THE RELATIONSHIP BETWEEN AIR AND STREAM TEMPERATURE AT MULTIPLE TIME SCALES-EFFECTS OF SUMMER RAINFALL EVENTS (Invited Presentation)

A complex set of factors acting at multiple temporal scales affects stream temperatures. In order to understand the controls on stream temperature, we applied wavelet coherence analysis to a X-year continuous (1 hr interval) stream stage, precipitation, stream and air temperature dataset collected at two low-order mountainous streams in northwestern North Carolina: Boone Creek (urban) and Winkler Creek (forested). The main advantage of wavelet coherence analysis approach is the ability to analyze non-stationary dynamics for the temporal covariance between air and stream temperature over time and at multiple time scales (e.g., hours, days, weeks, months, seasons). We find that air temperature co-varies with stream temperature at 1 day and >1 week time periods. The coherence is poor at 2-4 day time periods, especially during summer storm events. Northwestern North Carolina experiences frequent intense precipitation events, especially afternoon convective storms between April and October, and in urbanized settings runoff from these storms may induce sudden increases in stream temperature, or temperature surges, which we define as an increase in stream temperature by >1°C in less than 15 minutes. Boone Creek, for example, may experience increases in discharge by nearly two orders of magnitude and increases in stream temperature of >6°C in less than 15 minutes due to the effects of these storms. These effects are not observed in forested catchments in the region. Four years of Boone Creek stream temperature monitoring produced records of 71 temperature surges. Empirical models relating air temperature to stream temperature fail at these time scales across the measurement period, likely as a response to the frequent temperature surges. Inclusion of stream stage or rainfall as an additional factor might improve the model predictions. Finally, our results show better correlation between air and stream temperatures in Winkler Creek than in Boone Creek, indicating that summer rainfall has a minor impact on stream temperature in the forested watershed relative to the urban watershed.