Human-influenced streamflow during extreme drought: identifying driving forces, modifiers, and impacts in an urbanized catchment in central Poland

This study investigates the response of the streamflow to an extremely hot and dry summer 2015 in the urbanized catchment of the Utrata River in central Poland. The objectives were to: reveal changes in the flow regime, assess anomalies in summer river flows, estimate the natural and wastewater effluent contribution and provide an ecological context for the in-stream conditions. The mean annual flow rate in the period 1996–2015 increased by 0.61 m3/s as compared to 1951–1970. The mean annual wastewater inflow rate to the river in 2015 was approximately 0.770 m3/s, constituting 39% of the observed flow. Almost the entire period of August this contribution approached 100%. The optimum river water temperature threshold for warm-water fish species was exceeded. Streamflow modifications are attributed to an increase of wastewater discharge, urban impact through an increase in imperviousness, and the variability of climatic driving forces.