Wahania poziomów wody jezior w Polsce w latach 1956–2015 = Water-level fluctuations in Polish lakes in the 1956–2015 period

Abstract

Water-level fluctuations are among the primary factors determining the functioning of lakes. The volume to which lake basins are filled with water is of major importance to the courses of many processes and phenomena. A particular amount of water in a lake, and water-table stability, are also important from the point of view of human activity, as these elements help determine the quantity and accessibility of the water resources lakes have to offer, and therefore the possibilities for them to be used by different branches of the economy, e.g. industry, agriculture or tourism. The work detailed here is thus a presentation of trends as regards water-level fluctuations in 16 lakes in Poland, over the period 1956–2015. The study results, obtained for the first time in relation to such a long time scale and extending to around a dozen lakes, aim to point to the scale and direction of water-level fluctuations in times of the intensive transformation of the natural environment. They were obtained by reference to water-level observations made by the Institute of Meteorology and Water Management – National Research Institute (IMiGW-PIB). Specifically, data referring to the (November-October) hydrological year were analysed for trends as regards mean annual water levels using the Mann-Kendall test. Results point to major variability in the courses noted for these levels over the analysed multiannual period. Nevertheless, three overall situations could be designated from within the group of cases analysed, i.e. increase, decrease or lack of a trend. The first group includes Lakes Sławskie, Jamno, Łebsko, Nidzkie, and Studzieniczne (where increases were statistically significant at p=0.05); the second, Lakes Ostrzyckie and Ełckie (decreases significant at p=0.05); and the last group all remaining lakes, i.e. Charzykowskie, Jeziorak and Rajgrodzkie, Biskupińskie, Drwęckie and Białe, Gopło, Roś, and Wigry. It was, however, noted that in many cases analysed periods of alternating increase and decrease in water level were to be observed. The causes of such fluctuations were complex, but inter alia reflected droughts of several years’ duration, periods featuring higher-than-average precipitation, and local conditions. In general, water-level fluctuations in lakes result from natural and anthropogenic factors determining the hydrological conditions in catchments. And in the context of the lakes considered here, the courses of water-level fluctuations were mostly a reflection of local, rather than wider climatic conditions – a fact i.a. illustrated by the lack of cohesive regional designations. The situation is different from that of, for example, the thermal or ice regimes of Polish lakes, in relation to which observed similarities in properties are seen to be determined mainly by climatic factors. Information of this kind may be of key importance to the (quantitative and qualitative) management of water resources in the context of the climate change being observed currently.