Hunting for heat pulses: streamwater temperature responses to summer storms as tracer for urban water sources

Streamwater temperature is an important water quality parameter, and controls metabolic and other reaction processes. In urban systems, streamwater temperature has been shown to depend substantially on the near-stream land cover: Riparian buffer zones may cool urban streams, while effluent from wastewater treatment plants is known to raise streamwater temperatures. Groundwater contributions can decrease summer and increase winter streamwater temperature, essentially acting as a temperature buffer. In consequence, streamwater temperature is highly dependent on the specific layout of the urban system.

Streamwater temperature fluctuates on annual, seasonal, daily, and diurnal basis, however, storm events may additionally impact streamwater temperature on short time scales. The timing and patterns of streamwater temperature responses to storm events relative to the hydrograph may differ from event to event. Because urban infrastructure is typically designed to rapidly route water to the sewer network to avoid flooding, streamwater temperature response patterns are likely related to event- and site-specific water sources contributing to streamflow. Changes in temperature patterns could thus be linked to changes in water release processes. If we disentangle the various empirical relationships revealing potential physical controls on how water is conveyed to streams in urban areas, temperature could potentially be used as a cheap tracer of water sources and pathways in urban systems, which are typically difficult to assess.

We investigated and quantified different streamwater temperature response patterns to stormflow, to understand predictors of diverse streamwater temperature responses to summer storms. We found that streamwater temperature shows varied response patterns to storms, including temperature increases and decreases. Some of the temperature increases may take the shape of rapid “heat pulses”, a short but relatively high magnitude temperature increase and subsequent drop at the start of the hydrograph. Streamwater temperature responses to storms were event-specific and could be clearly linked to event characteristics. Understanding the streamwater temperature response can thus aid in understanding urban source contributions to streamflow, because the mixing of sources – and the timing of this mixing process – causes a unique streamwater temperature pattern. Likely sources contributing to the streamwater temperature patterns are ponded surface waters and storm drains, or cooler water from the shallow subsurface. These findings indicate that streamwater temperature may be used as a cheap but effective tracer informing the contributions from different source zones in urban catchments.