Developing an empirical model from Landsat data series for monitoring water salinity in coastal Bangladesh

This study aims to develop an empirical model from Landsat data series to monitor the water salinity of coastal Bangladesh efficiently. Such a model can substitute expensive conventional techniques for assessing remote water quality. A set of equations connecting sensors 5 TM and 8 OLI were generated using multiple regression analysis. Radiometric and atmospheric corrections were carried out to enhance the quality of satellite images. Total 13 compositions of different bands including blue, green and red were considered to find the Coefficient of Determination (r²) with the field level EC (electrical conductivity) values collected from 74 sampling locations. Salinity data mainly EC values of coastal water were collected from primary and secondary sources. Considering the r² values, significant band compositions were identified and then employed to generate linear equations. Such equation for Landsat 5 TM could detect water salinity (i.e. EC) accurately of around 82%. Similarly, the r² value for Landsat 8 OLI was found as 0.76 that can confirm the applicability of Landsat data series to detect the change of salinity level of coastal water for a long period. The availability of coastal water was delineated by NDWI whereas salinity level was assessed using the developed equations for the year 2001 and 2019. Interestingly, it was observed that coastal areas having lower level of EC almost vanished whereas those of having higher level of EC were increased significantly between 2001 and 2019. Such increase in coastal water salinity is the result of combined effects of climatic and anthropogenic factors, which can pose a considerable risk to the coastal inhabitants including freshwater scarcity, food insecurity, and health hazard.

Evaluation of climate-induced waterlogging hazards in the south-west coast of Bangladesh using Geoinformatics

Climate-induced waterlogging has been significantly affecting the lives and livelihood of people in the south-west coastal region of Bangladesh for a couple of decades. The objective of this study is to investigate the waterlogging hazards of Tala, a south-western coastal Upazila of Bangladesh by analyzing satellite image. An empirical model based on velocity of flow, depth of flow, and inundation depth has been proposed to assess waterlogging hazard in pre monsoon, monsoon, and post monsoon period. Landsat TM images for the years 1989, 2000, and 2011 were analyzed by using Geoinformatics including GIS and remote sensing techniques to quantify the water hazard. Three dominant land use classes such as "water," "vegetation," and "bare lands and others" were selected to identify the land use land cover change. Both FGD (focus group discussions) and KII (key informant interviews) were also accomplished to assess the waterlogging hazard. It was revealed that 0.7% of the study area (246 ha) was under water in 1989, which increased alarmingly to 34% (11,525 ha) in 2011. There was an increase in 62.9% of water bodies during 1989 to 2000, which was further expanded to 77% during 2000 to 2011. Satellite image analysis between 2011 and 2015 also showed that nearly 89% of the waterlogged area including floodplain is inundated by tidal saline water that supports shrimp cultivation. On the contrary, 11% of the waterlogged area was occupied by trapped rain water. Confirmation of saline water and fresh water was done by measuring electrical conductivity and conducting "mouth taste" during field visit. The decreasing rate of "bare lands and others" category indicates that there is around 69.4% of reduction in this category to accommodate the increasing water covering areas. The hazard model shows that the middle part of the Tala along with the flood plain of the Kabodak River usually have to suffer waterlogged in both pre monsoon and post monsoon period. It was found that low flow in the upstream side, siltation in the Kabodak River, elevated riverbed, and recent increase in total annual rainfall have resulted in waterlogging problem. Combining inputs obtained from FGD, KII, and economic study of drainage, the TRM (tidal river management) followed by re-excavation of silted riverbed may provide a sustainable solution to solve the prolonged waterlogging situation.

Temporal dynamics of land use/land cover change and its prediction using CA-ANN model for southwestern coastal Bangladesh

Change analysis of land use and land cover (LULC) is a technique to study the environmental degradation and to control the unplanned development. Analysis of the past changing trend of LULC along with modeling future LULC provides a combined opportunity to evaluate and guide the present and future land use policy. The southwest coastal region of Bangladesh, especially Assasuni Upazila of Satkhira District, is the most vulnerable to natural disasters and has faced notable changes in its LULC due to the combined effects of natural and anthropogenic causes. The objectives of this study are to illustrate the temporal dynamics of LULC change in Assasuni Upazila over the last 27 years (i.e., between 1989 and 2015) and also to predict future land use change using CA-ANN (cellular automata and artificial neural network) model for the year 2028. Temporal dynamics of LULC change was analyzed, employing supervised classification of multi-temporal Landsat images. Then, prediction of future LULC was carried out by CA-ANN model using MOLUSCE plugin of QGIS. The analysis of LULC change revealed that the LULC of Assasuni had changed notably during 1989 to 2015. "Bare lands" decreased by 21% being occupied by other land uses, especially by "shrimp farms." Shrimp farm area increased by 25.9% during this period, indicating a major occupational transformation from agriculture to shrimp aquaculture in the study area during the period under study. Reduction in "settlement" area revealed the trend of migration from the Upazila. The predicted LULC for the year 2028 showed that reduction in bare land area would continue and 1595.97 ha bare land would transform into shrimp farm during 2015 to 2028. Also, the impacts of the changing LULC on the livelihood of local people and migration status of the Upazila were analyzed from the data collected through focus group discussions and questionnaire surveys. The analysis revealed that the changing LULC and the occupational shift from paddy cultivation to shrimp farming were related to each other. Around 31.3% of the total respondents stated that at least one of their family members had migrated. Climate-driven southwestern coastal people usually migrate from the vulnerable rural areas towards the nearest relatively safe city due to adverse effects of natural disasters. To control the unplanned development and reduce the internal migration in Assasuni and other coastal areas, a comprehensive land use management plan was suggested that would accommodate the diversified uses of coastal lands and eventually lessen the threats to the life and livelihood of the local people.