**4. Results**

#### *4.1. Retrieval Method of Chlorophyll A Concentration*

By utilizing an empirical chlorophyll model created by Mishra and Mishra (2012), pre-processed Sentinel-2 images were used to derive the normalized differential chlorophyll index (NDCI). Because of backscattering and Chl-a absorption, these bands are most vulnerable to reflection. The spectral difference between bands 665 and 705 is used to calculate NDCI, which is then normalized by the sum of those two bands.

$$NDCI = \frac{\left[ (705) - (665) \right]}{\left[ (705) + (665) \right]}$$

Further, Chl-a was quantified using following equation as modelled by Kravitz et al. 2020 

$$\text{Chl}-\text{a}\left(\frac{\text{mg}}{\text{m}3}\right) = 17.441e^{(4.7038 \times NDCI)t}$$

Based on the data coordinates, the values for LST and chlorophyll a from the image bands of the relevant locations were determined (Figure 2). The values were used for further analysis and derivation of dissolved oxygen.

### *4.2. Dissolved Oxygen (DO)*

The sampling data from Wadhwana and Timbi Wetlands in April 2018 matched the Sentinel-2 image. This resulted in 24 in situ water sample matches on 18th April 2018. The in situ DO values ranged from 7.0 to 13.5 ppm in the Wadhwana wetland, while it ranged from 11 to 14.6 ppm in the Timbi Wetland. Field derived DO was further investigated for its relationship with Chl-a and temperature.

#### *4.3. Analysis for Water Quality of Wadhwana and Timbi Wetland*

The Chl-a and temperature data for June, 2018 were utilized with field generated dissolved oxygen were used.

From Table 1, correlation between DO and chlorophyll a (Chl-a) shows −0.7204. This means that there is a strong inverse relationship between Chl-a and DO. R-squared ( *R*2) equals 0.519. Dissolved oxygen and chlorophyll a are correlated and closely related, since a decrease in dissolved oxygen leads to an increased quantity of algae (chlorophyll a) in wetlands [14]. The correlation between DO and temperature shows −0.8478. This means that there is a very strong inverse relationship between temperature and DO R-Squared (*R*2) equals 0.72. According to the results of multiple linear regressions, there is a very strong overall significant relationship between Chl-a, temperature, and DO (F(2,21) = 28.98, *p* and <0.001, *R*<sup>2</sup> = 0.73, *R*<sup>2</sup> adj = 0.71). R-squared ( *R*2) is 0.734071. The multiple correlation coefficient (R) is 0.85. This indicates that the predicted and observed data are highly correlated. The squared R-adjusted value is 0.71.

**Table 1.** Relationship between Chla, Dissolved Oxygen and temperature in Wadhwana and Timbi Wetlands.


#### *4.4. Predictive Models for Water Quality of Wadhwana and Timbi Wetland*

A comparison of time series data from 2018 to 2022 for the Wadhwana (Figure 3a) and Timbi (Figure 3b) was performed to verify the proposed MLR model's accuracy. The correlation values between Sentinel-2 DO and Chl-a, as well as between the Sentinel-2 DO and temperature, were measured and the test results exceeded 0.6 and 0.89, respectively. Figure 3a,b shows the simulation results in comparison to the measurements. Data prediction by the model was successful. The results of all simulations are good and reasonable when measurement errors and spatiotemporal variations are taken into account. The RMSE for each variable ranges from 0.6 and 1.24.

The Multiple Linear Regression model could effectively simulate interannual dynamics. This model depicts the temporal trend of chlorophyll a with DO and temperature as shown in Figure 3. Increased algal blooms in wetlands and rising temperatures resulted in significant decreases in oxygen, which had far more detrimental effects on the lake's ecosystem than a decrease in algal blooms. Time series data (Figure 3) showing that during the summer months, i.e., as temperatures rose in March–May 2018–2020, dissolved oxygen levels were found to be low and chlorophyll levels high. A significant algal bloom was observed in the Ramsar Wadhwana wetland in 2021/2022, preceded by March to February. Timbi Wetland, which followed a similar pattern, showed a shift in high temperatures from March to February 2021. From these results, it is evident that the concentration of oxygen-consuming algae always increased after the concentration of dissolved oxygen decreased when compared to the concentration of chlorophyll a. In both wetland types, dissolved oxygen was an indicator that could be used to interpret chlorophyll a levels. The

chlorophyll a measurement is necessary because it shows the level of eutrophication that could endanger reservoirs around the world.

**Figure 3.** Time series analysis with Chl-a, temperature and modelled DO for (**a**) Wadhwana wetland (**b**) Timbi Wetland.

### **5. Conclusions**

Based on the findings of this research, Sentinel-2 data could be used effectively to accurately map dissolved oxygen levels in wetlands such as the Timbi and Wadhwana Ramsar sites. Accurate mapping of DO, an indicator of water quality parameters, can be used to provide a complete picture of the variability in algal blooms (chlorophyll a) concentrations due to their significant impact on water quality status.

**Author Contributions:** This research article has been written by two authors: A.M. and U.P.; A.M. involved in data downloading, processing and analysis, draft preparation, review and editing; U.P. in draft paper and review. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not Applicable.

**Data Availability Statement:** This study used Sentinel-2 multispectral imagery that is publicly available at https://scihub.copernicus.eu/ (accessed on 22 November 2022) and Landsat-8 multispectral imagery that is publicly available at http://earthexplorer.usgs.gov (accessed on 22 November 2022). **Conflicts of Interest:** The authors declare no conflict of interest.
