Lacustrine groundwater discharge (LGD) can substantially impact ecosystem characteristics and functions. Fibre optic distributed temperature sensing (FO-DTS) has been successfully used to locate groundwater discharge into lakes and rivers at the sediment-water interface, but locating groundwater discharge would be easier if it could be detected from the more accessible water Lacustrine groundwater discharge (LGD), that is, the discharge of groundwater (GW) into lakes, can substantially impact ecosystem characteristics and functions (Baker et al., 2014;Ridgway & Blanchfield, 1998;Warren, Sebestyen, Josephson, Lepak, & Kraft, 2005). Upwards directed GW flow is sometimes called upwelling, especially in the context of hyporheic zones, where commonly both upwelling and downwelling occur along river reaches. In the present manuscript, we use the term upwelling solely for upward transport processes in the water column; this definition is adopted from limnophysics. On the one hand, upwelling of warm water in cold lakes can be caused by natural processes such as GW flow across the lake bed into the cold lake water body during winter conditions (LGD; Lewandowski, Meinikmann, Ruhtz, Pöschke, & Kirillin, 2013) or thermal springs in volcanic lakes (Cardenas et al., 2012). On the other hand, it can be related to thermal pollution ------------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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.