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

Abstract

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