Thermal infrared imaging for the detection of relatively warm lacustrine groundwater discharge at the surface of freshwater bodies

Thermal infrared (TIR) imaging has been previously applied to survey relatively large thermal footprints in coastal zones, lakes, reservoirs and rivers. In freshwater systems, the buoyancy of relatively warm groundwater during the winter months allows for the surface identification of groundwater discharge or thermal pollution using TIR imaging. However, information regarding the performance of TIR for resolving this warm groundwater upwelling is limited, particularly at fine spatial scales and variable discharge rates. In order to evaluate the suitability of TIR to trace warm groundwater upwelling at the water surface of lakes, ponds and reservoirs (e.g. lacustrine groundwater discharge (LGD) in shallow near-shore zones) we conducted a mesocosm experiment with a TIR camera situated 4 m above the water surface to capture thermal patterns in response to different groundwater discharge rates, weather conditions and the diurnal cycle. A fiber optic distributed temperature sensing system (FO-DTS) installed at 2 cm below the water surface was used to ground-truth spatial patterns observed in TIR images. Results show the impacts of both the diurnal cycle of net radiation and prevailing weather conditions on the accuracy of TIR imaging for resolving warm groundwater discharge. Most reliable results were obtained under overcast weather conditions and during the night. The results of our study provide guidance for those looking to use TIR for conducting thermal tracing of LGD at the surface of freshwater bodies during winter.