TEMPERATURE SURGE CHARACTERISTICS IN A DYNAMIC AND URBANIZED HEADWATER STREAM

Boone Creek is a dynamic, low to moderate-gradient headwater stream running through an urbanized area of Boone, North Carolina, USA. The 5.2 km²catchment has a variable impervious surface coverage that increases downstream from 13.4% to 24.3% over a study reach length of approximately 1.5 km. In this study we characterize temperature surges within the stream, which we define as a stream temperature increase of 1 °C within a 15-minute period. We collected stream temperature and discharge data at a location near the midpoint of the study reach between September 2012 and September 2013. Rating curves were used to convert stream stage to stream discharge based on data measured at an automated stream gauge. The data suggest that a total of 90 temperature surges occurred during the monitored period and that these surges had a mean start time of 12:27, a mean temperature increase duration of 25.08 minutes, a mean temperature increase of 2.66 °C, a mean surge temperature increase of 2.38 °C, and a mean surge duration of 3.17 hours.

The primary focus of our study is to determine the quickflow temperature required to create the temperature surges that we measured in the stream. We used the local minimum hydrograph separation method to determine the baseflow component of total stream discharge. After the baseflow component was separated we were able to subtract the baseflow from the stream flow to obtain the quickflow component. We used a similar technique to separate baseflow temperature during temperature surge events. We then used a simple energy balance equation to determine the quickflow temperatures for each surge event based on known stream and baseflow temperatures and calculated components of flow. The analysis indicates the temperatures needed to produce the temperature surges of the individual storm events. The maximum quickflow temperature calculated with the energy-balance model is 44.32 °C. Stream temperatures rose above 20 °C, a critical temperature threshold for cold-water species such as trout, in 17 surge events that had a mean maximum stream temperature of 22.17 °C and a mean maximum quickflow temperature of 30.13 °C. Our study suggests the need for mitigation of quickflow during temperature surge events in order to limit rapid temperature increases above the temperature threshold of cold-water species.