Annual hydrological report 1999
A year of great hydrological variation
The hydrological conditions in Finland in 1999 can best be described as exceptionally variable. At the beginning of the year the water resources were abundant throughout the country, but by late summer record-breaking deficits were recorded in southern
and central Finland. The winter season included periods of record frosts, which led to ice dam flooding in many areas in the spring. The summer was unprecedentedly dry and hot - except in northern Lapland, where the summer season was very wet. Towards the
end of the year the water resources increased, but in the large lakes and groundwater formations of the eastern Lake District the amount of water was still low. Snow cover
At the beginning of 1999 the watercourses of Finland contained high amounts of water. Accumulation of snow in the southern coastal region during January and February was higher than in the preceding years – but still not exceptionally high
compared with the long-term seasonal mean. In the county of Uusimaa the water equivalent of the snow cover in March was 125% of the seasonal mean, although higher relative values were recorded along the south coast. Throughout most of the remainder of the
country, accumulation of snow was approximately normal. In southern and central parts of the country overall ice thicknesses were generally high, due mainly to a layer of snow ice. Lake ice thickness in Lapland was below average, but heavy accumulation of
river ice was recorded in some areas. Spring flood
Water resources were still above the seasonal mean in April. The first appreciable spring floods for many years, with a return fequency of about 20 years, occurred in the southern coastal zone due to rapid melting of the snow and simultaneous heavy
rainfall. In northern Finland a rapid increase in water level caused ice dam flooding e.g. in the rivers Simojoki, Ounasjoki and Tornionjoki in April and May. The total number of reported ice dam floodings was 35. Precipitation
Summer weather with rapidly decreasing water levels began in sothern and central Finland during April. By contrast, May was cold and unusually dry.
Rainfall during the summer months was only half the seasonal mean in southern and south-western Finland. The weather was very hot and the figures for evaporation represented a new record, first from from the soil and from vegetation and after warming
of the waters from lakes and rivers. Evaporation of water from the lakes of southern Finland during the summer period was estimated at 600 mm. After drying of the ground layers to an unprecedentedly severe water deficit, the rate of evaporation from the
soil eventually decreased to a low level. The hydrological statistics for southern and central Finland do not include another instance of such rapid loss of water from watercourses and soil layers during a single summer. Averaged over the whole area the
water level decreased by approximately 400 mm in just a few months, both by draining to the sea and by evaporation. Groundwater and discharge
The waters of both lakes and groundwater formations reached their lowest ever recorded levels at the end of September in many parts of southern Finland, from half a metre to a full metre below the seasonal mean. For example the discharge of the river
Kokemäenjoki - approximately 50 m3/s - in September 1999 was the lowest ever recorded for such a long period of time since the beginning of observations in 1931. The drought occurring throughout the south of the country during the summer of 1999 probably
had a return frequency of more than 100 years in many places.
In contrast to the south of the country, water resources in Lapland were abundant. The level of lake Inari increased almost to the maximum regulation level at about the middle of August and again during early November due to heavy rainfall in the
area.
Many wells dried up in late summer and autumn particularly in south-western Finland. Some tens of thousands of wells were estimated to have dried up throughout southern Finland. Later during the autumn the area of extreme water deficit moved to eastern
Finland.
By the end of the year the low water resources in southern and western Finland were mainly replenished. In some of the larger lakes of eastern and central Finland the water levels were still exceptionally low, as was the groundwater level. In these
areas some water availability problems were experienced at the end of 1999. Ice
Due to the warm summer and autumn, freezing over of watercourses occurred about two weeks later than normally. By the end of the year ice thicknesses were 5-15 cm less than the seasonal mean. The Tehinselkä reach of lake Päijänne did not freeze until
19.1.2000. Snow
At the end of the year the snow cover was heavier than normal in southern and central Finland, where the water equivalent of the snow cover was in many areas about 1.5-fold the seasonal mean. Evaporation
Annual precipitation for 1999 in northernmost Finland was about 120% of the long-term mean and in other parts of the country somewhat below the mean. The considerable decrease in water resources during the summer in southern and central Finland was
therefore due more to record-breaking evaporation during the hot summer than to low rainfall. The rather heavy precipitation towards the end of the year was mainly retained in the snow cover - ready to affect the water reserves during the year 2000. Water level
From the beginning to the end of the year the surface of lake Pielinen decreased by 74 cm and corresponding figures in some other lakes were: Kallavesi 18 cm, Saimaa 84, Päijänne 28 and Oulujärvi 18 cm. By contrast the levels of the northern lakes
Lokka and Inari increased over the whole year by 7 and 27 cm, respectively.
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