This is a summary of a review article in the International Journal of Remote Sensing. Geoger had nothing to do with the article but it is an interesting topic and one that might be of interest to a wider audience. The original paper can be accessed here. The full reference is:
Chong Y, Khan A, Scheelbeek P, Butler A, Bowers D, and Vineis P (2014) Climate change and salinity in drinking water as a global problem: using remote-sensing methods to monitor surface water salinity, International Journal of Remote Sensing, 35, 3-4, 1585-1599
A recent report from the Intergovernmental Panel on Climate Change (IPCC) has estimated a global sea-level rise of up to 0.6m by 2100, threatening many coastal areas but in particular deltas. Worldwide, approximately 300 million people live in 40 major deltas with the average population density of the Asian mega-deltas (e.g. Ganges) is 500 people/km. A major impact of sea-level rise in these areas will be an increase in the salinity of freshwater resources through a process of saline intrusion. With such high population levels, it is important to map and understand the salinity of surface waters and estuaries in these areas. Satellite imagery and data can provide a continuous dataset, with near global coverage, to help in the monitoring of estuarine salinity.
Salinity can be mapped by recording changes in emissivity from water of microwave frequencies. This concept is used by SMOS but the ground resolutions are too course for effective use in estuarine areas. As salinity has no direct colour signal that can be observed in the records of higher resolution optical sensors a proxy is required. This is most often chromophoric dissolved organic matter (CDOM). CDOM is produced by the decay of organic material, either on land and washed into a water-body or in the water-body itself. Blue wavelengths of light are strongly absorbed strongly by CDOM, with more reflection of light at longer wavelengths. This differential can be analysed using remote sensing and then estimates of salinity inferred from the results.
Most of the methods used to infer salinity are empirical, although semi-empirical techniques can be used to draw in understanding of the physics involved in attenuation of light. In non-complex estuarine environments, a simple linear relationship between salinity and CDOM can be expected (as long as certain assumptions hold).
With saline intrusion effects likely to be magnified by climate change and rising sea-levels, the use of remote sensing can help map and monitor salinity changes, not just in the open ocean but also in coastal and estuarine areas. This could be critical in understanding threats to drinking water supply and human health (through increased intake of sodium chloride).
If you are able to access it, I recommend that you read the original article.