A brief introduction to (ocean color) remote sensing of water quality
Jeremy Werdell Joel Scott, Ryan Vandermuelen NASA Goddard Space Flight Center Ocean Ecology Laboratory
NASA Water Quality Workshop 27 September 2017 Why NASA? Why satellites?
http://www.chesapeakebay.net
routine data collection since 1984 12-16 cruises / year
49 stations 19 hydrographic measurements
algal biomass Chesapeake Bay Program water clarity dissolved oxygen 1-day of MODIS-Aqua
satellites complement in situ sampling with routine, synoptic, & consistent views of our critical marine ecosystems
NASA Water Quality Workshop [email protected] 2 What is “ocean color”?
clear water sediment-rich water
)
l
(
s
r
R Water Reflectance Water
Spectral Wavelength (l) color
the spectral distribution of reflected sunlight can be used to infer the contents of the water
NASA Water Quality Workshop [email protected] 3 Measurements of ocean color are based on electromagnetic energy emitted by sunlight, transmitted through atmosphere, and reflected by Earth’s surface.
Water-leaving Radiance, Lw
AIR SEA
SCATTERING (bb)
Organic Matter There are two Detritus possible things that Phytoplankton can happen to a photon in water ABSORPTION (a)
NASA Water Quality Workshop [email protected] 4 400 500 600 700
400 500 600 700
400 500 600 700
400 500 600 700
400 500 600 700
NASA Water Quality Workshop [email protected] 5
chlorophyll-a (algal biomass) Ocean color data products )
3 -
diffuse light attenuation dissolved organic matter a a m (mg
(water clarity, turbidity) absorption (runoff) - chlorophyll ratio of blue:green radiances
and, many others, including: phytoplankton community composition (including HABs) particle size distributions (water composition) particulate (in)organic carbon (productivity) euphotic depth (visibility, water clarity) particle backscattering water temperature (MODIS, VIIRS) red light reflectance (sediment load) (sediment load) NASA Water Quality Workshop [email protected] 6 Steps for deriving ocean color data products from space
the satellite views the spectral light SATELLITE field at the top-of-the-atmosphere
TOP-OF-THE-ATMOSPHERE
1. remove atmosphere from total 3. spatially / temporally signal to derive estimate of light bin and remap the field emanating from sea surface satellite observations (remote sensing reflectance, Lw)
SEA SURFACE the water signal is often less than 10% of the total signal measured by the satellite 2. relate spectral Lw to a chlorophyll-a concentration (or PHYTOPLANKTON geophysical product of interest)
NASA Water Quality Workshop [email protected] 7 Different instruments & missions offer different capabilities Landsat 8 OLI, 26 Oct 2016, Queensland, Australia
different algal groups (spectral bands)
image artifacts dark ocean compared to bright targets
atmospheric correction (spectral bands + instrument performance)
temporal repeatability ground sample contamination distances by Sun glint
NASA Water Quality Workshop [email protected] 8 Heritage & future missions
NASA Water Quality Workshop [email protected] 9 Applications for water quality monitoring
NASA Water Quality Workshop [email protected] 10 Towards demystifying the use of satellite ocean color data
https://oceancolor.gsfc.nasa.gov
NASA Water Quality Workshop [email protected] 11
Further demystifying the use of satellite ocean color data
seadas.gsfc.nasa.gov https://
NASA Water Quality Workshop [email protected] 12 Thank you
NASA Water Quality Workshop [email protected] 13