Quantifying streamflow response to climate variability, wastewater inflow, and sprawling urbanization in a heavily modified river basin.

Considering the growing recognition that human activity and climate variability are critical stressors influencing river regimes, there is an urgent need to identify the contribution of these fundamental factors. Here we examine the runoff changes of a wastewater-effluent dominated river flowing in a heavily modified urban environment. The study concerns the Utrata River basin (727 km), central Poland, a challenging human-natural system for investigating changes in hydrological processes. The new insights into the present-day functioning of a system are provided through the quantification of runoff changes resulting from the following factors operating simultaneously: climate, wastewater, and urbanization. We adopted and applied the water balance and elasticity-based methods to assess the contribution of particular factors. Climatic data were provided by the gridded observational dataset, while runoff comes from observed daily streamflow values at the stream gauging station. Satellite-derived estimates of evapotranspiration acquired from Operational Simplified Surface Energy Balance (SSEBop) were assimilated into the conceptual framework to provide a robust representation of the system. To determine the changes in urban water budget components, two distinct periods were determined: the baseline period (1951-1960), with a relatively low level of urbanization (9.8%), and the change period (2007-2016), with controlled wastewater inflow and urbanization reaching almost 20%. The results show that in the change period, the mean annual runoff increment attributed to climate variability amounted to 68 mm; the contribution of wastewater inflow - 36 mm; and the contribution attributed to urban-induced changes - -43 mm. Thus, the relative contributions of human activity and climate were estimated as 54% and 46%, respectively. The results demonstrate that in a heavily modified environment, the hydrological effects of human activity can exceed those caused by climate variability. Moreover, wastewater inflow and climate impact can mask decreases in the streamflow induced by urban land fragmentation and land management practices.