Urbanization influences on stream temperature behavior within low-discharge headwater streams

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Urbanization has compromised water quality globally, especially stream temperature, by reducing shading and converting natural landscapes to impermeable surface coverage (ISC). We analyze stream-air temperature relationships in a low-order, moderate-gradient urban stream using three years of stream temperature data collected at nine monitoring sites. At the sub-catchment scale, ISC increases from 13.7% to 24.3% among sites, causing mean summer temperatures to increase 4–5°C or 0.37°C for each 1% increase in ISC. ISC at these spatial scales influences stream-air temperature relationships at daily-, weekly-, and monthly-averaged time scales. ISC at smaller spatial scales within a 25-m buffer of the stream, which ranges from 1% to 75%, does not correlate with mean stream temperatures at any temporal scale; however, buffer ISC does correlate with short-term temperature surge events, which we define as an increase of at least 1°C within 15 minutes. Mean surge amplitudes range from 1.90°C to 3.27°C in areas with low and high buffer ISC, respectively. Our results show that ISC influences stream temperatures at stream buffer and sub-catchment spatial scales and daily, weekly, and monthly temporal scales, and may render the concept of equilibrium temperature obsolete for predicting stream temperatures, especially in low-order, headwater streams.