ELEVATION GRADIENT AND LONG-TERM TREND OF THE WATER TEMPERATURE IN SURFACE STREAMS IN SLOVAKIA

In the first part of the paper the long-term trends of the water temperature are analysed of the high mountain river Bela, and the lowland river Morava. The stations were selected to cover wide range of elevations. The trends were studied on 50-year data series by Mann-Kendal test. The long-term mean annual water temperature of the Bela river was 4.2 degrees C at Podbansk station, and it corresponds very well with the mean annual air temperature, which was 4.9 degrees C. The long-term mean annual water temperature of the Morava river was 11.1 degrees C. The second aim of the study was to assess the water temperature gradient. The elevation gradient of the water temperature specifies the relation of the water temperature on elevation in natural conditions. It was determined for individual months and year seasons based on data series of 21 stations. There exists the negative relation between the water temperature and the elevation - with decrease of elevation the water temperature increases. The positive relation was observed only in the high mountain Jalovecky creek in specific winter conditions, and during short time periods. During the winter months the elevation gradient is the smallest. The biggest values of the water temperature gradient are observed in small streams during the summer season. The biggest gradient -1.06 degrees C per 100 m of elevation was estimated in August 2012.