# Remote Sensing of the Water Quality Parameters for a Shallow Dam Reservoir
## 2 Data, Methods, and Techniques
### Spectral Reflectance Curves  
To compare the data recorded by Sentinel-2 satellites at different times, they were normalized to the conditions above the reference surface. The reference area was established as a fragment of the lakeside dam crest; it shows a low reflectance, and its properties are stable in time. The air composition over the reference surface was assumed to be the same as that over the lake surface. To correct lake surface reflectance obtained from satellite data, the special corrective parameter was introduced. It is the ratio of the reflectance registered on 13 April 2016 (13 April 2016, base date) in the reference field and the actual reflectance at a given day in the field. The normalized reflectance for the lake surface was the product of the corrective parameter and the reflectance of a given point on the lake surface at a given date. Such normalization is considered as an atmospheric correction. It made it possible to accommodate changes in air quality over the reference surface and the lake surface.  
Spectral curves for the fragment of the dam crest are presented in Figure 6.  
Figure 5: Depth map of Lake Dobczyce.  
The base reflection curve for the fragment of the dam crest (reference area) is dated 13 April 2016 (Figure 6). Other spectral curves have a similar shape, but pass through points with different reflectance values, even though the properties of the dam crest surface have not changed; this was due to changes in the atmosphere composition over time.  
The specific course of the spectral curves for Lake Dobczyce (Figure 7) results from the combination (superposition) of partial spectral curves for the water surface, water column, and bottom of the reservoir. Backscattering in a blue band also results in high reflectance (Figure 7). It should be noted that even at a high concentration of, e.g., montmorillonite suspensions (500 g/m\({}^{3}\)), the spectral curve does not show strong local extremes for wavelengths over 440 nm (Figure 8). Such a concentration corresponds to a turbidity of 500 NTU, rarely found in surface waters (mostly after heavy rains). Starting from approximately 440 nm, the spectral curve drops down (Figure 8), as shown in the curves in Figure 7. At concentrations exceeding 500 g/m\({}^{3}\), the maximum, corresponding to 440 nm, shifts towards longer wavelengths. The spectral curves may take completely different shapes than the ones shown in Figures 7 and 8 for other types of the mineral suspensions.  
The spectral curves for the chlorophyll solution can also differ significantly. At low concentrations (2.97 mg chl/m\({}^{3}\)) (Figure 9), there are no strong local extremes on the spectral curve above 400 nm. Characteristic local extremes appear at higher concentrations (7.609 mg chl/m\({}^{3}\)) (Figure 10) and are similar to the curves in Figure 2. Therefore, it may be concluded that concentration of water components (chlorophyll, type of suspension) will have a decisive influence on the shape of the spectral curves.  
Figure 8: Spectral curve for the montmorillonite suspensions in water, USGS Spectral Library Version 7, [[https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water](https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water)]([https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water](https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water)) (accessed on 1 January 2022). Reprinted with permission from [39].  
Figure 9: Spectral curve of chlorophyll in water (2.97 mg chl/m\({}^{3}\)), USGS Spectral Library Version 7, [[https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water](https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water)]([https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water](https://crustal.usgs.gov/speclab/QueryAll07a.php?quick_filter=water)) (accessed on 1 January 2022). Reprinted with permission from [39].