Monthly hydrological report in May 1999
The first half of May 1999 was cold and almost without rain. High local rainfalls were recorded at the end of the month particularly in the county of Oulu. The surfaces of unregulated lakes in southern Finland began to decrease several weeks ahead of
their normal schedule. A second spring flood occurred in the rivers of Lapland towards the end of the month, during which seious damming by breaking ice was recorded in the river Tenojoki. Intense sunshine maintained water temperatures near their normal
levels despite the cold weather.
Precipitation was very low in southern and eastern Finland and in northern Lapland, only 10-20 mm. In other regions the levels of precipitation were more variable. In the basin of the river Oulujoki the monthly precipitation was almost normal, about 50
mm. The rainfall was strongly biased towards the end of the month.
The snow cover persisted in Lapland for a long time due to the cold weather, but mainly melted rather rapidly towards the end of the month. By the end of May, snow was still recorded only in some small areas of northernmost Lapland.
Evaporation was somewhat below the seasonal mean in most places.
Water levels. Melting of snow caused a second, rapidly developing spring flood in rivers of Lapland after May 20. Build-up of broken ice in the river Tenojoki against the island of Karnjargan during the period 20-23.5 increased water levels near the
villages of Niemelä and Karnjargan to 1.5 metres above the previous record high level from 1922. In central and southern Finland, however, water levels began to decrease from their spring maxima several weeks earlier and more rapidly than normally. Level
deviations from the seasonal mean in some major Finnish rivers were: Pielinen -13 cm, Kallavesi -24, Saimaa +36, Keitele -3, Päijänne +1, Pyhäjärvi (Säkylä) -5, Längelmävesi +16, Näsijärvi -12, Lestijärvi +1, Lentua -24, Oulujärvi +17, Lokka +19 ja Inari
Discharges in many rivers remained below the long-term seasonal mean, although in the lake chains of eastern Finland discharges were still above the mean. The discharge of the river Pielisjoki was 106% of the mean May discharge during the period
1961-1990, and corresponding figures in some other river watercourses were: Vuoksi 118%, Kymijoki 103, Paimionjoki 49, Kokemäenjoki 77, Kalajoki 35, Oulujoki 52, Iijoki 49, Kemijoki 74 and Tornionjoki 94%.
Groundwater was higher than normal at the beginning of May but decreased towards the end of the month, except in the north where level increases were recorded. Groundfrost melted almost throughout the country during May.
Break-up of ice, which had started earlier than normally, was delayed by the cold weather at the beginning of May. For example break-up of ice on lake Kallavesi occurred a few daysbehind schedule after the middle of the month. As a result of subsequent
rapid warming of the weather the ice cover left northern lakes even somewhat earlier than normally.
Due to the cold weather, surface water temperatures during May remained below the seasonal mean in most parts of the country. However, the deviation from the seasonal mean was not great because of intense incoming solar radiation. By the end of the
month lake water temperatures in the south were almost 15°C, which was close to the seasonal mean. Temperatures in lakes of central Finland varied around 10°C, and in Lapland the corresponding temperatures were close to 5°C. These figures are about 2°C
below the seasonal mean.