Scientific publications

A feature of modern changes in the hydrological regime of some Karelian rivers is a significant increase in the water content during the cold period of the year. Runoff in the study area has been increasing in the fall and in all winter months, and in synchrony with the rise in mean monthly surface air temperature. In Karelian rivers, the hydrological regime period called the “fall low water” has been gradually shrinking, sometimes to the point of disappearance. The fall low-water periods are gradually getting replaced by rainfallinduced freshets of varying volume and duration, which often last until February. The paper compares the river runoff in the fall period against the highest-water phase of the river’s hydrological regime – the spring high-water period – by processing data on daily discharges for 37 watercourses of Karelia over the period 1993–2022. The fall runoff volume is shown to exceed the water content during the spring high-water phase. The average duration of the fall freshets (non-low water period) is 128 days (125 for rivers of the Baltic Sea basin, 133 for rivers of the White Sea basin). On average, the fall freshets period for the rivers studied in the Baltic Sea basin begins on September 29 and ends on February 1, for river of the White Sea basin it begins on September 17, ending on January 29. During the period under study, the water content of the spring high-water phase has been gradually declining, and the water content in the fall has been growing. An average runoff depth of the fall freshets period is 21 % greater than that of the spring high-water period. Meanwhile, the highest water discharges in the fall are still 48 % lower than the highest water discharges of the spring high-water phase (according to the sample of averages). The presumed reason for the increase in runoff in the fall is that winters have become milder due to a warmer temperatures background, an increase in the number of thaws and a consequential reduction in soil freezing depth and an increase in soil water reserves.