Monthly hydrological report in October 1999
Temperatures during October were 1-2.5°C higher than the seasonal mean. Precipitation was heavy in western Finland, and river discharges in this region were high for a short period during the middle of the month. Lake- and groundwater levels also
increased in this area from the very low levels recorded at the beginning of the autumn season. An unusual deficit of water continued in the larger lakes and groundwater formations of eastern Finland. In Lapland, however, lake water resources were still
abundant. Accumulation of snow and formation of ice over watercourses began from the north of the country about two weeks later than normally. In southern and central districts, water temperatures were above the seasonal mean.
Precipitation during October in western Finland was high, exceeding 120 mm or double the seasonal mean in wide areas. In northern areas of eastern Finland, in northern Ostrobothnia and in western Lapland, however, the precipitation was at or even below
the seasonal mean, with recorded figures of 40-60 mm. A permanent snow cover began to accumulate in northernmost parts of the country only towards the end of the month; temporary coatings of snow were also recorded further south.
Evaporation from watercourses continued to be above the seasonal mean.
Water levels in rivers of western Finland were rather high at about the middle of the month, and lake levels also increased. In the larger lakes of central and eastern Finland water levels increased only slightly, and the level of lake Saimaa continued
to decrease. All lakes in the north of the country contained high volumes of water and lake levels were still increasing. The deviation of the level of lake Pielinen from the long-term seasonal mean at the end of the month was –53 cm and
corresponding figures in some other Finnish lakes were: Kallavesi –26 cm, Saimaa –38, Keitele –35, Päijänne -29, Pyhäjärvi, Säkylä –9, Längelmävesi –1, Näsijärvi +2, Lestijärvi –2, Lentua –7, Oulujärvi –14,
Lokka +71, Ounasjärvi +4 and Inari +40 cm.
Discharges of rivers draining from the Lake District of central and eastern Finland were still unusually low. Rather high instantaneous peak discharges were however observed in small and medium-sized river watercourses of western Finland at about the
middle of the month. The discharge of the river Pielisjoki was 61% of the mean October discharge for the period 1961-1990 and corresponding figures in some other Finnish rivers were: Vuoksi 83%, Kymijoki 61, Paimionjoki 85, Kokemäenjoki 106, Kalajoki 48,
Oulujoki 64, Iijoki 88, Kemijoki 99 and Tornionjoki 97%.
Groundwater levels increased considerably in moraine areas of western Finland, even reaching the seasonal mean after being exceptionally low only one month earlier. Water levels in groundwater formations of eastern Finland were still 10-75 cm below the
seasonal mean, whereas in the north of the country the situation was approximately normal.
Surface water temperatures of watercourses exceeded the seasonal mean in some areas by as much as 2°C. Formation of ice covers over the lakes began from the north of the country about two weeks later than normally.