Comparing remote sensing signatures of upwelling events and submarine groundwater discharge in coastal areas

Submarine groundwater discharge (SGD) is the flow of groundwater from coastal aquifers into the oceans. Sustainable groundwater management recognizes SGD as a resource that may be used for human consumption, especially in coastal regions facing water shortages. Most attempts to identify SGD occurrences are based on in situ measurements (site specific).

SGD can be linked to changes in sea surface temperature (SST), since groundwater typically has a constant and lower temperature than sea water, originating plumes of anomalous cold water. SGD contains high concentrations of nutrients, which contribute to primary productivity, thus promoting algae growth and, consequently, the increase in chlorophyll concentration and changes in ocean color (OC). Thus, SST and OC together may constitute an indicator of SGD. However, coastal upwelling events are also known to cause changes in SST and OC, as the nutrient enriched cold waters rise to the surface. To date, the identification of possible SGD occurrences on a large scale through remote sensing has been neglecting this conflicting signature with upwelling events.

We propose to use remote sensing visible (VIS) and thermal infrared (TIR) data acquired from Landsat 8 to compare the signatures of these two phenomena. While identifying plumes where SST and OC are characteristic of cold and nutrient enriched waters is not sufficient to distinguish between SGD and upwelling, we hypothesize that the location, area, and temporal variability of those plumes contribute to that goal. We identify possible SGD or upwelling plumes through a combination of derivative and cluster analyses of TIR and VIS spectra. Then, we determine the area of the plumes, and their location in relation to the coast line and isobaths. This process is repeated on images acquired across time to distinguish between seasonal and perennial events. These methods are applied to the southwest coast of Ireland, an area of known SGD and coastal upwelling occurrences.