# Remote Sensing of the Water Quality Parameters for a Shallow Dam Reservoir
## 4 Discussion  
The authors developed the model to calculate the concentration of chlorophyll a and turbidity in the water. In the case of Lake Dobczyce, the chlorophyll a model takes into account reflectance corresponding to the middle wavelengths 665, 705, 740, and 842 nm, while the bottom effect is related to the wavelengths 665 and 705 nm. The model for turbidity considers the reflectance corresponding to the middle wavelengths 705, 740, and 783 nm, while the bottom effect is related to the wavelength 705 nm. To eliminate a minor reflectance related to other wavelengths from the pseudo-linear model (3), the Student's \(t\)-test was been used for both chlorophyll a and turbidity. It cannot be predicted in advance whether the models for different lakes will always use the reflectance for the same  
Figure 21: Water turbidity of Lake Dobczyce, 21 October 2021 (imagery by Sentinel 2). Figure 20: Water turbidity of Lake Dobczyce 9 May 2021 (imagery by Sentinel 2). wavelengths. If so, the model coefficients may probably be different due to different water characteristic and other properties of the bottom sediments. In the case of Lake Dobczyce, all of the discriminants that are quotients of reflectance differences or reflectance quotients (Table 1) were not used in the models because they did not improve their quality. The average relative error of the model for chlorophyll a is about 0.216, while the average error is about 2.01 mg Chl a/m\({}^{3}\). Graphs for errors are shown in Figures 22 and 23. The greatest errors of the model relate to concentrations of about 15 mg Chl a/m\({}^{3}\) (Figure 22) and the highest relative errors concentrations of about 6 mg Chl a/m\({}^{3}\) (Figure 23). For low and high chlorophyll a concentrations, the relative errors are the smallest. Sometimes, the existence of greater errors is caused by the unrepresentativeness of point measurement relative to the area represented by one raster. In addition, a greater number of data with average values of chlorophyll a concentration increases the likelihood of a greater error. The reflectance corrections for 665 nm were smaller than for 705 nm (Figure 24). This may be due to the fact that the wavelength of emissions for chlorophyll a is 663 nm, after it was stimulated using a 430 nm radiation. Backscattering with a length of 665 nm, recorded by the satellite, therefore had to be more characteristic of chlorophyll a than for other substances contained in water. Backscattering with a length of 705 nm is characteristic of minerals, in particular montmorillonite (Figure 8), and this likely explains why the reflectance corrections were greater. Reflectances for other wavelengths, occurring in the model for chlorophyll a, can be characteristic of other substances contained in water and atmosphere ingredients. Figure 23: Model relative errors for chlorophyll a versus measurement data. Figure 22: Model errors for chlorophyll a versus measurement data. The average relative error of the model for turbidity is about 0.184, while the average error is about 0.629 NTU. Graphs of errors are shown in Figures 25 and 26. The greatest errors for the turbidity model concern the turbidity of about 7 NTU (Figure 25) and the greatest relative errors of turbidity of about 2-4 NTU (Figure 26). Sometimes, the existence of greater errors is caused by the unrepresentativeness of point measurement relative to the area represented by one raster. In addition, a greater number of data with average turbidity increases the likelihood of a greater error. Figure 24: The reflection corrections graph versus the product of the depth and turbidity for the chlorophyll a model. Figure 25: Model errors for turbidity versus measurement data. The reflectance corrections for 705 nm for the turbidity model were approximately constant and amounted to around 0.0146. The remote determination of water quality parameters requires corrections of the reflectance measured by the satellite, which may pose some problems.