Ocena temperatury wody w jeziorach i długości sezonu kąpielowego na pojezierzach w Polsce

Abstract

In the 2002-2015 period, 15 lakes of Poland’s Pomeranian, Masurian and Wielkopolska Lakelands were characterised in terms of multiannual mean, monthly, and daily maximum and minimum values for water surface temperature. The relevant calculations were based on data provided by the PISHM Bulletin of the National Hydrological and Meteorological Service, as published by Poland’s Institute of Meteorology and Water Management. Determined for each individual lake were the average dates of the beginning and end of the bathing season (and hence duration), in relation to water temperatures of 15°, 18° and 20°C. Relationships involving monthly water and air temperatures, as well as mean depths of lakes, were also analysed. From May through to mid-March of a following year, the mean monthly water temperature in lakes is higher than mean monthly air temperature by 1.0–2.0°C. Water and air temperatures are comparable at other times. Variation in monthly values for water temperature and the length of the bathing season in selected lakes of the Pomeranian Lakeland was also analysed. In most of the lakes under study, water temperatures in the warmest month ranged between 21.0° and 22.0°C, which is higher than air temperature by approximately 2.0°C. In summer (June–August), the greatest variations in water temperature occur in July, the most limited in August – as is exemplified by Lakes Komorze and Sławskie. Moreover, August temperatures are characterised by a significant upward trend through the 2002-2015 period. From among the three morphometric features of lakes considered (i.e. area, volume and depth), it was mean depth that displayed the most significant relationship with water temperature in the warm half-year, and especially in April and May. From February through to September, and sometimes October, mean monthly values for water temperature along the Baltic coast are lower than those recorded in lakes of the Pomeranian Lakeland, particularly in May and June – on average by between 3.1 and 4.4°C. In the remaining period of each year, water temperature in the Baltic Sea is higher than in the lakes by between 0.5 and approximately 1.0°C. Across the Lakeland area, in directions from southwest through to northeast, or from south to north, the length of the bathing season generally decreases. However, different morphometric features of given lakes may be associated with significant variations in the lengths of the bathing season, even between lakes in close proximity to one another (like Lakes Jasień and Raduńskie Górne). In most lakes of the three Lakelands, the average length of the potential bathing season with water temperature 15°C is approximately 120 to 145 days. In turn, the seasons with temperatures 18°C or 20°C last respectively 76-100 and 50-70 days. From the thermal point of view, the most favourable bathing conditions in the Lakelands characterise the Lubuskie Lakeland and the southwest region of the Pomeranian and Masurian Lakelands. The least favourable conditions are those noted in the Kaszubskie and Suwalskie Lakelands.