# Remote Sensing of the Water Quality Parameters for a Shallow Dam Reservoir
## 2 Data, Methods, and Techniques  
### Remote Sensing Methods for a Chlorophyll Content  
In surface waters, algae and bacteria contain many pigments that can be analyzed using spectral methods. Listed according to color, these are, e.g., chlorophylls a, b, c, c1, c2, d, e, f, and g--green; caroten--orange; xanthophyll--yellow; phycoerythrin--red; phycoocyanin--blue; and fucoxanthin--brown. The dominant pigments in photosynthetic organs are chlorophyll a (blue-green) and chlorophyll b (yellow-green). Chlorophyll a is the most frequently used indicator of surface water quality. The surface spectral reflectance curves for water with different chlorophyll a concentrations are presented in Figure 1. Blue and far-red light ranges are strongly absorbed by chlorophyll, while the reflectance peaks are recorded at the wavelengths of approximately 566 and 688 nm (Figure 1). Figure 1: Surface remote sensing reflectance spectrum for the waters with different concentration of chlorophyll a, collected in the study area (\(119^{\circ}52^{\prime}\)-\(119^{\circ}54^{\prime}\) E, \(26^{\circ}16^{\prime}\)–\(26^{\circ}19^{\prime}\) N) at 10:00–15:40, 2 June 2003. Reprinted with permission from [15]. Mineral and organic suspensions may pose a serious difficulty in studying the concentration of chlorophyll a in surface waters. Both suspensions result in the reflectance peaks at approximately 550, 712.5, and 805 nm (Figure 2), so a shift in the peaks only takes place relative to the peaks for chlorophyll a. The suspensions may also enhance the reflectance in relation to that of chlorophyll a and therefore, models describing the relationship between chlorophyll a and the reflectance should consider several wavelengths. The presence of clay or dusty particles in water results in a specific spectrogram (Figures 3 and 4) [18]. High reflectance in the range of 580-690 nm and reflectance near 810 nm will strongly distort the chlorophyll a spectrogram. The first range is characteristic for a specific type of suspension. In the case of lake bottom sediments that are composed of clay or dusty suspensions, they will have a similar spectrogram. The reflectance pattern around 810 nm is similar for clay and dusty suspensions and therefore, the reflectance around 810 nm can be used to estimate the suspension concentration. Figure 3: Reflectance for water with a clay suspension [g/m\({}^{3}\)]. Reprinted with permission from [18]. Figure 2: Relative contributions of chlorophyll and suspended sediment to a reflectance spectra of the surface water, based on in situ laboratory measurements made 1 m above the water surface by the authors of [16; 17]. Reprinted with permission from [16; 17]. Mathematical models using the reflectance R to determine the concentration of C\({}_{\text{chl-a}}\) chlorophyll a in water are developed for a given type of surface water. These are the most abundant types of empirical models. Models such as GlobColour or Modis Aqua can be used globally or adapted to local reservoir conditions. They can take the form of polynomial dependence, as shown in the research of [19; 20], dependence, as a form of the products or quotients of expressions [21; 22], or a logarithmic form [23; 24]. The estimation of model parameters can be carried out in different ways--using multiple linear recreation, support vector machine regression (SVR), or genetic algorithms. Examples of models used to determine the chlorophyll a content in the water are presented in Table 1.  
The models are characterized by a different precision of results. [PERSON] et al., 2013 [25], testing global models GlobColour or Modis Aqua, obtained low R\({}^{2}\) determination coefficients of 0.25-0.51; on the other hand, for local conditions, this indicator may reach up to