Influência dos constituintes opticamente ativos na reflectância espectral da água do reservatório Passo Real, RS

Abstract

The construction of dams results in hydrological impacts as they alter the natural flow of water of a river. Remote sensing is a tool capable of helping the monitoring of water quality by enabling acquisition of information for spatial and temporal analysis of its composition. This paper presents the analysis of the Influence of the optically active constituents on the spectral reflectance of the water of Passo Real reservoir. For this, limnological and spectral data were collected in October 2009 and March 2010 in 31 sampling stations in the reservoir. The limnological variables total of suspended solids (TSS), chlorophyll a (Chl a), dissolved organic carbon (DOC) and water transparency were determined on field and laboratory. Spectral data were acquired on field by using FieldSpec® HandHeld spectroradiometer. Results showed the most concentration of TSS and Chl a in October may be related with greater occurrence of rain in that period. TSS was greater in October resulting a reduction of water transparency. DOC maintained similar average in both analyzed months, still it was found that sampling points location with above average concentration were located in water compartments with higher concentration of Chl a. Exclusively using Chl a as indicator of trophic state we found environments between oligotrophic to hypereutrophic. Reflectance was higher in October 2009 in all wavelengths, because of high concentration of TSS. Two features of absorption were observed in both dates, centered in 610 and 675 nm due to the presence of Chl a. On account of the great concentrations in some sampling points in October, the peak in the infrared region presented higher magnitude than the peak in the green region and in some cases the spectrum was similar to the earth vegetation spectral response. DOC did not present enough concentrations to produce, significantly, spectrum alterations. Each information extraction technique in spectral reflectance better showed certain spectral characteristics. First derivative showed itself efficient on the characterization of Chl a effects, this technique contrasted the influence of this pigment especially on the wavelengths of 438, 566, 685 and 700 nm and the absorption band through pure water in 740 nm. By applying statistics tests, we could prove the influence of limnological variables on the reservoir water spectral characteristics and identify wavelengths with strong correlation between variables. In October we verified the joint acting of Chl a with TSS in water reflectance. Also, it was proved the influence of Chl a on the spectral response of the reservoir water, especially on greater concentrations.