Research on the Features of Chlorophyll-a Derived from RapidEye and EOS/MODIS Data in Chaohu Lake

Chaohu Lake is one of the largest freshwater lakes in China, however, with the rapid expansion of Hefei, the water environment of Chaohu Lake shows significantly deterioration. In this paper, RapidEye and MODIS imagery were used to monitor the chlorophyll alpha (Chla) distribution in Chaohu Lake. After analyzing the correlation between in-situ measured Chla concentrations and each channel of RapidEye and MODIS imagery, the optimal band combination for the establishment of Chla concentration was determined. For RapidEye imagery, the red edge channel and near infrared channel are relatively more sensitive than other channels. The inversion model using the feature of (B5+B4)/(B2+B1) worked the best with the square value of correlation coefficient reaching up to 0.745. A similar procedure was applied to MODIS imagery. The experiments show that RapidEye with its red edge channel is an effective data source for water environment monitoring, it could provide high spatial resolution of Chla distribution thematic map, MODIS with its higher temporal resolution is also an effective data source in dynamically monitoring water environment. The Chla concentration in the western half of Chao Lake is higher than that in the eastern half. 1. Background Chaohu Lake is the largest freshwater lake in the Anhui Province, and is also the fifth largest freshwater lake in China. Situated in the depression between the Yangtze River and the Huaihe River in the middle of Anhui Province, Chaohu Lake (31°25′28′′-31°43′28′′ N and 117°16′54′′-117°51′46′′ E) is one of the large shallow lakes of the middle and lower reaches of the Yangtze River, covering 760 square kilometers with an average depth of 2.5 m. Surrounded by the cities such as Hefei City, Chaohu City and Liu’an City, Chaohu Lake assumes the responsibility of water supply, irrigation, fishery, transportation, and it is also the carrier for shipping and tourism in Anhui [1]. Several rivers including Nanfei River, Hangbu River, Fengle River, Baishishan River, flow into Chaohu Lake, and flow into Yangtze River by passing through Yuxi River in the east of Chaohu Lake. Since the mid-20th century, with the rapid development of industrialization and urbanization in the 1 Corresponding author: Xiongfei Wen, Tel.: +86-27-82926550; fax: +86-27-82820076 35th International Symposium on Remote Sensing of Environment (ISRSE35) IOP Publishing IOP Conf. Series: Earth and Environmental Science 17 (2014) 012111 doi:10.1088/1755-1315/17/1/012111 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 Chaohu Lake basin, especially the rapid expansion of Hefei City, the water environment of Chaohu Lake shows significantly deterioration, and a series of eco-environmental problems, such as the bluegreen algal bloom pollution and eutrophication have gradually become more and more serious that cannot be ignored [2]. Remote sensing technique, with its synoptic view, repetitive coverage, cost effectiveness and availability, is an effective approach to dynamically monitor the lake’s water environment [3-5]. In this paper, two kinds of satellite imagery were collected to analyze the Chla concentration distribution in Chaohu Lake. 2. Introduction of experiment data The RapidEye constellation of five Earth observation satellites equipped with identical sensors and located in the same orbital plane stands apart from other has been in operation since February 2009. It provides of satellite-based geospatial information in their unique ability to acquire high-resolution, large-area image data on a daily basis. The RapidEye constellation collects an unprecedented 4 million square kilometers of data per day at 6.5 meter nominal ground resolution [6]. RapidEye’s Multi-Spectral Imager (MSI) acquires imagery in five different spectral bands, and the presence of a red edge band is a unique feature that distinguishes RapidEye’s satellites from most other multispectral satellites. The red edge band is located between the red band and the near-infrared (NIR) band without overlap. In the typical spectral response of green vegetation, the red edge band covers the portion of the spectrum where reflectance drastically increases from the red band towards the NIR band. Several studies suggested that the transition between the absorbance in red band and the reflection in the NIR band is able to provide additional information about vegetation, and is a useful instrument for water quality parameters extraction in productive inland water bodies with relatively higher phytoplankton content [7]. MODIS is the keystone instrument onboard a series of satellites commissioned as part of the National Aerospace and Space Administration (NASA) Earth Observation System (EOS). MODIS data provides the spectral information in 36 spectral bands ranging in wavelength from visible to thermal-infrared and at varying spatial resolutions, with the temporal resolution of 1–2 days, which is a valuable data resource in the application of meteorological and land/sea-surface feature inversion such as water environment dynamically monitoring [8]. The MODIS data used in this paper are provided by Goddard Space Flight Center of NASA. The RapidEye imagery captured on Oct 17, 2012 and MODIS imagery captured on Oct 10/11/17, 2012 were collected in this paper.