Satellite-based assessment of water quality and associate land cover changes of selected five Villus in Wilpattu National Park, Sri Lanka

Abstract

This study was conducted in relation to Sri Lanka's distinctive Villu ecosystem, situated within the expansive Wilpattu National Park (WNP), a Ramsar site. Villus are natural lakes which are filled with fresh or brackish water. Remote sensing techniques assessed water quality parameters and analyzed vegetation and land cover changes from 2019 to 2023. The incorporation of ground data enhanced the study's comprehensiveness. Fifteen sites across five villus within the WNP were strategically selected for mapping and spectral analysis. Precise distinctions in land use were achieved using the Normalized Difference Vegetation Index (NDVI), Normalize Difference Water Index (NDWI), and supervised classification. NDWI analysis highlighted pronounced water level fluctuations, peaking from November to December (Borupan Villu-3.09%, Kokkare Villu-19.41%, Mhapatessa Villu-8.59%, Kumbuk Villu-3.87%, and LunuVila Villu-7.09%) and sharply declining from July to August (Borupan Villu-0.80%, Kokkare Villu-8.89%, Mhapatessa Villu-4.00%, Kumbuk Villu-1.31%, and LunuWila Villu-3.74%). NDVI revealed the highest presence of sparse vegetation from March to April (Borupan Villu-64.94%, Kokkare Villu- 57.35%, Kumbuk Villu- 47.84%, LunuVila Villu- 67.91%, Mahapatessa Villu-68.53%), whereas the lowest occurred from July to August (Borupan Villu-6.39%, Kokkare Villu - 9.96%, Kumbuk Villu- 6.81%, LunuVila Villu - 6.45%, Mahapatessa Villu-6.41%). Conversely, dense vegetation exhibited an opposite seasonal pattern compared to sparse vegetation. Water quality parameters, assessed by Moran's Index, displayed distinct spatial clusters independent of Villus' proximity. All parameters, except nitrate concentration, exhibited significant spatial disparities among the five villus during both periods. Kokkare Villu showed high salinity, while others ranged from slightly saline to freshwater. Total phosphate concentration and pH exceeded aquatic life's ambient limits during rainy and dry periods. Intriguingly, changes in Villu-associated shrub and grassland areas were not recorded as a significant relationship with any water quality parameter. This study underscores the importance of incorporating satellite-based analysis for ongoing wetland management, serving as an invaluable tool for monitoring water areas and associated vegetation.

KEYWORDS:     Wilpattu National Park, Normalize Difference Vegetation Index, Normalize Difference Water Index, water quality, Land cover