Trends and Characteristics of Hydrological Time Series in Finland

Abstract

Hydrological time series analyses made in Finland up to 2001 show the following: 1) Precipitation has been increasing in southern and central Finland, and also in the north in winter, during the period 1911-2000. There are, however, no harmonized analyses of areal precipitation to show the exact increase. 2) The annual maximum of the areal water equivalent of snow has been increasing in eastern and northern Finland but decreasing in the south and west during the period 1947-2001. 3) The winter runoff has generally been increasing strongly in southern and slightly in central Finland during the 20th century. In northern Lapland there are no signs of increase in winter or annual flow. Annual discharge in the south and west has also increased to some extent. 4) The existing analyses show no signs of long-term trends in annual evapotranspiration. 5) Long-term fluctuations of water stage have been observed in the major groundwater formations. 6) The series of the date of ice break-up in the river Tornionjoki - starting in 1693 – shows that in recent decades the ice cover of the river has broken up about two weeks earlier than in the beginning of the period. 6) Lake ice maximum thickness series show no noticeable trend. 7) Lake water temperature in south-eastern Finland seems to have been increasing slightly during the period starting in 1924; in central and northern Finland no trends in water temperature have been observed.