Anthropocene Trends in Water Flow of Small and Medium-Sized Rivers in the East of the East European Plain: The Forest-Steppe and Steppe Zones

Based on long-term series of observations of water flow of 22 small and medium-sized rivers in the forest-steppe and steppe east of the East European Plain, an analysis of its trend changes in 1961–2022, i.e., in the time interval of the Anthropocene with the most progressive climate change in the study region, was carried out. The main quantitative hydrological parameters studied were annual average water discharge, annual maximum water discharge (Qmax), minimum water discharge (Qmin-CP) during the ice-covered period of the riverbed (cold period, mainly December–March), minimum water discharge (Qmin-WP) during the ice-free period of the riverbed (warm period, mainly April–November), as well as some of their ratios, which provide a clear idea of changes in the intra-annual variability of water flow. The principal methodological toolkit used was a standard set of statistical tests applied to time series analysis. A summary of the study results showed that statistically significant trend changes in the annual water flow of most of the analyzed rivers were not observed for the specified period. At the same time, statistically significant intra-annual changes in the flow were revealed: a significant reduction in Qmax (especially in the forest-steppe zone) and a significant increase in Qmin-CP and Qmin-WP. Thus, the ratio between Qmax and Qmin-CP (Qmin-WP) decreased between the baseline climatic periods 1961–1990 and 1991–2020 by an average of 4.1 (4.0) times in the rivers of the forest-steppe zone, and by 5.2 (5.3) times in the rivers of the steppe zone. Climate change is considered the leading cause of the observed intra-annual changes in river water flow in the study region, with the main factor being an increment in annual air temperature, especially during the cold season.