Human-influenced streamflow during extreme drought: identifying driving forces, modifiers, and impacts in an urbanized catchment in central Poland Urszula Somorowska & Maksym Laszewski Department of Hydrology, Institute of Physical Geography, Faculty of Geography and Regional Studies, University of Warsaw, Warsaw, Poland Keywords drought; impacts; modified streamflow; Poland; urbanized catchment. Correspondence Urszula Somorowska, Department of Hydrology, Institute of Physical Geography, Faculty of Geography and Regional Studies, University of Warsaw, Warsaw, Poland. Email: usomorow@uw.edu.pl doi:10.1111/wej.12249 Abstract 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 m 3 /s as compared to 1951–1970. The mean annual wastewater inflow rate to the river in 2015 was approximately 0.770 m 3 /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. Introduction Drought is considered to be one of the most damaging natu- ral hazards, having several direct and indirect environmen- tal, societal and economic consequences (Tallaksen & van Lanen 2004). Low groundwater levels and stream flows are just two among the environmental effects of prolonged meteorological droughts in summer, which are caused by long rainless periods and enhanced by heat waves. Stream- flow drought is dependent on climate and catchment charac- teristics, two important driving factors (van Lanen et al. 2013; van Loon 2015). In human-altered catchments the occurrence of hydrological droughts can be significantly modified by undertaking several types of activities (O’Driscoll et al. 2010). Groundwater and surface water abstraction amplify low flows. In turn, urban water discharge to rivers increases observed flows. Both modifiers change the streamflow regime by changing its magnitude and timing. Low flows have a significant impact on surface water qual- ity and the health of the river ecosystem (Bradford & Heino- nen 2008; Van Vliet & Zwolsman 2008). During a low flow period an increase in river water temperature is observed. It is often accompanied by an increase in the concentration of biogenic nutrients, which, if in excess, can lead to an over- growth of algae and aquatic plants in streams. This, in conse- quence, leads to a reduction of dissolved oxygen in water, water stagnation and a degradation of the stream habitat (Garc  ıa & Pardo, 2016; Pardo & Garc  ıa 2016). All anthropo- genic stressors result in changes in regard to natural water balance, flow regime and the ecological status of rivers (Sp€ anhoff et al. 2012). Consequently, adaptive environmen- tal flow management is required to reduce pressure on river ecosystems (Nilsson & Ren € of € alt 2008; Momblanch 2015). Historical analysis of the largest drought events in Europe from 1950 to 2012 has proved the gradual increase of Euro- pean areas prone to drought (Spinoni et al. 2015). 2015 saw the occurrence of a long-lasting summer drought, which also encompassed the territory of Poland (Van Lanen et al. 2016). In consequence, in central Europe, including Poland, river stages reached an absolute minimum. Selected runoff projections for the far future (2071–2100) indicate that the probability of the occurrence of severe droughts under the conditions of climate change might increase even further (Meresa et al. 2016). With the possible consequences of streamflow regime alter- ation in mind, the challenge is to identify driving forces, modi- fiers and impacts in human-altered catchments (Van Loon et al. 2016). This paper aims at evaluating the response of the streamflow to an extremely hot and dry summer in the low- land catchment of the Utrata River in central Poland. In particu- lar, the following questions were addressed: (1) how extreme was the 2015 summer hydrological drought in terms of driving 1 Water and Environment Journal (2017) V C 2017 CIWEM. Water and Environment Journal. Print ISSN 1747-6585