Development of Algorithms for Remote Evaluation of Chlorophyll «a» in the Water Bodies of the Dniester River Delta Using Sentinel-2 MSI
DOI:
https://doi.org/10.26565/2410-7360-2026-64-12Keywords:
Dniester River Delta, ecological status of water bodies, chlorophyll «a», remote sensing, Sentinel-2 MSI, C2RCC, atmospheric correction algorithms, functional modelsAbstract
Problems Statement and Objective. Intensive economic activity in the Dniester River basin and overregulation of its flow via excessive reservoirs provoke eutrophication and deterioration of the water quality affecting ecosystem biodiversity and threatening public health. To achieve sustainable water quality management, it is proposed to use remote sensing (RS) to estimate chlorophyll «a» concentration as a tool for water bodies monitoring. To consider optical properties of these objects and to determine functional dependences, the concentrations calculated with different algorithms based on RS data are compared with field measurements. The main objective of this study is to select and validate the algorithms for remote estimation of chlorophyll «a» concentration in water bodies of the Dniester River Delta using measurements from the Sentinel-2 Multispectral Instrument (MSI).
Data and Methods. Chlorophyll «a» concentrations obtained from the Dniester Delta water bodies from April to September of 2017-2021 have been used. Also, two types of MSI reflectance were used: top-of-atmosphere (TOA) and surface reflectance (SR). During the selection of optimal algorithms, a literature review was conducted. It was decided to select two types of MSI Sentinel-2 spectral band functional combinations. Also, calculations of chlorophyll «a» concentration by the C2RCC (Case 2 Regional Coast Color) processor in SNAP Sentinel Toolbox platform were used.
Results and Discussion. Exponential-type functional dependences were established during development of the selected two-channel and three-channel models. The proposed functional dependences were verified by comparing the calculated values, as well as obtained by the C2RCC processor, with the chlorophyll «a» concentration in the field samples. Comparison showed an advantage of using the TOA coefficients (compared to SR) for both the two-channel and three-channel models: Rp<0.001=0.92, MAPE=28.5, RMSE=14.4 mg/m3 and Rp<0.001=0.93, MAPE=27.9, RMSE=14.3 mg/m3, respectively. An underestimation of the chlorophyll «a» concentration calculated by the C2RCC processor was found.
Conclusions. The functional dependences based on two- and three-channel TOA combinations of the Sentinel-2 satellite’s MSI spectral bands have been proposed for calculation of chlorophyll «a» concentration in complex inland waters of the Dniester River Delta and the Dniester Estuary. Manual and semiautomatic filtering of cloudy, mist, cloud shadowed, and aquatic vegetation-covered areas from imagery has been proposed as a mandatory calculation step. Chlorophyll «a» concentration calculation by the C2RCC processor in the C2X-COMPLEX version for turbid inland waters should be assessed critically due to the ongoing development of atmospheric correction algorithms. Nevertheless, analysis of the selected functional dependences for various types of complex waters, combined with improvements to the atmospheric correction module, offers significant potential of the C2RCC processor, which uses trainable neural networks, to be used in remote monitoring of inland and coastal waters.
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