**4. Results and analysis**

### **4.1 Field measurement**

The results of field sample analysis showed that the lowest concentration of chlorophyll-A was in station number 9 with 0.11 mg/m3 and the highest level was in station number 6 with 6 mg/m3 (due to severe algae) and in the rest of the stations. Chlorophyll content is below 1 mg/m3 . The diagram below shows the trend of chlorophyll-A changes in the stations (**Figure 4**).

## **4.2 Chlorophyll-A validation received from satellites with field data**

The results of chlorophyll-A evaluation show that the use of OC2 bio-optical algorithm to estimate chlorophyll-A concentration using Landsat-8 data has acceptable results and has a high correlation with field data. **Table 2** shows the result of statistical parameters shows the correlation between field data and chlorophyll derived from Landsat-8 data.

**Figure 4.** *The trend of field chlorophyll changes.*


#### **Table 2.**

*Results of RMSE, R2 and Landsat-8 modeled coefficients with field data.*

#### **4.3 Seasonal estimation of chlorophyll-A concentration with Landsat-8**

After obtaining the appropriate results from the OC2 algorithm in Landsat-8 Remote Sensing data using Eq. (2) and the modeled coefficients presented in the table above, a chlorophyll-A concentration map was prepared in the seasons of 2019. The chlorophyll-A concentration estimation map is shown in **Figure 5**.

$$\mathbf{C} = \mathbf{10}^{\left(\mathbf{a}\mathbf{0}\ast\mathbf{a}\mathbf{1}\ast\mathbf{R}\ast\mathbf{a}\mathbf{2}\ast\mathbf{R}2\ast\mathbf{a}\mathbf{3}\ast\mathbf{R}3\right)}$$

$$\mathbf{R} = \log\left(R\mathbf{rs}\,\mathbf{4}\,\mathbf{90}\,/\,R\mathbf{rs}\,\mathbf{5}\,\mathbf{5}\,\mathbf{5}\right) \tag{4}$$

**Figure 5.** *Time series map of chlorophyll-A concentration in Tiab estuary prepared from Landsat-8 images [22].*

The coefficients a0, a1, a2, a3, a4 were determined for Landsat OC2 algorithm −0.20094, −1.3018, 0.52631 and 1.4235, Rrs is the reflection of distance measurement at blue and green wavelengths, R is the band ratio and C, chlorophyll-A concentration to The unit is milligrams per cubic meter.
