River thermal seasons in the Central European Plain and their changes during climate warming

Abstract

Temperature is one of the most important properties of rivers. Interest in river temperature (RT) has been rapidly increasing in the current period of climate change. It is related to among others the application of models for the development of scenarios of RT increase. The article presents an attempt to identify one of the most important elements of fluvial regime: river thermal seasons (RTS). No attempts have been undertaken to date to determine such seasons, although they are among the most important elements of fluvial regime. This paper proposes another approach to the issue of changes in RT. It involves the designation of river thermal seasons RTS based on unique daily data from a period 1961–2020. The thermal season was defined as a sequence of days with intrinsic similarity resulting from mean multiannual water temperature, its variability in the multiannual period, and autocorrelation. A total of 18 types of RTS were designated in 5 groups of RTS: very cold (0.0–1.0 °C), cold (1.1-4.0 °C), moderate (4.1–10.0 °C), warm (10.1–15.0 °C), and very warm (15.1–20.0 °C). Moroever, two sub-periods were designated: namely 1961–1992 and 1993–2020, permitting the identification of the character of RTS in both sub-periods, and the assessment of changes in their distribution. The applied method allowed for conducting comparative analyses between rivers of various sizes. The occurrence of different thermal seasons was evidenced, as well as various directions and effects of changes resulting from climate warming. The greatest contribution in a year was reached by the group of very warm seasons (34.9%). In comparison with sub-period 1961–1992, it increased by 5.1%. A smaller share was determined for the group of cold seasons (29.0%) which also increased (by 10%) in comparison to the earlier sub-season. An increase in the contribution of cold seasons resulted from a considerable reduction of the duration of seasons from the moderate (from 27.5% to 16.5%) and very cold group (from 9.1% to 1.7%). The proposed study method is universal. It can be applied to analyses of river temperature in other climate zones.