Trends in the Water Regime Changes of the Styr River (Lutsk Hydrological Station) During 1963-2022 Period

The article investigates trends in long-term dynamics and intra-annual distribution of river discharge at the Styr River (Lutsk hydrological station) during 1963-2022 in relation to precipitation and air temperature. The research established that mean annual and maximum discharge are decreasing (trends are statistically significant). Summer-autumn discharge is decreasing while winter discharge is increasing (linear trends are not significant). The data series for mean annual discharge is homogeneous, as evidenced by the coefficient of variation (VϬ = 26.8%) and the constructed integral curve. The data series for maximum and minimum discharge is heterogeneous. A decrease in river discharge is observed for all months except January and February (all trends are statistically insignificant, except for March and April). The highest discharge occurs in March-April, and the lowest in August and September. Spring runoff accounts for 34% of the mean annual runoff, winter for 24%, and summer and autumn for 21% each. The cycles of low-water (with relatively low-water) and high-water (with relatively high-water) years for the Styr River last approximately 15 years. During 1963-2022, two nearly complete water cycles of the Styr River are traced (two phases of discharge increase and two phases of decrease), in precipitation fluctuation – two phases of annual totals increase and one phase of decrease, in mean annual air temperature fluctuation – one phase of increase and one phase of decrease. The years 1982, 1997, and 2013-2014 were breakpoints in the river's discharge fluctuations. The duration of water regime phases is about 15-16 years. Therefore, a further decrease in mean annual and minimum discharge of the Styr River can be predicted until approximately 2028.