Modelling chlorophyll-a concentrations in a continental aquatic ecosystem of the Brazilian semi-arid region based on remote sensing¹

Modelagem da concentração de clorofila-a em um ecossistema aquático continental do seminário brasileiro baseada em sensoriamento remoto

ABSTRACT

Remote-sensing data are essential to evaluate dynamic processes such as eutrophication and increases in the concentration of suspended sediments in continental aquatic systems. The aim of this study, therefore, was to develop models to estimate chlorophyll-a concentrations from remote-sensing data in continental waters of the Brazilian semi-arid region. The study area corresponds to the Orós reservoir, located in the state of Ceará. The models were developed based on measurements taken at 20 sampling points. Water samples were collected from the reservoir to i) analyse the chlorophyll-a, electrical conductivity, pH and turbidity; ii) take optical measurements in situ of water transparency and spectral radiance. Radiance measurements were carried out using an ASD FieldSpec^®3 Hi-Res spectroradiometer. The spectral data were later transformed into reflectance values and used to test the performance of several models found in the literature for estimating chlorophyll-a. The results showed that for the three-band model, the maximum value for the coefficient of determination (R²), of 0.88, was obtained using the λ₁ = 660 nm, λ₂ = 690 nm and λ₃ = 717 nm spectral bands. The model employing two spectral bands presented the best performance (R² = 0.87) in the λ₁ = 660 nm and λ₂ = 690 nm bands. An absolute mean error of 5.35 and 5.00 µg L^-1 was found for the three- and two-band models respectively. The developed models are reliable, showing that chlorophyll-a concentrations can be quantified from remote-sensing field data with a high degree of accuracy.

Key words:
Surface reservoirs; Models; Remote quantification; Eutrophication.