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Abstract
Wetlands are important ecosystems on Earth. However, global wetland coverage is being reduced due to both anthropogenic and natural effects. Thus, assessment of temporal changes in vegetative coverage, as a measure of the wetland health, is critical to help implement effective management plans and provide inputs for climate-related research. In this work, 596 moderate-resolution imaging spectroradiometer (MODIS) 250-m resolution images of the Hongze Lake national wetland nature reserve from 2000 to 2009 were used to study the veg- etative coverage (above the water surface) of the reserve. Three vegetation indices (normalized difference vegetation index (NDVI), enhanced VI (EVI), and floating algae index (FAI)) were compared to evaluate their effectiveness in assessing relative changes. FAI was less sensitive than NDVI and EVI to aerosol effects and showed less statistical error than NDVI and EVI. Long- term FAI data revealed clear seasonal cycles in vegetative coverage in the 113-km 2 core area of the reserve, with annual maximal coverage relatively stable after 2004. This suggests that the national wetland nature reserve was well protected through the study period. However, vegeta- tive coverage decreased due to the flooding event in 2003. Moreover, correlation analysis showed that annual sunshine duration collectively played a significant role in affecting the wet- land vegetative coverage. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JRS.7.073589)
Highlights
Wetlands are one of the most important ecosystems on Earth
To evaluate each vegetation indices (VIs)’s effectiveness in removing noise while covering sufficient dynamic range, data from the study area were first averaged from the individual image, and 596 average VI values were obtained to form a time series
The same time-series analysis applied to the noisier enhanced VI (EVI) data resulted in similar results in accumulated vegetative coverage areas (AVCAs) and significant vegetative coverage days (SVCDs), and similar conclusions in the evaluation of the wetland nature reserve, suggesting the effectiveness of the EVI method
Summary
Wetlands are one of the most important ecosystems on Earth. They usually contain thousands of species in their aquatic and terrestrial environments, provide important habitat to animals, and play a special function in hydrology.[1]. Based on the most recent advances in remote-sensing technology and algorithm development, as well as the need for the assessment of the effectiveness of wetland protection, we initiated a study to assess the vegetative coverage of the Hongze Lake national wetland nature reserve using MODIS medium-resolution data between 2000 and 2009. NDVI saturates at high leaf-coverage area and is susceptible to variable environmental and observing conditions.[34,35,36,37] EVI was introduced to avoid those problems.[11,37,38,39] FAI was designed to detect floating algae in the open-ocean environments using medium-resolution (250- and 500-m) MODIS data and was shown to be more effective to separate algae from water and less prone to influence by aerosols than NDVI and EVI.[33] it remains to be evaluated which index is more suitable for assessing temporal changes in wetland vegetative coverage. Comparison with data from two other stations farther from our study area showed temporal consistency among all stations, suggesting that these data could represent the meteorological conditions of our study area
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