Water Discharge Change in the Rivers of the South of the Boreal Forest Zone of Eastern European Russia at the End of the Late Holocene and in the Anthropocene: The Vyatka River

Abstract

Based on long-term observations of the water discharge (WD) from 1878 to 2018, the main trends and patterns of its change in the basin of one of the largest rivers in the east of the East European Plain, the Vyatka River, were revealed using a set of standard statistical procedures and a graphical analysis of the WD probability curves. Three main phases of the annual river WD were identified, corresponding to the periods 1878–1929, 1930–1977, and 1978–2018. The first and third periods were characterized by increased WD (by 22–23%) relative to the medium period, and the differences in the average annual WD characteristics between these periods were statistically significant. It is also noteworthy that the difference in the average annual WD between the first and last periods of increased WD was very small and statistically insignificant. A gradual increase in the share of the so-called normal annual WD and a decrease in the abnormal (including extreme) annual WD were noted from 1878–1929 to 1978–2018 in the predominantly northern half of the river basin. At the same time, in the predominantly southern half of the basin, according to the analysis of only the periods 1930–1977 and 1978–2018, the so-called normal and abnormal WD remained almost unchanged; only a slight increase in positive anomalies of the WD was noted. Increased intra-annual variability in the WD was characteristic of the period of its reduced average annual value. Between the three identified periods, a gradual reduction in the water runoff coefficient was observed during the warm (mainly summer) season in the Vyatka River basin. The specified long-term variability in the water discharge of the Vyatka River was chiefly due to long-term changes in climatic factors, primarily the ratio of the air temperature and precipitation, expressed as the De Martonne aridity index. The duration of the principal cycle of these changes was 82 years. An additional contribution could also have been made by the widespread reduction in cropland in the study region in recent decades. The obtained results can be preliminarily considered representative of the entire southern sector of the boreal forest zone (taiga zone) of the east of the East European Plain.