ASSESSMENT OF ALGAL BLOOMS IN KURUNEGALA LAKE USING INDICES DERIVED WITH OPTICAL REMOTE SENSING DATA

摘要

Kurunegala lake is located in the heart of Kurunegala city and serves as the supplementary water source for city water supply. In 2002, 2003 and 2009, algal blooming conditions have occurred in the lake. When surface waters become enriched with nutrients such as Phosphorous, the phytoplankton community often dominates by algae species. In 2002, dominant species was Cylindrospomosis rasiborskii, 2003 dominant species were Microcystis aeruginosa and Anabaena sp and in 2009 the dominant species was Botrycoccus brunnaii and the lake was identified as hyper-eutrophic. After this situation, rehabilitation of the lake has started in 2003 and has taken about 3 years for the completion. Routine water quality analysis will help to identify algae bloom conditions with the use of traditional methods which are time consuming and costly. Optical remote sensing can be used to develop different indices based on reflectance from the water surface to establish relationships with the concentrations of algae. Objective of this study was to develop indices/ band ratios using optical remotely sensed data to assess the algal bloom conditions in Kurunegala lake. Therefore, two periods were identified with respect to rehabilitation of the tank to develop these indices/ band ratios. Six Landsat satellite images were selected to represent the two periods (Landsat ETM+: 24-Aug-2002, 28-Nov-2002, 31-Jan-2003, 16-Feb-2003 (before rehabilitation), Landsat 8: 21-Jan-2014, 22-Feb-2014, 10-Mar-2014, 11-Apr-2014 (after rehabilitation). Six band ratios/ indices were developed. These index/ band ratio values were statistically assessed to identify possible differences between pre and post rehabilitation. Land use/ land cover change around the lake was also assessed to identify the impact on water quality. According to the results, there is a statistically significant different observed in the index/ band ratio values before and after rehabilitation. When considering land use change inside the 500 m buffer from the tank from 2001 to 2014, area of settlements has increased by 30% and vegetation cover has decreased by 21.5%. High concentrations of Nitrates and Phosphates were observed in the lake before rehabilitation. To identify the best index/band ratio to be used in the future, this study needs some modifications such as photosynthetic pigment validation using ground truth and laboratory data. These indices/ band rations coupled with ground truth data can be successfully used to identify the changes occur in the lake.