Our group at CU is developing and testing (this year) a small microwave radiometer operated on a UAV for river, estuary, and coastal salinity mapping. We have presented this concept at IGARSS 2012 and earlier conferences, and are in a build stage this year. The spatial resolution will be of order several tens of meters, but the range limited to far below that of Aquarius. - AJG [email protected]

Given the inverse relationship between CDOM and salinity, CDOM fluorescence or absorption seems an easier way to get at salinity. I believe Arnone's group at NRL used that to map salinity.

I agree with Emmanuel that the relationship to CDOM is probably a more promising approach to take. Our Aquatic Remote Sensing Group at CSIRO recently published work on the relationship of CDOM to salinity for river plumes impacting the Great Barrier Reef. Check out:

Schroeder, T., Devlin, M., Brando, V., Dekker, A., Brodie, J., Clementson, L., and McKinna, L. (2012). Inter-annual variability of wet season freshwater plume extent into the Great Barrier Reef lagoon based on satellite coastal ocean colour observations. Marine Pollution Bulletin, 65: 210-223.

Abstract:

Riverine freshwater plumes are the major transport mechanism for nutrients, sediments and pollutants into the Great Barrier Reef (GBR) lagoon and connect the land with the receiving coastal and marine waters. Knowledge of the variability of the freshwater extent into the GBR lagoon is relevant for marine park management to develop strategies for improving ecosystem health and risk assessments. In this study, freshwater extent has been estimated for the entire GBR lagoon area from daily satellite observa- tions of the Moderate Resolution Imaging Spectroradiometer (MODIS) between 2002 and 2010. To enable a reliable mapping of freshwater plumes we applied a physics-based coastal ocean colour algorithm, that simultaneously retrieves chlorophyll-a, non-algal particulate matter and coloured dissolved organic mat- ter (CDOM), from which we used CDOM as a surrogate for salinity (S) for mapping the freshwater extent.

Microwave estimates are more accurate. However resolution of space-borne microwave sensors is too coarse to resolve river plumes. If you are interested in river plumes, you might want to use CDOM proxies.

Raphe Kudela's group has a good article on predicting river plume salinity from MODS cdom absorption estimates:

'Development of synthetic salinity from remote sensing for the Columbia River plume' jgr, 2009.

Thanks for the information.

Vic K.

Vic,

Agree on the CDOM; but sediment is good for the simple question of: where is the plume? CDOM is much more conservative than sediment (it doesn't settle out of the water it came with), which makes it a better tracer.

However, care is needed. The CDOM concentration varies with drainage basin and with discharge, and worse, the estimation of CDOM in a sediment-rich plume will be convolved with the particulate non-algal absorption, because the latter has the same spectral shape. So the "CDOM" will tend to be over-estimated in sediment-rich waters (and decrease rapidly as the sediment settles until the true CDOM dominates). Having said that, I would use a CDOM estimate as a relative salinity indicator in coastal water. I've looked at a fair bit of CDOM salinity products (Arnone's group's work) for the northern Gulf of Mexico, and they are useful, if treated cautiously. For blackwater rivers CDOM is good (even better if the rivers are from the same region). The Big Bend region of Florida is a good example, the river water is like black tea, and you can readily trace the plumes (e.g. the Suwannee) and mixing by the CDOM. Until the CDOM decays, in sediment-poor rivers, CDOM traces the river water.

Oh, and to go way beyond your question, we don't have enough degrees of freedom in the available satellite data (and probably even with hyperspectral data) to deconvolve the dissolved and particulate (non-algal) pigments with any meaningful reliability. The spectral absorption of these pigments are boring monotonic functions of the same form, so the absorptions at all wavelengths are highly correlated.

--rick

Rick,

An approach to separate sediment contribution from CDOM involves using scattering (NIR reflectance) to get at sediments (turbdity, e.g. yours and Nechad approach), and using an assumed specific absorption for particle to separate CDOM absorption from that of sediments. It is doable with the current degrees of freedom.

Best,

Emmanuel

Emmanuel,

Good point to raise, and that's how I would (and occasionally do) determine a_cdom, not that you need my endorsement! The specific absorption in a particular scene can be tuned well by inspecting the results across the sediment plume. For any others reading, the two absorption quantities are linked, overrestimate of one leads to underestimate of the other. Makes it tough for automated processing because we have to assume that specific absorption (even if from another source); calculating it exhausts our degrees of freedom in a zero-sum game. (Oh, and I'm thinking of places with lots of stuff in the water.) Anyway, the particulate absorption may be a complexity in the CDOM-salinity relationship determination, if sediment loads are particularly high. On the other hand, the sediment absorption may be a problem where the salinity is low enough that it doesn't matter.

I prefer algal blooms, much easier to work with.

Always good to hear from you.