EXPLORING LINKS BETWEEN SEA SURFACE TEMPERATURE AND COLOR IN COASTAL AREAS UNDER THE EFFECT OF SUBMARINE GROUNDWATER DISCHARGE

Submarine groundwater discharge (SGD) is the flow of groundwater from coastal aquifers into the oceans. This poorly-known discharge carries nutrients that may locally influence the ecology of the ocean, as well as other dissolved solids that add to the ocean’s stockpiles. Sustainable groundwater management recognizes SGD as a resource that may be used for human consumption, especially in coastal regions facing water shortages.

SGD can be linked to changes in sea surface temperature (SST), since groundwater typically has a constant and lower temperature than sea water, thus originating a plume of anomalous cold water. To date, the identification of possible SGD occurrences requires prior knowledge of coastal hydrogeology, biogeochemical analyses of coastal waters, or the processing and interpretation of indirect measures, such as surface geophysics or remote sensing images.

We propose to use remote sensing visible (VIS) and thermal infrared (TIR) data acquired from Landsat 8 to pinpoint possible occurrences of SGD. Changes in SST are expected to correspond to changes in TIR, while VIS data refers to the ocean color (OC). SST and OC have been linked through the influence of temperature on the proliferation of algae and dissolved organic matter. This link is particularly relevant in coastal areas where there is SGD. SGD contains high concentrations of nutrients, thus they contribute to algae growth. For instance, a decrease in SST, caused by SGD cold waters, may lead to an increase of chlorophyll concentration, which in turn would lead to a greener surface. Therefore, SST and OC together may constitute an indicator of SGD.

Here, we compute second order derivatives of spectra composed by four VIS bands (ultra blue, blue, green, and red). It has been demonstrated that the second derivative may have features associated with the presence of chlorophyll. The change of the signal of the second order derivative is combined with a threshold in the TIR band to highlight the areas that are likely under the influence of SGD. The procedure is experimented in a set of case studies with different climatic and atmospheric conditions.