and discharge

Due to the low precipitation, water levels were generally decreasing during September. However, towards the end of the month the levels began to increase particularly in northern and eastern Finland. In central and western regions lake levels were generally below the seasonal mean, and lakes Kallavesi and Pielinen in the east were also low.

Monthly discharges of the main watercourses were mainly below the seasonal mean, particularly near the western coast, as a result of dry weather both during September and earlier in the summer. In some smaller watercourses a few days with heavy precipitation towards the end of the month resulted in transient level increases, but renewed dry weather later caused a return of the decreasing trend.

The deviation of the level of lake Pielinen from the seasonal mean at the end of September was -43 cm, and corresponding figures in some other lakes were: Kallavesi -19 cm, Saimaa +5, Keitele -24, Päijänne -13, Pyhäjärvi, Säkylä -16, Längelmävesi -19, Näsijärvi +0, Lappajärvi -3, Lammasjärvi -28, Oulujärvi -10, Lokka -61 and Inari -11 cm. The monthly discharge of the river Pielisjoki was 68% of the mean for August during the reference period 1971-2000, and corresponding figures in some other watercourses were: Vuoksi 100%, Kymijoki 74, Karjaanjoki 127, Kokemäenjoki 49, Siikajoki 32, Oulujoki 74, Iijoki 81, Kemijoki 76 and Tornionjoki 71%.

Groundwater level

Groundwater levels in most parts of the country were 10–45 cm below the seasonal mean. In parts of southern Finland deviations of 0–20 cm above the mean were observed, although the levels were decreasing. In areas with low precipitation the dry weather was reflected in diminished water reserves, and the levels were low e.g. in eastern regions.

Surface water temperature

The cooling of surface water levels which had begun in August halted at the beginning of September due to the warmer weather, particularly in southern and central Finland. Water temperatures were exceptionally high in some areas during the first half of the month. Cooler weather at the end of September caused considerable decreases in water temperature. In Lapland, water temperatures decreased steadily throughout the month. Surface water temperatures at the end of September were 8–13°C in southern and central Finland and 4–7°C in Lapland. These temperatures were typical for the time of year.

Evaporation

Evaporation from the warm watercourses was rather high during September, although the dry conditions limited evaporation from soil layers and from vegetation. Evaporation figures for the whole month on the basis of watercourse model calculations were generally 60–100 mm from the larger lakes and 20–40 mm from soil layers.

Maps, graphs and tables

Published 2014-12-29 at 13:09, updated 2014-12-29 at 13:09