| dc.description.abstract |
Variations of salinity and Chlorophyll-a concentration are important parameters to identify the possible opportunity for effective management of the Meghna estuary. Traditional in situ measurement systems in the large Meghna estuary are very challenging for real-time monitoring. Moreover, the major limitation in terms of spatial-temporal acquisition of reliable data. The remote sensing technique is a very popular and convenient system for real-time monitoring of salinity and chlorophyll-a concentration. This study investigates the applicability of Landsat TM and Landsat OLI images for estimating these parameters in the Meghna Estuary. Multiple Landsat images from 1989 to 2022 were processed to extract Top of Atmospheric Reflectance (ToA) corresponding data from 18 Bangladesh Water Development Board (BWDB) stations. Since mostly clear scenes were used, atmospheric correction was ignored in situ. For regression analysis, several band or band combinations were investigated to determine which band makes a strong correlation. However, it is noticed that no strong correlation was developed in relation to in situ data and ToA reflectance. Qualitative analysis with Landsat OLI band 4 elucidates that high sediment concentration in the Meghna estuary strongly correlates with the in situ data and ToA reflectance. So, Landsat data is not suitable for salinity mapping in the high sediment concentrate Meghna Estuary. The Inverse distance weighted interpolation method was used to quantify the salinity gradient mapping. Also, investigate the changes in salinity levels for different years and observe that salinity levels increase over the years. The maximum average salinity level was found in 2021 for all eight stations. The highest mean salinity level observed in Tajumuddin station was 7.64 ppt in 2021, which was 0.15 ppt in 2000. Another Chlorophyll-a concentration can also be measured by using a bio-optical algorithm. This analysis was conducted through the conversion of Digital Numbers to remote sensing reflectance. This bio-optical algorithm is a formula of the combination of blue R_rs (490) and green R_rs (555) band from Landsat TM & Landsat OLI. This algorithm elucidated the spatial and temporal variation of chlorophyll-a concentration in Meghna Estuary from 1989 to 2022. Chlorophyll-a concentration is high in the northeastern part of the estuary along the Chittagong coast with an average concentration of 4-6 mg/m3, where the maximum concentration is about 10 mg/m3, and the minimum concentration is 2 mg/m3. |
en_US |