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Coupling Suspended Sediment Dynamics and Light Penetration in the Upper Chesapeake Bay. Charles L. Gallegos Smithsonian Environmental Research Center. Photos by Grace Cartwright. Outline. Review Model Refinement Field Work Potomac findings Baywide extension Summary. - PowerPoint PPT Presentation
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Coupling Suspended Sediment Dynamics and Light Penetration in the Upper
Chesapeake Bay
Charles L. Gallegos
Smithsonian Environmental Research Center
Photos by Grace Cartwright
Outline• Review
• Model Refinement
• Field Work–Potomac findings
–Baywide extension
• Summary
Elements of Radiative Transfer Modeling in Natural Waters
• Radiative Transfer Equations perform an energy balance on an infinitesimal solid angle, accounting for gains and losses
• First-order effects are governed by the magnitude of inherent optical properties (IOPs): absorption coefficient, scattering coefficient, and scattering phase function (principally, backscattering:total scattering ratio)
• Primary water quality determinants of the IOPs are colored dissolved organic matter (CDOM), phytoplankton pigments, and other particulates (detritus, suspended minerals).
Inherent Optical Properties
esParticulat algal-NonPhytoCDOMwatertotal aaaaa
Absorption Coefficient
•Water: From published tables (Pope and Fry 1997)
Laboratory
In Situ (total-water)•WETLabs Spectral ac-9
•CDOM: Spectrophotometric with long path cell•Particulate: Filter pad method
Inherent Optical Properties
tp acb Particulate Scattering
Beam Attenuation
Total Absorption
Scattering Coefficient
Also measured using WETLabs ac-9 in situ or in laboratory.
Inherent Optical PropertiesScattering Phase Function, ()/b
• Probability distribution of scattered photons
• Strongly peaked in the forward direction
• Well specified by the ratio of backscattering:total scattering, bb/b
• Measured in situ by HOBILABS Hydroscat-6 or WETLabs ECO-VSF-3
Backscattering,
bb
Forward scattering,
bf
bf
bf
bb
bbb
Kd Relationship Revisited:Parameterize effects of variations in…
• Solar incidence angle, µ0
• Relative proportion of scattering and absorption, principally b:a ratio
• Backscattering ratio, bb:b
• Optical depth, c·z
• Direct and diffuse incident light*--New complexity
New Bio-optical Model is Based On*:
0 b
zd
baK
Depends on: optical depth, backscattering ratio, solar zenith angle, and scattering:absorption ratio.
Ratio backscattering:total scattering
Cosine in-water solar zenith angle
*—Albert, A., and C. D. Mobley. 2003. An analytical model for subsurface irradiance and remote sensing reflectance in deep and shallow case-2 waters. Optics Express 11: 2873-2890.
Implementation
• Algorithm has 33 equations due to multiple dependencies of z on other parameters (i.e. bb/b, b/a, etc.)
• Some of the governing parameters, e.g. backscatter ratio, are poorly represented in data
• Solution at each cell and time step would place excessive drag on complete model
Look-up Table Approach• Used bio-optical model to generate Kd values for
a range of inherent optical properties in nested loops of:– I: Cosine solar angle (8 bins, 0.71-0.91)– J: CDOM (20 bins, 0.1-8 m-1)– K: Chlorophyll absorption (35 bins, 0.02-18.4 m-1)– L: Particulate scattering (35 bins, 0.6-570 m-1)– M: Particulate absorption:scattering ratio (10 bins,
0.06-0.24)
• Resulting array consists of 1,960,000 values of Kd(PAR)
Lookup Table Approach cont’d.
• Initialize:– Read in Kd(PAR) array
– Read in season- and segment-specific inherent optical properties
• From date, segment, and water quality (CDOM, chlorophyll & TSS), search bins to find I, J, K, L, & M
• Look up Kd(PAR) in array
Lookup Table Approach cont’d.Must Determine…
• Absorption by CDOM• Absorption by chlorophyll
– Product of specific-absorption coefficient and chlorophyll concentration
• Scattering by particulates– Product of specific-scattering coefficient and TSS
concentration
• Absorption by non-algal particulates– Product of scattering and absorption:scattering ratio
Need: CDOM absorption, and specific-absorption and –scattering coefficients on segment and season basis
CDOM Concentrations: Potomac
0 20 40 60 80 100 120 1400.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Distance Below Fall Line (km)
ag(4
40
) (m
-1)
Mar04 Jun04 Aug04 Mar05 Jun05 Sep05
• Seasonal variability>>Spatial
• Current sampling program expected to be adequate characterization
Bay-wide Extension: Sample Coverage
CDOM Absorption: Bay-wide
Chlorophyll-specific Absorption: Potomac
0 2 4 6 8 10 12 14 16 180.0
0.1
0.2
0.3
0.4
0.5
a (6
75)
(m-1
)
Chlorophyll (mg m-3)
March June Aug-Sep
a*(675) = 0.028 m2 mg-1 Mar-Jun
0.018 m2 mg-1 Aug-Sep
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.0902468
101214161820222426283032
Fre
quen
cya
*(675) m2 (mg Chl)-1
Ensemble mean: 0.030 m2 mg-1
Forced zero-intercept Regressions
*Point-estimate a(675)/[Chla]
*—Necessary when sample size is small
Chlorophyll-specific Absorption: Bay-wide
Particulate Scattering: Potomac
0 50 100 150 200 250
0
20
40
60
80
100
120
bp(5
55)
(m-1)
TSS (g m-3)
Mar JunAug-Sep
20 40 60 80 100 1200.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
b p* (555
) (m
2 g-1)
km Below Fall Line
•Forced zero-intercept: = 0.57 m2 g-1
•Ensemble average of point-estimates = 0.94 m2 g-1
•Varies systematically along river axis
Particulate Scattering: Bay-wide
NAP Absorption:Scattering Ratio: Potomac
0 2 4 6 8 1012141618202224262830323436380.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
a p-(
440)
(m
-1)
bp(555) (m-1)
Mar Jun Aug-Sep
555
440
p
p
b
aPCI Define “Particle Color Index”, PCI,
•From forced zero-intercept regression, PCI=0.088
•Ensemble average, PCI=0.12
•Variability principally temporal
2 3 4 5 6 7 8 9 100.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
PC
I
Month
NAP Absorption:Scattering Ratio: Bay-wide
Light Attenuation Model EvaluationBased on Measured Coefficients—Potomac River
0 1 2 3 4 50
1
2
3
4
5
Sim
ulat
ed K
d(P
AR
) (m
-1)
Measured Kd(PAR) (m-1)
• 2004 data
• Slope (model vs. obs.) = 0.98, r2=0.75
• Used measured, station-averaged coefficients
Light Attenuation ModelBay-wide Extension Based on Look-up Table Approach
1 10
1
10
Model Ref Reps
Mod
eled
Kd(
PA
R)
(m-1)
Measured Kd(PAR) (m-1)Chesapeake Bay Program Data
5,873 Observations
1995-1999
Backscatter Fraction: Potomac
8 8
3 2 22 2
2 35
3 3 3
LE2.
3
LE2.
2
PR
01
PR
02
PR
03
PR
04
PR
05
PR
06
PR
07
PR
08
PR
09
PR
10
PR
11
0.000
0.005
0.010
0.015
0.020
0.025
Bac
ksca
tter
Fra
ctio
n, b
b/b
Station
• Difficult to measure in RET/TF sections of river due to high concentrations of TSS saturating instrument
• Available data indicates systematic longitudinal gradient
• Direct measurements unavailable elsewhere, except CB4MH (average=-.0125)
LE2.
3
LE2.
2
PR
01
PR
02
PR
03
PR
04
PR
05
PR
06
PR
07
PR
08
PR
09
PR
10
PR
11
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
Pre
dic
ted
Kd(P
AR
) (m
-1)
Station
Implications for Prediction of Kd(PAR)
Constant Water Quality Assumed:
CDOM=0.5 m-1; Chla=6 mg m-3; TSS=12.7 g m-3; based on 6/15/05, PR04
• Ca. 30% change in simulated Kd(PAR) due to changes in bb/b alone
• For assumed condition, change crosses SAV Tier II habitat requirement
To Do
• Re-generate Kd for Lookup Table varying backscatter ratio, bb/b
• Modify Lookup Table routine to take advantage of further bb/b data as it becomes available
Symbols for diagrams courtesy of the Integration and Application Network (ian.umces.edu/symbols), University of Maryland Center for Environmental Science.
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