Water Flashcards
The interaction of EMR and water has lots in common with the interaction between EMR and what?
Atmosphere and vegetation
Attenuation =
Absorption and Scattering
What is the Lt (Total radiance) recorded by a RS system over water a function of?
Electromagnetic energy received from:
- Lp, atm path radiance
- Ls, free-surface layer (glint)
- Lv, Subsurface volumetric reflectance, water column (water constituents)
- Lb, Bottom reflectance (substrate, bathymetry)
Which spectrum penetrates the deepest in water?
Blue
Why does IR have a low reflectance for water?
Much of the IR is absorbed by water
- Assume it is like glint and use it to calibrate Ls (specular reflection)
Ls
- Radiance that reaches the air-water interface (free-surface layer or boundary layer)
- glint
- Waves make glint on one side
Glint
- Specular reflection on the surface of the water
- Waves can make glint on one side and increase all wavelengths radiance proportionally/equally
Lv
- Radiance that penetrates the air-water interface
- Interacts with organic/inorganic constituents
- Exits water before reaching bottom
- Subsurface Volumetric Radiance
- Provides information about the internal bulk characteristics of water column
What is the main Radiance variable that influences the water colour?
- Lv
- Function of water optical constituents and light
What are the water constituents that affect attenuation (absorption and scattering of light) for Case 2 waters?
- Water
- Colour Dissolved Organic Matter (CDOM)
- Phytoplankton (case 1 waters)
- Detritus and other organic material
- Inorganic material
Case 2 waters
- Coastal and inland
- Complex
- CDOM, Inorg seds, Phyto etc.
Case 1 waters
- Oceanic
Why does pure water appear blue?
- Blue spectrum exhibits low absorption
- UV and Yellow through IR heavily absorbed (<400nm and >580nm)
- IR has negligible scattering
- Some scattering at dark-blue spectrum is slightly more than absorption
Which Landsat bands would be better for displaying/delineating water bodies?
- Not visible, water can get lost in other information
- Use Infrared and Red, water will appear very dark due to high IR absorption and no real reflectance
CDOM stands for?
Colour Dissolved Organic Matter
- Dissolved component leftover after plant/animal break down
- Strong light absorber
Sources of CDOM
- Phytoplankton w/in photic depth of water column convert consumed nutrients into organic matter using photosynthesis (primary production), then bacterioplankton decompose this organic matter
- Humic substances from decomp of terrestrial organic matter
Primary production
- Phytoplankton photosynthesis of nutrients converting to organic matter
What does decomposition of organic matter in water lead to?
Leads to Dissolved Organic Matter (DOM) in oceanic, near-shore and inland water bodies
Sunlight penetrates the water column to what depth?
- Photic depth
If there is sufficient DOM in water, what may happen to penetration of light?
- Light penetration will reduce in depth
How does CDOM impact water?
- Impacts absorption of light in water column
- Changes water colour
- Can block light penetration and compete with creatures that require that light for primary production (eelgrass, phytoplankton etc.)
Absorption is the opposite of what?
- Reflectance, assuming transmission is low
What colour does water with significant CDOM appear? Why?
- Reddish Brown
- High CDOM = black tea coloured water
- CDOM heavily absorbs Low wavelengths (blue) and decreases abs towards red
How is CDOM measured?f
- Filter out particulates, 0.2micrometer filter
- Measure absorption of filtrate w/ spectrophotometer
- Absorption at 440nm is proxy for CDOM concern (m^-1)
Why is a(440) used to approximate aCDOM?
- Because CDOM will heavily absorb in this wavelength
- Low CDOM = low a440
- High CDOM = high a440
Inorganic Suspended Material
- Minerals found in suspension are common
- Size range varies from clay, silt, to fine and course sands
- Variety of sources
- Concentrated near inland and nearshore waters
- Rare in deep offshore oceans (less than 1 micrometer in diameter, smaller than clay)
What are some potential sources of inorganic suspended particles?
Agriculture, erosion, weathering, shore erosion (waves/boat traffic), volcanic ash
What happens to reflectance vs. wavelength as concentration of Inorganics increases?
- Scatter’s in all wavelengths, independent of nm, but generally more in short wavelengths
- Higher inorganic conc’n = higher scatter, independent of wavelength
How is TSS measured?
- Filter water
- Measure conc’n of material left on filter by weighing
- Weight 1 = x (clean filter)
- Weight 2 = x plus y (filter after 1L water, then dried, then weighed)
- Conc’n in mg/L
Compare measurement of CDOM vs. TSS
- CDOM uses dissolved filtrate and absorption as proxy for conc’n
- TSS uses particulate weight per L of water to get conc’n
Why does really pristine water look purple?
- Short wavelengths reflected, rest absorbed, scatters purple light at short wavelengths and Blue through Green a bit
Clay vs. Silt size reflectance
- Clay scatters more in red spectrum and reflects more IR than clear water
- Silt scatters more in red and green, and IR over clear water
- Increase conc’n of particles and all wavelengths will increase regardless of size but some scatter/reflect more than others
- Size of particle determines if spectrum will be reddish or green
Why does glacial outwash/lakes appear turquoise?
- Silt size particles in water
What is the correlation between conc’n of TSS vs. Reflectance
- Highly correlated
- As TSS increases, Reflectance increases
Secci Disk
- Measures turbidity/visibility in water
- Black and white quadrants
- Lower into water and when it is no longer visible that is the depth that light can no longer penetrate
Amazon River, water turbidity overview
- 1 river w/ heavy mining increases sediments, discharges into Amazon
- Amazon appears brown
- Measure and track conc’n of seds through system using Red reflectance
- Use Landsat time series to track change
Amazon River, water turbidity implications
- Less light for primary production = less phytoplankton = less fish
- Changes ecosystem
What did tracking the change of sediment conc’n in Amazon reveal over time?
- Landsat time series revealed correlation of sed conc’n with price of gold
- Gold increases, mining increases, sediment input increases
How is sediment in river (Amazon example) measured remotely?
- Using classification of remote image
- Find areas of increased sediment input
- Use optical index (B3-B2) x 100
Fraser Plume, impacts
- Visible = Milky whitish from seds
- Photic zone depth is shallower
- Scatters photosynthetically active so less is available to creatures
- Salish Sea is more productive on periphery of plume
Plankton
- Term describes all living organisms present in water-body that cannot resist the current
- Plants and animals
What do all phytoplankton in water contain?
- Photosynthetically active chlorophyll a pigment
- Also some chl b
What are 2 other phytoplankton photo-synthesizing agents besides chl-a
- Carotenoids, beta-carotene
- Biliproteins (phycocyanin, phycoerythrin)
Why are phytoplankton important?
- Productivity b/c base of food chain
- Carbon Sink
- Use to infer primary productivity in waters
What is a reasonable surrogate for phytoplankton productivity?
Chlorophyl a
Dominant chl a wavelengths of absorption
435 and 670-680nm
What is red tide caused by?
- Red or brown phytoplankton Dinoflagellates in the water
- Reflects red from phycoerythrin pigment absorbing more in B and G
- Kills shellfish and causes illness
What does the reflectance of algae-laden water look like? What happens when suspended sediment is added?
- Algae absorbs strongly in blue and red, reflects green
- Add sediment and shift reflectance from Green to Red
- Seds can distort reflectance of phytoplankton and affect productivity analysis, open ocean is better w/o coastal influence
What colour is water when algae is dominant factor?
- Green water because of chlorophyl
Fluorescence
- Phytoplankton can emit/reflect right before IR spectrum
- Peak can be used to evaluate phytoplankton when sediments distort reflectance but may be less accurate measurement
What satellite platform is designed to monitor productivity?
- MODIS-Aqua
- Measures chlorophyl with bands designed for B, G, R and Fluorescence
- Help monitor salmon in SoG and if they are getting what they need
What is one method to aid in modelling chlorophyl in the Salish Sea?
- Sensor’s on ferry measuring chl-a
- Compare w/ satellite and find algorithm is pretty good for measuring chl-a
- But ferry only measures on routes and satellite can measure large areas
Lb
- Radiance that reaches the bottom of waterbody, if it can get that far
- Reflects from bottom, propagates through water column, exits column
- Radiance is value of the bottom (depth, substrate type, algae)
Can IR be used for Lb?
- No b/c is heavily absorbed by water molecules
In what order does light stop penetrating the water, when clear?
IR, R, O, UV, Y, G, V, B
- Blue penetrate deepest, usually
What is the usual affect of light in clear water? What happens when CDOM is involved?
- IR and R stop penetrating the deeper it goes and blue goes deepest
- When CDOM involved it is difficult for light to penetrate and B and G is now absorbing, R may go further than B
- No light left to reflect after water abs IR and R, and CDOM abs B and G
Which SPOT band would be best for substate mapping? Why not other bands?
- Band 1, Green b/c interacts w/ corals
- Can’t use Band 3 b/c IR abs by water
- Band 2 Red only shows some info
- Blue Band would only be useful if no CDOM absorbing B
Irradiance vs. Depth: Clear water
- Deepest light penetration in clear water
- B and G deepest, scattered, use to detect substrate
- R absorbed by water molecules, doesn’t go as far
Irradiance vs. Depth: Chl-a
- G penetrates furthest, then Red, then Blue b/c of chl-a absorbance
- Max depth btwn 9 -10m
Irradiance vs. Depth: CDOM
- R penetrates furthest b/c B and G heavily absorbed by CDOM
- Max R penetration 5cm, B max at 10cm, G at 20cm
- Tea-coloured almost black water colour
Irradiance vs. Depth: Chl-a and some CDOM
- B doesn’t penetrate far
- G and R penetrate a few meters
- G lowers w/ addition of CDOM b/c it absorbs some green, but mostly blue
Where is CDOM usually highest?
- Inland waters
- Red gives most penetration
How to separate intertidal using Landsat TM5
- Use mix of Visible and IR
- Use IR band 5 and Visible bands 1 and 2
- Mix b/c IR good for land and doesn’t show much water, can use this to separate the land from intertidal in B and G
