Identification of thermal seasons of selected rivers in central Poland

The aim of this study is to identify thermal seasons (RTS) in seven rivers located in central Poland. This was done using daily, unique monitoring data of river water temperature (RT) from the available measurement period (1985–2014). These series come from the database of the Institute of Meteorology and Water Management – National Research Institute (IMWM-NRI). The applied procedure allowed the identification of compact sequences of similar days within each catchment as part of the average long-term annual thermograph, referred to as thermal seasons (RTS). The similarity was assessed based on the daily average temperatures on corresponding days, as well as their variability and autocorrelation over a multi-year period. Each identified thermal season is characterised by statistically significant differences in these features compared to other seasons and significant internal similarity. The delimitation of RTS allowed for characterising the thermal regime of the studied rivers in terms of environmental factors and anthropogenic influences. In the group of rivers in central Poland, 12 RTS were identified, among four groups of seasons: cold, moderate, warm, and very warm. Each river had four different seasons, and their occurrence varied in the range of 6 to 16 within a year. The fewest occurrences of four RTS were noted in the Kamienna river (6). Therefore, the average annual arrangement of seasons in this river is the least diversified. The highest number of different RTS occurrences was recorded in the Prosna river (16). The duration of an individual thermal season varies significantly, ranging from a minimum of 10 days, which occurred several times in the studied rivers, to 118 days in the Warta river. The least variation in the number and character of RTS was observed during the periods with moderate water temperatures (moderate group). The rivers showed the most diversity in RTS when low water temperatures prevailed. In the cold season group, four seasons were identified, and two of them were characteristic of individual rivers. Seasons in this group usually exhibit significant multi-year temperature variability and sometimes a statistically significant increasing temperature trend (e.g. the Prosna and Pilica rivers) or decreasing trend (the Ner and Kamienna rivers). In the periods of low water temperatures (cold season group), physical-geographic conditions and water management in river basins significantly affect water temperatures, as well as the duration and arrangement of thermal seasons.