Urban Stormwater Temperature Surges: A Central US Watershed Study

Impacts of urban land use can include increased stormwater runoff temperature (Tw) leading to receiving water quality impairment. There is therefore a need to target and mitigate sources of thermal pollution in urban areas. However, complex relationships between urban development, stormwater runoff and stream water heating processes are poorly understood. A nested-scale experimental watershed study design was used to investigate stormwater runoff temperature impacts to receiving waters in a representative mixed-use urbanizing watershed of the central US. Daily maximum Tw exceeded 35.0 °C (threshold for potential mortality of warm-water biota) at an urban monitoring site for a total of five days during the study period (2011–2013). Sudden increases of more than 1.0 °C within a 15 min time interval of Tw following summer thunderstorms were significantly correlated (CI = 95%; p < 0.01) to cumulative percent urban land use (r2 = 0.98; n = 29). Differences in mean Tw between monitoring sites were significantly correlated (CI = 95%; p = 0.02) to urban land use practices, stream distance and increasing discharge. The effects of the 2012 Midwest USA drought and land use on Tw were also observed with maximum Tw 4.0 °C higher at an urban monitoring site relative to a rural site for 10.5 h. The current work provides quantitative evidence of acute increases in Tw related to urban land use. Results better inform land managers wishing to create management strategies designed to preserve suitable thermal stream habitats in urbanizing watersheds.