Rainstorms impacts on water, sediment, and trace elements loads in an urbanized catchment within Moscow city: case study of summer 2020 and 2021

In 2020 and 2021, the city of Moscow, Russia, has experienced two historical rainfall events that had caused major flooding of small rivers. Based on long-term observation datasets from the surrounding weather stations, regional mesoscale COSMO-CLM climate model results, and a detailed hydrological and water quality monitoring data, we performed a pioneer assessment of climate change and urbanization impact on flooding hazard and water quality of the urban Setun River as a case study. Statistically significant rise of some moderate ETCCDI climate change indices (R20mm and R95pTOT) was revealed for the 1966-2020 period, while no significant trends were observed for more extreme indices. The combined impact of climate change and increased urbanization is highly non-linear and results in as much as a fourfold increase in frequency of extreme floods and shift of water regime features which lead to formation of specific seasonal flow patterns. The rainstorm flood wave response time, involving infiltrated and hillslope-routed fraction of rainfall, is accounted as 6 to 11 h, which is more than twice as rapid as compared to the non-urbanized nearby catchments. Based on temporal trends before and after rainfall flood peak, four groups of dissolved chemicals were identified: soluble elements whose concentrations decrease with an increase in water discharge; mostly insoluble and well-sorted elements whose concentrations increase with discharge (Mn, Cs, Cd, Al); elements negatively related to water discharge during flood events (Li, B, Cr, As, Br and Sr); and a wide range of dissolved elements (Cu, Zn, Mo, Sn, Pb, Ba, La, Cs, U) which concentrations remain stable during rainfall floods. Our study identifies that lack of research focused on the combined impacts of climate change and urbanization on flooding and water quality in the Moscow urban area is a key problem in water management advances.