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2016
Li, LH, Stramski D, Reynolds RA.  2016.  Effects of inelastic radiative processes on the determination of water-leaving spectral radiance from extrapolation of underwater near-surface measurements. Applied Optics. 55:7050-7067.   10.1364/ao.55.007050   AbstractWebsite

Extrapolation of near-surface underwater measurements is the most common method to estimate the water-leaving spectral radiance, L-w(lambda) (where lambda is the light wavelength in vacuum), and remote-sensing reflectance, R-rs (lambda),for validation and vicarious calibration of satellite sensors, as well as for ocean color algorithm development. However, uncertainties in L-w(lambda) arising from the extrapolation process have not been investigated in detail with regards to the potential influence of inelastic radiative processes, such as Raman scattering by water molecules and fluorescence by colored dissolved organic matter and chlorophyll-a. Using radiative transfer simulations, we examine high-depth resolution vertical profiles of the upwelling radiance, L-u(lambda) and its diffuse attenuation coefficient, K-Lu (lambda) within the top 10 m of the ocean surface layer and assess the uncertainties in extrapolated values of L-w(lambda) The inelastic processes generally increase L-u and decrease K-Lu in the red and nearinfrared (NIR) portion of the spectrum. Unlike K-Lu in the blue and green spectral bands, K-Lu in the red and NIR is strongly variable within the near-surface layer even in a perfectly homogeneous water column. The assumption of a constant K-Lu with depth that is typically employed in the extrapolation method can lead to significant errors in the estimate of L-w. These errors approach similar to 100% at 900 nm, and the desired threshold of 5% accuracy or less cannot be achieved at wavelengths greater than 650 nm for underwater radiometric systems that typically take measurements at depths below 1 m. These errors can be reduced by measuring L-u within a much shallower surface layer of tens of centimeters thick or even less at near-infrared wavelengths longer than 800 nm, which suggests a requirement for developing appropriate radiometric instrumentation and deployment strategies. (C) 2016 Optical Society of America

Lu, YC, Li LH, Hu CM, Li L, Zhang MW, Sun SJ, Lv CG.  2016.  Sunlight induced chlorophyll fluorescence in the near-infrared spectral region in natural waters: Interpretation of the narrow reflectance peak around 761 nm. Journal of Geophysical Research-Oceans. 121:5017-5029.   10.1002/2016jc011797   AbstractWebsite

Sunlight induced chlorophyll a fluorescence (SICF) can be used as a probe to estimate chlorophyll a concentrations (Chl) and infer phytoplankton physiology. SICF at approximate to 685 nm has been widely applied to studies of natural waters. SICF around 740 nm has been demonstrated to cause a narrow reflectance peak at approximate to 761 nm in the reflectance spectra of terrestrial vegetation. This narrow peak has also been observed in the reflectance spectra of natural waters, but its mechanism and applications have not yet been investigated and it has often been treated as measurement artifacts. In this study, we aimed to interpret this reflectance peak at approximate to 761 nm and discuss its potential applications for remote monitoring of natural waters. A derivative analysis of the spectral reflectance suggests that the 761 nm peak is due to SICF. It was also found that the fluorescence line height (FLH) at 761 nm significantly and linearly correlates with Chl. FLH(761 nm) showed a tighter relationship with Chl than the relationship between FLH(approximate to 685 nm) and Chl mainly due to weaker perturbations by nonalgal materials around 761 nm. While it is not conclusive, a combination of FLH(761 nm) and FLH(approximate to 685 nm) might have some potentials to discriminate cyanobacteria from other phytoplankton due to their different fluorescence responses at the two wavelengths. It was further found that reflectance spectra with a 5 nm spectral resolution are adequate to capture the spectral SICF feature at approximate to 761 nm. These preliminary results suggest that FLH(761 nm) need to be explored more for future applications in optically complex coastal and inland waters.

Li, L.  2016.  Characterization of distinctive features of oceanic light fields associated with inelastic radiative processes in the near-surface, euphotic, and mesopelagic layers. ( Stramski D, Ed.)., La Jolla: University of California San Diego   10.13140/RG.2.1.2072.5524   Abstract

A thorough understanding of the oceanic light fields is required to support studies of various biological, chemical, and physical processes and phenomena in the ocean. The interaction of light with seawater and its constituents involves absorption (change of radiant energy into another form of energy), elastic scattering (change in light propagation direction but not wavelength), and inelastic radiative processes (change in light wavelength and propagation direction). The absorption and elastic scattering have been the primary research focus for decades. The inelastic processes have been less investigated and often ignored in oceanographic studies or applications. The inelastic processes, including Raman scattering and fluorescence, have been demonstrated to significantly affect the oceanic light fields. However, a systematic examination of these influences within different ocean layers is lacking. I studied the effects of inelastic processes on oceanic light fields in the near-surface (0-10 m), euphotic (0-200 m), and mesopelagic (200-1000 m) layers.
I modeled the upwelling radiance within the top 10 m of the ocean surface layer. The inelastic processes dramatically affect the upwelling radiance and its attenuation coefficient in the red and near-infrared spectral regions, indicating that common approaches for estimating water-leaving radiance from extrapolating measurements of upwelling radiance are inadequate. A new strategy is proposed for more accurate in-situ determinations of water-leaving radiance, which is critical for ocean color applications. Using both a unique field dataset and radiative transfer modeling I examined the effects of inelastic processes in the euphotic layer. I demonstrate distinctive features caused by inelastic processes in the irradiance and radiance fields as well as apparent optical properties for realistic scenarios of optically non-uniform water column. I also demonstrate the role of inelastic processes in photosynthetically available radiation and heating within the upper ocean. Finally, I modeled the mesopelagic light field to comprehensively characterize its magnitude, spectral composition, and angular distribution, which is important for understanding the habitat of deep-sea animals. In contrast to common assumptions, my results show much higher magnitude of green and red light at mesopelagic depths primarily owing to Raman scattering. The results also show a nearly-asymptotic regime of light field below ~400 m

2015
2014
Li, L, Stramski D, Reynolds RA.  2014.  Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations. Deep-Sea Res. Pt. I. 87:53–69.   10.1016/j.dsr.2014.02.005   AbstractWebsite

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200–1000 m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by ~9–10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475 nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565 nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (40K) indicates that the solar irradiance dominates over the 40K-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400–500 m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly-asymptotic regime at depths exceeding ~400–500 m.

Haag, JM, Roberts PLD, Papen GC, Jaffe JS, Li L, Stramski D.  2014.  Deep-sea low-light radiometer system. Optics Express. 22(24):30074-30091.   10.1364/OE.22.030074   Abstract

Two single-waveband low-light radiometers were developed to characterize properties of the underwater light field relevant to biological camouflage at mesopelagic ocean depths. Phenomena of interest were vertical changes in downward irradiance of ambient light at wavelengths near 470 nm and 560 nm, and flashes from bioluminescent organisms. Depth profiles were acquired at multiple deep stations in different geographic regions. Results indicate significant irradiance magnitudes at 560 nm, providing direct evidence of energy transfer as described by Raman scattering. Analysis of a night profile yielded multiple examples of bioluminescent flashes. The selection of high-sensitivity, high-speed silicon photomultipliers as detectors enabled measurement of spectrally-resolved irradiance to greater than 400 m depth.

2013
Li, L, Li L, Song K, Li Y, Tedesco LP, Shi K, Li Z.  2013.  An inversion model for deriving inherent optical properties of inland waters: Establishment, validation and application. Remote Sens. Environ. 135:150-166.   10.1016/j.rse.2013.03.031   AbstractWebsite

The inherent optical properties (IOPs) of natural waters are the most significant factors affecting light propagation within water columns, and thus play indispensable roles on estimation of aquatic biomass, primary production, and carbon pools. Despite its importance, no IOPs retrieval model was specifically developed for inland water bodies, although significant efforts were made on oceanic inversion models. In addition, for inland waters, an IOPs-based model is often preferred for estimating chlorophyll-a (Chl-a) concentration, an application of IOPs, over empirical and some semi-empirical algorithms. Then developing a model for estimating both IOPs and Chl-a is of significance for understanding the bio-optical properties and occurrence of algal blooms in eutrophic reservoirs, lakes and estuaries. In this paper, an IOPs Inversion Model of Inland Waters (IIMIW) for deriving natural water IOPs and estimating Chl-a is proposed and validated. The results indicate that this model can be used to accurately retrieve absorption coefficients at 443 nm and 665 nm with R2 = 0.8347 and R2 = 0.7550 respectively for Indiana study sites, and to estimate Chl-a from the derived absorption coefficients at high accuracies (R2 = 0.9292 and a mean relative error 21.65%) with samples collected from eight different study sites in the world and in different seasons. The model was also applied on Airborne Imaging Spectrometer for Application (AISA) images to map IOPs and Chl-a. Through validation by in situ measured Chl-a, results directly show that IIMIW can predict Chl-a with good accuracy even using the AISA bands, to as well indirectly prove that non-water absorption coefficients are retrieved accurately, at least within red and near-infrared region. Further biogeochemical information can be derived from these maps as well. These promising mapping results reveal possible remote routine surveillance of bio-optical states of inland waters.

2012
Li, L, Li L, Shi K, Li Z, Song K.  2012.  A semi-analytical algorithm for remote estimation of phycocyanin in inland waters. Sci. Total. Environ. 435-436:141-150.   10.1016/j.scitotenv.2012.07.023   Abstract

Phycocyanin (PC) is the unique and important accessory pigment for monitoring toxic cyanobacteria in inland waters. In this study, a semi-analytical algorithm combining both three band indices and a baseline algorithm (TBBA) was developed to estimate PC concentrations and then tested in three eutrophic and turbid reservoirs. TBBA does not need to optimize wavelengths as either the traditional baseline algorithm or three-band algorithms does when it is used across different study sites. TBBA evidently corrects some effects of absorptions due to colored detritus matter and other pigments and backscattering of water column. TBBA accurately estimated PC concentrations with R(2)=0.8573 and rRMSE=31.4% for water samples with the PC range from 1.4 mgm(-3) to 146.1 mgm(-3). Particularly, TBBA outperformed three-band algorithms and a previously proposed semi-empirical algorithm for the prediction of low PC (PC

Song, K, Li L, Wang Z, Liu D, Zhang B, Xu J, Du J, Li L, Li S, Wang Y.  2012.  Retrieval of total suspended matter (TSM) and chlorophyll-a (Chl-a) concentration from remote-sensing data for drinking water resources. Environ. Monit. Assess. 184:1449-1470.   10.1007/s10661-011-2053-3   Abstract

The concentrations of chlorophyll-a (Chl-a) and total suspended matter (TSM) are major water quality parameters that can be retrieved using remotely sensed data. Water sampling works were conducted on 15 July 2007 and 13 September 2008 concurrent with the Indian Remote-Sensing Satellite (IRS-P6) overpass of the Shitoukoumen Reservoir. Both empirical regression and back-propagation artificial neural network (ANN) models were established to estimate Chl-a and TSM concentration with both in situ and satellite-received radiances signals. It was found that empirical models performed well on the TSM concentration estimation with better accuracy (R (2) = 0.94, 0.91) than their performance on Chl-a concentration (R (2) = 0.62, 0.75) with IRS-P6 imagery data, and the models accuracy marginally improved with in situ spectra data. Our results indicated that the ANN model performed better for both Chl-a (R (2) = 0.91, 0.82) and TSM (R (2) = 0.98, 0.94) concentration estimation through in situ collected spectra; the same trend followed for IRS-P6 imagery data (R (2) = 0.75 and 0.90 for Chl-a; R (2) = 0.97 and 0.95 for TSM). The relative root mean square errors (RMSEs) from the empirical model for TSM (Chl-a) were less than 15% (respectively 27.2%) with both in situ and IRS-P6 imagery data, while the RMSEs were less than 7.5% (respectively 18.4%) from the ANN model. Future work still needs to be undertaken to derive the dynamic characteristic of Shitoukoumen Reservoir water quality with remotely sensed IRS-P6 or Landsat-TM data. The algorithms developed in this study will also need to be tested and refined with more imagery data acquisitions combined with in situ spectra data.

Song, K, Li L, Li S, Tedesco L, Hall B, Li L.  2012.  Hyperspectral remote sensing of total phosphorus (TP) in three central Indiana water supply reservoirs. Water, Air, & Soil Pollution. 223:1481-1502.: Springer Netherlands   10.1007/s11270-011-0959-6   AbstractWebsite

The connection between nutrient input and algal blooms for inland water productivity is well known but not the spatial pattern of water nutrient loading and algae concentration. Remote sensing provides an effective tool to monitor nutrient abundances via the association with algae concentration. Twenty-one field campaigns have been conducted with samples collected under a diverse range of algal bloom conditions for three central Indiana drinking water bodies, e.g., Eagle Creek Reservoir (ECR), Geist Reservoir (GR), and Morse Reservoir (MR) in 2005, 2006, and 2008, which are strongly influenced anthropogenic activities. Total phosphorus (TP) was estimated through hyperspectral remote sensing due to its close association with chlorophyll a (Chl-a), total suspended matter, Secchi disk transparency (SDT), and turbidity. Correlation analysis was performed to determine sensitive spectral variables for TP, Chl-a, and SDT. A hybrid model combining genetic algorithms and partial least square (GA-PLS) was established for remote estimation of TP, Chl-a, and SDT with selected sensitive spectral variables. The result indicates that TP has close association with diagnostic spectral variables with R 2 ranging from 0.55 to 0.72. However, GA-PLS has better performance with an average R 2 of 0.87 for aggregated dataset. GA-PLS was applied to the airborne imaging data (AISA) to map spatial distribution of TP, Chl-a, and SDT for MR and GR. The eutrophic status was evaluated with Carlson trophic state index using TP, Chl-a, and SDT maps derived from AISA images. Mapping results indicated that most MR belongs to mesotrophic (48.6%) and eutrophic (32.7%), while the situation was more severe for GR with 57.8% belongs to eutrophic class, and more than 40% to hypereutrophic class due to the high turbidity resulting from dredging practices.

2011
Li, L, Li L, Song K, Li Y, Shi K, Li Z.  2011.  An improved analytical algorithm for remote estimation of chlorophyll-a in highly turbid waters. Environ. Res. Lett. 6:034037.   10.1088/1748-9326/6/3/034037   Abstract

An analytical three-band algorithm for spectrally estimating chlorophyll-a (Chl-a) has been proposed recently and the model does not need to be trained. However, the model did not consider the effects of the absorption due to colored detritus matter (CDM) and backscattering of the water column, resulting in an overestimation when Chl-a < 50 mg m − 3 and an underestimation when Chl-a ≥ 50 mg m − 3 . In this letter, an improved three-band algorithm is proposed by integrating both backscattering and CDM absorption coefficients into the model. The results demonstrate that the improved three-band model resulted in more accurate estimation of Chl-a than the previously used three-band model when they were applied to water samples collected from five highly turbid water bodies with Chl-a ranging from 2.54 to285.8 mg m − 3 . The best results, after model modification, were observed in three Indiana reservoirs with R 2 = 0.905 and relative root mean square error of 20.7%, respectively.

2010
Li, L, Li L, Song K.  2010.  A bio-optical approach to estimating chlorophyll-a concentration from hyperspectral remote sensing. Proc. SPIE. 7809:78090E.   10.1117/12.859484   Abstract

Eagle Creek Reservoir is one of three central Indiana reservoirs supplying drinking water for the residents of Indianapolis. The occurrence of blue-green algae blooms resulting from high nutrient input has been a major public concern so that estimation of chlorophyll-a concentration of this reservoir is significantly important for assessing the reservoir's water quality. Empirical and semi-empirical methods were used in our previous studies for estimating CHL. Due to limitations to empirical and semi-empirical methods, a bio-optical model is tested in this study. Field campaigns were carried out in Eagle Creek Reservoir in central Indiana, and water samples analyzed for water quality parameter concentrations and their inherent optical properties (IOPs). A bio-optical model parameterized with these derived IOPs is used to estimate CHL concentration through a matrix inversion of hyperspectral data, and its performance is compared with those for empirical and semi-empirical models. The result demonstrates that the bio-optical model results in a higher correlation than empirical and semi-empirical models do.

2009
Li, L, Qu L, Ying S, Liang D, Hu Z.  2009.  Use of Google SketchUp to implement 3D spatio-temporal visualization. Proc. SPIE. 7492:74920Y.   10.1117/12.837572   Abstract

Geovisualization is an important means to understand the geographic features and phenomena. Urban space, especially buildings, keeps changing with social development. However, traditional 2D visualization can only represent the plane geometric description, which is unable to support 3D dynamic visualization. Only with 3D dynamic visualization can the buildings' spatial morphology be exhibited temporally, including buildings' creation, expansion, removing, etc. But these buildings' changes are impossible to be studied in traditional 2D and 3D static visualization systems. As a result, it becomes urgent to find an effective solution to implement 3D spatial-temporal visualization of buildings. Inspired by 2D spatial-temporal visualization methods, like snapshot and event-based spatio-temporal data model(ESTDM), we propose a new data model called Spatio-Temporal Page Model(STPM) and implement 3D spatial-temporal visualization in Google SketchUp based on STPM. This paper studies 3D visualization of real estate focusing on its spatio-temporal characteristics. First of all, 3D models are built for every temporal scenario by the Google SketchUp. And every Geo-object is identified by a unique and permanent ObjectID, the linkage of Geo-objects between different time spots. Then, each temporal scenario is represented as page . After having the page series, finally, it is possible to display its spatial-temporal changes and create an animation. Underlying this solution, we have built a prototype system on part of real estate data. It is proven that users are able to understand clearly the real estate's changes from our prototype system. Consequently, we believe our method for 3D spatial-temporal visualization definitely has many merits.