Remote Sensing of Water
Resources
Lecture Notes 10
prepared by R. Lathrop 11/00
Revised 10/07
Learning Objectives
• Remote sensing science concepts
– Understand fundamental interactions of light and water
along with the effect of various constituents that
determine water color
– Understand how LiDAR works and potentially used to
map bathymetry
– How wetlands are classified
• Skills
– Photo-interpretation of various classes of wetlands
Reflectance of a water body
depends on
•
•
•
•
•
•
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Lu: reflectance signal at sensor
Lv: subsurface volumetric scattering
La: subsurface volumetric absorption
Lb: bottom backscattering
Lp: Atmospheric scattering
Ls: water surface scattering
Lu = [(LV + Lb )– La] + Ls + Lp
Reflectance of a water body
•
•
•
•
•
•
•
Lu: reflectance signal at sensor
Lv: subsurface volumetric scattering
La: subsurface volumetric absorption
Lb: bottom backscattering
Lp: Atmospheric scattering
Ls: water surface scattering
Lu = [(LV + Lb )– La] + Ls + Lp
Lu
Lp
Incoming sunlight
Ls
La
Lv
Lb
Reflectance of a water body
depends on
• Subsurface volumetric scattering & absorption
determined by H2O and constituents in the water
• Water absorbs a large % of the incoming radiation,
only a small % is reflected, therefore water appears
dark in color and CIR imagery.
• Atmospheric scattering due to haze can add greatly
to the water reflecting signal.
• Specular reflection off the water surface (sunglint)
can obscure the water volume reflectance but can
provide information on the surface sea state (wave
heights and patterns).
Absorption & scattering
characteristics of pure water
• Minimal absorption( highest transmission) in
the blue spectral wavelengths - minima near
460-480 nm
• Scattering increases with shorter wavelength maxima in the ultraviolet
• Combined effects of absorption/scattering:
highest reflectance in the blue visible region
decreasing to very low reflectance in near ir
and mid ir regions
Spectral reflectance curve of water
Graphic from
http://ceos.cnes.fr:8100/cdrom/ceos1/irsd/pages/intro2c.htm
Water Color
a function of organic and inorganic constituents
• Suspended sediment/mineral: brought into water
body by erosion and transport or wind-driven
resuspension of bottom sediments
• Phytoplankton: single-celled plants also
cyanobacteria
• Dissolved organic matter (DOM): due to
decomposition of phytoplankton/bacteria and
terrestrially-derived tannins and humic
substances
Water Color
a function of organic and inorganic constituents
• Suspended sediment/minerals: increases
volumetric scattering and peak reflectance shifts
toward longer wavelengths as more suspended
sediments are added
• Near IR reflectance also increases
• Size and color of sediments may also affect the
relative scattering in the visible
Relationship between suspended sediment concentration (SSC) in a
water body and spectral reflectance: SSC
Green & Red
Graphic from http://www.ga.gov.au/map/east_coast/fig2.gif
Suspended Sediment Plume
Water Color
a function of organic and inorganic constituents
• Phytoplankton: contain photosynthetically
active pigments including chlorophyll a
which absorbs in the blue (400-500nm) and
red (approx. 675nm) spectral regions;
increase in green and NIR reflectance
• Suspended sediment and DOM will
confound the chlorophyll signal. Typical
occurrence in coastal or Case II waters as
compared to CASE I mid-ocean waters
Absorption coefficients for chlorophyll-rich green oceanic
waters. aw is the absorption spectrum for clear water
Decreasing
reflectance
Increasing
absorption
Note that the
absorption
coefficient is
inverted as
compared to
the spectral
reflectance:
as CHL
absorption
the green,
absorption
in the blue
and red.
Graphic from http://www.ga.gov.au/map/east_coast/fig2.gif,
originally from Morel and Prieur, 1977
Water Color
a function of organic and inorganic constituents
• Dissolved organic matter DOM: strongly
absorbs shorter wavelengths (e.g., blue)
• High DOM concentrations change the color
of water to a ‘tea-stained’ yellow-brown
color
Strong absorption by Dissolved Organic Color in the shorter
blue to green wavelengths: DOC
Blue Abs
Blue Refl
Graphic from http://www.aquabotanic.com/paper2-6.html
Water in Other Forms:
Clouds and Snow
• Clouds: generally high reflectance due to
scattering across the visible, NIR and
MidIR spectral regions
• Snow: High reflectance across the visible
and NIR but low in the MidIR due to
absorption
Spectral
Profile:
Snow
<>
http://speclib.jpl.nasa.gov/forms/asp/water.htm
Note how
reflectance
decreases as the
snow ages and
compacts
http://cires.colorado.edu/~maurerj/albedo/snow_reflectance.gif
Spectral
reflectance of
ice for remote
sensing here on
Planet Earth
and beyond:
H20, CO2 and CH4
Graphic from
http://speclab.cr.usgs.gov
/PAPERS.reflmrs/refl4.html
Remote sensing of water depth
and bottom type
• If water is shallow and clear enough, light will be
transmitted through the water column, reflect off the
bottom and back through the water column to be
received by the sensor.
• Under these conditions the bottom type can be
determined. Bright sandy bottoms and coral reefs are
highly reflective and appear light. Submerged aquatic
vegetation beds appear dark.
• The change in water color/reflectance as one moves
from shallow to deep water under consistent water
volume and bottom type conditions, can be used to
provide information on water depth (bathymetry).
Remote Sensing of the Benthos
• Passive (Visible wavelengths) vs. Active
(lidar or sonar)
• Bathymetric mapping: mapping differences
in depth
• Bottom type mapping: detecting and
differentiating different benthic habitat,
from submerged aquatic vegetation to coral
reefs
Example Imagery: Barnegat Bay New Jersey
Quickbird Satellite Imagery (Fall 2004)
Aerial Photography (Spring 2003)
Light Detection and Ranging
(LIDAR)
• LIDAR uses pulsed laser light
to measure travel time from the
laser transmitter to a target and
back to determine ground
surface elevation with high
vertical accuracy, available as
2-D image
graphic from
http://soundwaves.usgs.gov/
Bathymetric LiDAR
• Scanning Hydrographic Operational Airborne
Lidar Survey (SHOALS) system.
•SHOALS fires 2 lasers into the water:
NIR & blue-green.
•The NIR pulse is reflected off the
water surface.
•The blue-green pulse penetrates the
surface and is reflected off the
seafloor.
•The water depth is then calculated
from the time difference between the
NIR surface return and the blue-green
bottom return. http://shoals.sam.usace.army.mil/Pages/Welcome_to_shoals.htm
SHOALS Project Examples
• Lake Worth, Florida navigational channel:
11 meters deep (dark blue/magenta) shoaling (cyan)
http://shoals.sam.usace.army.mil/Pages/lake_worth.htm
NASA's Experimental Advanced
Airborne Research Lidar (EAARL)
•EAARL is a "multiple reflection" lidar instrument that measures
the full "waveform" of the returned signal.
•The unique shape of the waveform reveals where—in the space
between the ground and the top of the canopy —the foliage, trunk,
and branches are concentrated.
Image by Robert Simmon
http://earthobservatory.nasa.gov/Library/VCL/VCL_2.html
• NASA EAARL
Equipped with a green
laser, EAARL is also
useful for bathymetric
and underwater
topographic mapping
graphic from
http://gulfsci.usgs.gov/tampabay/data/1mapping/LiDAR/lidar.html
Ongoing Research Question:
Can we use the EAARL system to map seagrass beds
location and density in Barnegat Bay?
Wetlands
lands where saturation with water is a
dominating factor
• Lands with predominantly hydrophytic
(water-loving) vegetation cover
• soil that is predominantly hydric
• for areas without vegetation or soil, the land
is flooded or saturated at some time during
the growing season each year
• Aerial photo interpretation is an important
method for mapping wetlands
Wetlands classification system
Cowardin et al. 1979
USF&WS National Wetland Inventory
• 1st level(System): marine, estuarine,
riverine, lacustrine, palustrine
• 2nd level (Subsystem):
M/E -subtidal, intertidal
R - tidal, perennial, intermittent
L - limnetic, littoral
• 3rd level (Class)
Wetlands classification system
Class types
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rock bottom
unconsolidated bottom
aquatic bed
unconsolidated shore
moss-lichen wetland
emergent wetland
scrub-shrub wetland
forested wetland
Rock Bottom/Shore
Door Peninsula, WI
Tierra del Fuego, Chile
Unconsolidated Bottom/Shore
Little Egg Harbor, NJ
Aquatic Bed
Eelgrass bed, Barnegat Bay, NJ
Emergent wetlands
Broad-leaved herbaceous vegetation
Grass-sedge-rush vegetation
Emergent wetland: saline marsh
Sheepshead Meadows, NJ
Great Salt Lake, Utah
Emergent Wetlands:
Phragmites around the world
Great Salt Lake, UT
Cheesequake, NJ
Hampshire coast, UK
Scrub/shrub wetland
Everglades, FL
Scrubby tree islands
Coastal Mangrove swamps
Forested Wetland
Coniferous-dominated
(A. white cedar swamp)
Pine Barrens, NJ
Deciduous-dominated
(red maple swamp)
Pine Barrens, NJ