Spatial distribution and temporal changes in river water temperature in the Baltic States

The thermal regime of rivers in the Baltic States (Estonia, Latvia and Lithuania) is not sufficiently studied. The presented research describes the spatial distribution and temporal variation of river water temperature (WT) in the Baltic States using a unified methodology. The object of the research is the WT of rivers during the warm season (May–October) and the warmest month of the year (July). The contour maps of WT were compiled for the rivers of the Baltic States using data series for the period 1961–1990 from 17 water measuring stations in Estonia (EST), 36 stations in Latvia (LV) and 40 stations in Lithuania (LT). These maps allow the evaluation of WT for the unmonitored rivers. Analysis of long-term (1951–2010) changes of river WT showed that during the first half of this period (1951–1980), river WT decreased significantly (0.003 °C/warm season). Meanwhile, only positive trends prevailed over the last three decades (1981–2010) in the river WT of the Baltic States. During the latest period, rivers which show significant positive trends in WT warm up significantly – by 0.04 °C/warm season.