Temperature dynamics of stormwater runoff in Australia and the USA

Thermal pollution of surface waters by urban stormwater runoff is an often overlooked by-product of urbanization. Elevated stream temperatures due to an influx of stormwater runoff can be detrimental to stream biota, in particular for cold water systems. However, few studies have examined temperature trends throughout storm events to determine how these thermal inputs are temporally distributed. In this study, six diverse catchments in two continents are evaluated for thermal dynamics. Summary statistics from the data showed larger catchments have lower maximum runoff temperatures, minimum runoff temperatures, and temperature variability. This reinforces the understanding that subsurface drainage infrastructure in urban catchments acts to moderate runoff temperatures. The catchments were also evaluated for the presence of a thermal first flush using two methodologies. Results showed the lack of a first flush under traditional assessment methodologies across all six catchments, supporting the results from a limited number of studies in literature. However, the time to peak temperature was not always coincident with the time to peak flow, highlighting the variability of thermal load over time. When a new first flush methodology was applied, significant differences in temperature were noted with increasing runoff depth for five of the six sites. This study is the first to identify a runoff temperature first flush, and highlights the need to carefully consider the appropriate methodology for such analyses.