Spatial and temporal variation in the water temperature of Lithuanian rivers

Water temperature is one of the twelve physico-chemical elements of water quality, used for the assessment of the ecological status of surface waters according to the Water Framework Directive (WFD) 2000/60/ EC. The thermal regime of Lithuanian rivers is not sufficiently studied. The presented article describes the temporal and spatial variation of water temperature in Lithuanian rivers. Since a huge amount of statistical data is available (time series from 141 water gauging stations), the average water temperatures of the warm season (May-October) have been selected to analyse because that is the time when the most intensive hydrological and hydro-biological processes in water bodies take place. Spatial distribution of river water temperature is mostly influenced by the type of river feeding, prevalence of sandy soils and lakes in a basin, river size, and orography of a river basin as well as anthropogenic activity. The temporal distribution of river water temperature is determined by climatic factors and local conditions. The averages of the warm season water temperature for 41 WGS are 15.1°C in 1945-2010, 14.9°C in 1961-1990, and 15.4°C in 1991-2010. The most significant changes in water temperature trends are identified in the period of 1991-2010. For this period, the rates of increase of water and air temperature are 0.04 and 0.06°C/warm season, respectively. Therefore, air temperature is one of the most significant factors affecting the water temperatures of Lithuanian rivers. Keywords Water temperature * Spatial distribution * Trends * Lithuanian rivers