Parametrization and calibration of a quasi-analytical algorithm for tropical eutrophic waters

Abstract

Quasi-analytical algorithm (QAA) was designed to derive the inherent optical properties (IOPs) of water bodies from above-surface remote sensing reflectance (R rs). Several variants of QAA have been developed for environments with different bio-optical characteristics. However, most variants of QAA suffer from moderate to high negative IOP prediction when applied to tropical eutrophic waters. This research is aimed at parametrizing a QAA for tropical eutrophic water dominated by cyanobacteria. The alterations proposed in the algorithm yielded accurate absorption coefficients and chlorophyll-a (Chl-a) concentration. The main changes accomplished were the selection of wavelengths representative of the optically relevant constituents (ORCs) and calibration of values directly associated with the pigments and detritus plus colored dissolved organic material (CDM) absorption coefficients. The re-parametrized QAA eliminated the retrieval of negative values, commonly identified in other variants of QAA. The calibrated model generated a normalized root mean square error (NRMSE) of 21.88% and a mean absolute percentage error (MAPE) of 28.27% for a t (k), where the largest errors were found at 412 nm and 620 nm. Estimated NRMSE for a CDM (k) was 18.86% with a MAPE of 31.17%. A NRMSE of 22.94% and a MAPE of 60.08% were obtained for a u (k). Estimated a u (665) and a u (709) was used to predict Chl-a concentration. a u (665) derived from QAA for Barra Bonita Hydroelectric Reservoir (QAA_BBHR) was able to predict Chl-a accurately, with a NRMSE of 11.3% and MAPE of 38.5%. The performance of the Chl-a model was comparable to some of the most widely used empirical algorithms such as 2-band, 3-band, and the normalized difference chlorophyll index (NDCI). The new QAA was parametrized based on the band configuration of MEdium Resolution Imaging Spectrometer (MERIS), Sentinel-2A and 3A and can be readily scaled-up for spatiotemporal monitoring of IOPs in tropical waters.

Estimation of Chlorophyll-a Concentration from Optimizing a Semi-Analytical Algorithm in Productive Inland Waters

“…The semi-analytical algorithm produced good results compared to other bio-optical models parameterized from the BB1 and BB2 datasets [22]. The performance of the numerical optimization was better at estimating the chl a content than the empirical approach.…”

“…They are the first and second reservoirs in the low and middle course of the Tietê River, and close to point sources of wastewater discharge in big urban centers. The high trophic level associated with anthropic tasks leads to algal blooms in these reservoirs, especially cyanobacteria dominating the phytoplankton community [2,21,22], although variations in phytoplankton species are closely linked to water column mixing events and residence time [23].…”

“…Values of b bp (560) were obtained from the inversion of the semi-analytical model. The spectral power of b bp (Y) was modeled from Equation (12) [39], used by [22] in a quasi-analytical algorithm (QAA) applied to the BB reservoir.…”

Glint Removal Assessment to Estimate the Remote Sensing Reflectance in Inland Waters with Widely Differing Optical Properties

“…Water quality parameter variations from up-to-downstream inside a unique continuum system (Tietê River) are dependent on the time residence of the water, inputs from tributaries along the cascading reservoirs, auto-depuration, the availability of solar irradiation, and the activities developed within the sub-basin areas that contribute to wastewater discharges. Barra Bonita, for instance, is near to São Paulo (capital of São Paulo State) and it is the second most populous city in Brazil [51]. The Bariri reservoir receives discharge from the river that flows throughout Bauru city, an urban area that recently adopted wastewater treatments, and Ibitinga is near Environmental Protection Areas, which contribute to the organic matter as a result of the vegetation coverage [26].…”

“…The wide variation in OSC's absorption affects the spectral features in the R rs curves (Figure 4). We observed absorption features of Chl-a at 440 nm and 675 nm in all reservoirs [52], and the phycocyanin absorption near 620 nm [51] can be realized in BB and BAR fieldworks (Figure 4a-d). The iron absorption features (450-550 nm) and high CDOM absorption coefficients (<500 nm) resulted in flat Rrs spectra at shorter wavelengths (<550 nm), mainly in curves obtained in BAR, IBI, and NAV fieldworks [53][54][55].…”