Estimation of Secchi Transparency in Turbid Coastal Waters

Available online at www.sciencedirect.com

ScienceDirect Aquatic Procedia 4 (2015) 1114 – 1118

INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN ENGINEERING (ICWRCOE 2015)

Estimation of Secchi Transparency in Turbid Coastal Waters Anuj Kulshreshthaa and Palanisamy Shanmugam*a Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai – 600036, India

Abstract The estimation of water clarity in terms of Secchi disk transparency is important for assessing water quality in coastal and various in-land water bodies. In this work, we have developed a new method to estimate underwater visibility depth, commonly known as the Secchi disk transparency (Zsd). The input parameters for this model include turbidity (NTU), vertical diffuse attenuation coefficient Kd (m-1), and beam attenuation coefficient c (m-1). The turbidity parameter is used to redefine the coupling coefficient. The successful validation of this model is achieved at a wavelength of 520 nm using independent dataset obtained from relatively clear and turbid waters. The performance of the present model was evaluated in terms of various statistical parameters which included MRE~-0.012 and RMSE~0.073, with approximately bias and slope of 0.004 and ~1 respectively © by Elsevier B.V.by This is an open access article under the CC BY-NC-ND license © 2015 2015Published The Authors. Published Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015. Peer-review under responsibility of organizing committee of ICWRCOE 2015 Keywords: Coastal and in-land waters; water clarity; diffuse attenuation coefficient; beam attenuation coefficient; Turbidity

*Corresponding author. Tel.: 91-44-22574818;. E-mail address: [email protected]

2214-241X © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015 doi:10.1016/j.aqpro.2015.02.141

Anuj Kulshreshtha and Palanisamy Shanmugam / Aquatic Procedia 4 (2015) 1114 – 1118

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1. Introduction The adequate level of light intensity is necessary to support various aquatic lives and plays a crucial role for the growth of phytoplankton (micro-organisms) in the water column. This also implies that the regulation of rate of photosynthesis and diverse bio-geochemical phenomenon are dependent on the propagation trend of light intensity governing the underwater geometric light filed structure in the water column (Kirk, 1994; Marra, Langdon, and Knudson, 1995; Mcclain et al., 1996). The accurate estimation and assessment of index of water clarity in various regional water bodies like ponds, river, lakes, etc. is crucial to ascertain its credibility for recreational usage and diversified eco-biological activities. The trend of light penetration in a given water column varies regionally owing to the in-water components. The light intensity entering the water column penetrates to different depth, being higher in clear waters and lower in turbid waters. The physical boundary conditions for parameter setting are shown in Fig. 1 (Coker and Jennifer Eileen, 2000).

Fig 1. The concepts of Secchi disk depth measurements

The underwater target detection system and imaging sensors (either active or passive) essentially depend on the underwater geometric illumination conditions of the light field and depth of propagation of light intensity. The spectral downwelling irradiance Ed(λ) in the selected waveband region of visible spectra (400nm-700nm) plays a pivotal role in ascribing the optical characteristics of the water column, and therefore, its investigation is necessary prior to the development of the underwater visibility model. The IOPs were measured employing various underwater optical instruments, which included AC-S (absorption and attenuation sensors), BB9 (backscattering sensors), FLNTU (turbidity and fluorescence chlorophyll sensors) and CTD (conductivity-temperature-depth) sensors. The FLNTU simultaneously measured turbidity level in standard terms of Nephelometric Turbidity Unit (NTU) along with the concentration chlorophyll of water sample in μg/l, while the SBE Seabird CTD sensors were used to obtain the conductivity-temperature-depth profile data. The downwelling irradiance (Ed), upwelling irradiance (Eu) and upwelling radiance (Lu) were measured by the respective hyper spectral radiometers (i.e one ARC for measuring the upwelling radiance and two ACC radiometers for measuring the upwelling irradiance and downwelling irradiances respectively). The in-situ measurements of these optical properties were carried out both in relatively clear and turbid waters off Point Calimere, located on the east coast of southern India, in the region of Bay of Bengal over the spectral range of 400-700 nm. The optical data were obtained at 25 stations located adjacently on 5 transects in the water depth ranging from 10-14m. These sensors measured radiometric quantities in the visible and near-infrared (350-950nm) with an overall accuracy interval of 3nm The turbidity was measured by FLNTU, which records the turbidity by detecting the light scattered by 700nm LED at an angle of 140° with a sensitivity of 0-75 NTU. Figure 2 illustrates the spectral nature of the downwelling irradiance depicting a prime feature in the green-yellow waveband. For turbid waters, spectra attain the maximum peak in this region and tend to shift towards the lower end of the green wavelength. In relatively clear waters, higher magnitude and fluctuation of irradiance with more flat spectra is observed. However, the fluctuation in irradiance spectra tends to diminish in case of turbid waters. Hence, the waveband region of green-yellow is important in developing an accurate algorithm for estimating water clarity.

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Anuj Kulshreshtha and Palanisamy Shanmugam / Aquatic Procedia 4 (2015) 1114 – 1118

Furthermore, most of the underwater sensor networking system operates in this region. The objective of this work is to develop and validate an algorithm for estimating the Secchi disk transparency in coastal and in-land waters. The selection of the specific wavelength of 520 nm to model the visibility equation is attributed to the fact that the penetration of the light intensity in the water column occur maximum at this wavelength.

(b)

(a)

Ed(1 m)

Ed(2 m)

Ed(3 m)

Ed(8 m)

Ed(9 m)

Ed(10 m)

Ed(4 m) Ed(11 m)

Ed(5 m) Ed(12 m)

Ed(6 m) Ed(13 m)

Ed(7 m) Ed(14 m)

Fig 2 Spectral variation of the measured downwelling irradiance (Ed(λ)) in the visible range from 400-700nm for (a) clear water and (b) turbid waters

2. Model description The Secchi disk depth (Zsd) serves as a robust depth transparency parameter and acts as a proxy to determine underwater visibility under various geometric illumination conditions. It is evaluated as a function of length attenuation coefficient (1/(c+Kd)), has a unit of per meter (m-1), and varies with undetermined coupling coefficient (Γ). The determination of coupling coefficient requires the knowledge of apparent contrast (C0) and threshold contrast (CT) to evaluate the relative reflectance of the disk based on the background illuminance of the water column (Davies-Colley, 1988; Doron et al., 2007; Hou, Lee, and Weidemann, 2007; Preisendorfer, 1986; Tyler, 1968).

Z sd v

Z sd

1 (c  K d ) * (c  K d )

(1) (2)

where, ‘Γ’ is the proportionality constant (a dimensionless parameter) and is known as the undetermined coupling coefficient. Based on the relative reflectance of the disk, the above equation can be written as

Z sd

ln(C0 / CT ) (c  K d )

* (c  K d )

(3)

The coupling coefficient is the ratio of the logarithmic variation of apparent contrast (C0) to threshold contrast (CT), and therefore, Secchi disk depth is calculated as the product of length attenuation coefficient and the undetermined coefficient which acts as surrogate to the underwater visibility parameter.

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Anuj Kulshreshtha and Palanisamy Shanmugam / Aquatic Procedia 4 (2015) 1114 – 1118

The new coupling coefficient (Γ) is composed of two sub-coefficients that define the spatial variability of the Secchi disk depth for a given solar illumination condition and is given by,

Z sd

D (c  K d )



E

(4)

(c  K d )

In equation (4), the parameter ‘α’ accounts for the logarithmic decay of the spectral downwelling irradiance Ed(λ) penetrating the water column under given solar illumination condition and ascribes the optical characteristics of the underwater light field, whereas the parameter ‘β’ determines the rate at which premature diffusion of light intensity occurs as it encounters the underwater light field geometric structure. Therefore, it accounts for the magnitude of the inherent optical factor that contributes to the variation in underwater visibility through the creation of diffused region of light intensity. 3. Results The modelled Secchi depth parameter is found to be in good agreement with measured values from a wide range of waters (typically from turbid to relatively clear waters). The minimum, maximum and mean values of the various measured optical properties are described in Table I. Table 1. The mean, maximum and minimum of the measured optical properties Statistics

Measured ZSD(m)

Kd (m-1)

c (m-1)

(c+Kd) (m1)

Turbidity(NTU)

Mean

2.53

0.76

7.07

7.87

4.17

Maximum

4.5

1.45

14.07

15.57

7.33

Minimum

0.5

0.08

1.33

2.01

1.01

No. of data points

39

The statistical measures are shown in Table II and the scatter plot in Fig. 3 shows the one to one correspondence of the estimated Secchi depth parameter with respect to the measured Secchi depth parameter. The statistics revealed that the present model has MRE~-0.012, RMSE~0.073, and approximately bias and slope of 10 0.004 and ~1 respectively. Modelled Zsd (m)

8 6 4 2 0 0

2

4

6

8

10

Measured Zsd (m) Fig 3. The scatter plot showing the close agreement of the modelled secchi depth with the measured data

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Anuj Kulshreshtha and Palanisamy Shanmugam / Aquatic Procedia 4 (2015) 1114 – 1118 Table 2. The statistical evaluation of the new model using the measured Secchi depth data. Wavelength (λ)

MRE

RMSE

Slope

Intercept

R2

Bias

520 nm

-0.012

0.073

1.0

-0.06

0.90

0.004

4. Discussion and Conclusion The above formulation can be applied to a wide range of waters to obtain the underwater vertical visibility in terms of Secchi depth. The undetermined coupling coefficient plays a critical role in determining the underwater vertical visibility parameter under various illumination conditions and can be considered to be composed of two integrals components. One of the factors governs the decay of underwater spectral downwelling irradiance(Ed) and other determines the rate at which the premature diffusion of the light intensity, penetrating the water column, occurs. The assessment of the water quality can be achieved by employing the fundamental concept of radiative transfer theory to determine the parameters affecting the coupling coefficient. The results revealed that the wavelength of 520mn is found to optimal in predicting the Secchi depth parameter in relatively clear and turbid coastal waters. Furthermore, the present model may assist the researchers in ascribing the index of water quality in such waters. In an alternative perspective, it might be helpful in understanding physical environment of the water column and the level of underwater flocculation that might be caused due to the passive movement of suspended sediments and other particulate matters in the shallow regions. References Coker., Jennifer, Eileen., 2000. Optical water quality of Lake Tahoe. Dissertation. University of California, Davis. Davies-Colley, R.J., 1988. Measuring water clarity with a black disk. Limnology and Oceanography, 33(4 part 1), 616-623. Doron, M., Babin, M., Mangin, A., Hembise, O., 2007. Estimation of light penetration, and horizontal and vertical visibility in oceanic and coastal waters from surface reflectance. Journal of Geophysical Research, 112(C6), C06003. doi:10.1029/2006JC004007 Hou, W., Lee, Z., Weidemann, A.D., 2007. Why does the Secchi disk disappear? An imaging prespective. Optics Express, 15(6), 2791-2802. Kirk, J.T.O., 1994. Light and photosynthesis in aquatic ecosystems. 2nd Edition. Cambridge: Cambridge university press, 528p. Marra, J., Langdon, C., Knudson, C.A., 1995. Primary production, water column changes, and the demise of a Phaeocystis bloom at the Marine Light-Mixed Layers site (59° N, 21° W) in the northeast Atlantic Ocean. Journal of Geophysical Research: Ocean, 100(C4), 6633-6643. Mcclain, C.R., Arrigo, K., Tai, K.S., Turk, D., 1996. Observation and Simulations of physical and biological processes at ocean weather station P, 1951-1980. Journal of Geophysical Research, 101(C2), 3967-3713. Preisendorfer, R.W., 1986. Secchi disk science: Visual optics of natural waters. Limnology and Oceanography, 3(5), 909-926. Tyler, J.E., 1968. The Secchi Disc. Limnology and Oceanography, 13(1), 1-6.