Detection of Mangrove Changes Using Multi Temporal Satellite Data: Effect of Shrimp Cultivation in North Western Province

摘要

Habitat encroachment, socio-economic exploitation are well-known factors contributing to the decline of mangroves in Sri Lanka and it is important to identify the area and rate of destruction of the mangrove forests. This study focused to observe the changes of mangrove land cover in surrounding area of Chilaw lagoon in North Western province in Sri Lanka using multi temporal satellite imageries, Landsat MSS, Landsat TM, Landsat ETM and Landsat OLI in 1975, 1987, 2000 and 2014 respectively. Land use and land cover of the area were derived by unsupervised classification technique and validated by visual interpretation through high resolution Google Earth imageries. Tasseled Cap Transformation (TCT) was applied to images to enhance the distinguishing characteristics of wet surfaces and mangrove vegetation in assigning the land use categories for improved accuracy in image classification. Overall accuracies of classified images of Landsat TM (1987), Landsat ETM (2000) and Landsat OLI (2014) were 78 %, 80 % and 90 % respectively. The results showed a drastic change of 11% depletion of mangrove extent throughout the study area during the period of 1987-2000 where there was no significant change observed during the period of 2000-2014. The estimated deforestation rate of mangrove cover of-0.43 % was high and the average annual loss was 1.55 ha over a 27 year period (1987-2014). Intensive shrimp farming and urbanization were identified as the main causes for depletion of mangroves. Shrimp farms in 2014 were 455 ha which mainly emerged in mangrove sites. A low ratio of 1:1.33 of existing mangroves to shrimp farms indicates an adverse impact of shrimp farming on mangroves which subsequently affects the environmental sustainability within the study area. Finally, this study proved the potential use of satellite imageries to area quantification of mangrove vegetation in Sri Lankan context.