Long-term changes in temperature of river waters in the transitional zone of the temperate climate: a case study of Polish rivers

The article discusses the range and course of changes in the thermal regime of 14 rivers in Poland over the period 1961-2010. Eleven rivers are located in the Central European Plain, and the others flow in the foothills of the Carpathians Mountains. Statistical analyses take into consideration the results of daily measurements of water temperature carried out at 16 hydrological stations by the Institute of Meteorology and Water Management-National Research Institute. In the first part of the analysed period (1961-1986) water temperature in most rivers declined in relation to its mean value for the entire study period (1961-2010). In 1987 there was a reverse trend: the temperature started rising. The fastest increase in water temperature was recorded in the western part of the study area, and it became slower towards the east. In the southern part of the study area (the foothills) changes of that kind were not observed. The mean yearly temperature of fluvial waters in the Central European Plain showed a positive trend, ranging from 0.17 to 0.27 degrees C (10 years)(-1), whereas it did not change in the rivers in the foothills of the Carpathians Mountains. Its fastest rise was recorded in spring, and it reached from 0.08 to 0.43 degrees C (10 years)(-1). The increase in water temperature correlated strongly with rising air temperature. The temperature of river waters in the lowlands is believed to be a good indicator of climatic changes.