Abstract
Thermal refuges in streams are essential for the survival of coldwater fish species such as Redband trout (Oncorhynchus mykiss) in landscapes with stressful or lethal stream temperatures. We utilized an uncrewed aerial system (UAS) mounted with thermal and natural color sensors to conduct hourly flights spanning approximately 15 minutes each over a 24-hour period in Little Jacks Creek, a desert stream home to Redband trout during late summer when temperatures were near seasonal maximums and streamflow was near seasonal minimums. We used the fine-resolution imagery to map stream temperatures and determine where water temperatures exhibit significant variability. Thermal imagery from 3 out of 24 flights showed no significant differences when compared to true water temperatures from in-stream temperature loggers, which appeared to be highly dependent on atmospheric conditions. The thermal imagery (range of 9.17 to 21.04°C) consistently underestimated HOBO logger stream temperatures (range of 13.6 to 17.1°C) during cooler, nighttime flights, and overestimated temperatures during hotter, afternoon hours, resulting in a global RMSE of 2.12°C. Between flight RMSE values ranged from 0.53°C to 4.00°C, within the error range of the thermal sensor. The sensor showed a wide range of stream temperatures during the day with differences of almost 7°C between flights. This study yields valuable lessons for future stream temperature data collection that will aid in the calibration of thermal sensors on UAS missions. Furthermore, our results provide insights into environmental stressors within desert streams vital for conservation efforts.