Seasonal structure of water stages on lakes in Northern Poland

Abstract The paper presents the characteristics of hydrological periods in an average annual cycle in Polish lakes. The types of period and their sequence determine the regime of water stages in lakes. The article applies the unsupervised approach to analysis of water level fluctuation patterns, where the regime is identified by grouping analytical parameters. Hydrological periods were designated by grouping elementary time units of the hydrological year (pentads) based on the similarity of their parameters, namely water level frequency distributions. The analysis covered daily water stages in 33 lakes in Poland from the period from 1984 to 2012. Five types of hydrological period were designated. The studied lakes differ in the number, type and sequence of hydrological periods in an average annual cycle. Most of the lakes (19 lakes) have a 4-period temporal structure of water stages with the course of water stages in a year characteristic of this geographical zone. No spatial patterns occurred in the location of lakes from particular groups. This suggests the dominant role of local factors in determining the seasonality of water stages.