Combining continuous spatial and temporal scales for SGD investigations using UAV-based thermal infrared measurements

摘要

Submarine groundwater discharge (SGD) is highly variable in spatial andtemporal terms due to the interplay of several terrestrial and marineprocesses. While discrete in situ measurements may provide a continuoustemporal scale to investigate underlying processes and thus account fortemporal heterogeneity, remotely sensed thermal infrared radiation shedslight on the spatial heterogeneity as it provides a continuous spatial scale. Here we report results of the combination of both the continuous spatial andtemporal scales, using the ability of an unmanned aerial vehicle (UAV) tohover above a predefined location, and the continuous recording of thermalradiation of a coastal area at the Dead Sea (Israel). With a flight altitudeof 65 m above the water surface resulting in a spatial resolution of 13 cmand a thermal camera (FLIR Tau2) that measures the upwelling long-waveinfrared radiation at 4 Hz resolution, we are able to generate a time seriesof thermal radiation images that allows us to analyse spatio-temporal SGDdynamics. In turn, focused SGD spots, otherwise camouflaged by strong lateralflow dynamics, are revealed that may not be observed on single thermalradiation images. The spatio-temporal behaviour of an SGD-induced thermalradiation pattern varies in size and over time by up to 155 % for focusedSGDs and by up to 600 % for diffuse SGDs due to different underlying flowdynamics. These flow dynamics even display a short-term periodicity of theorder of 20 to 78 s for diffuse SGD, which we attribute to an interplaybetween conduit maturity–geometry and wave set-up.