2018) Trophic state assessment of global inland waters using a MODIS-derived Forel-Ule index.Abstract 13 Eutrophication of inland waters is considered a serious global environmental 14 problem. Satellite remote sensing (RS) has been established as an important source of 15 information to determine the trophic state of inland waters through the retrieval of 16 optically active water quality parameters such as chlorophyll-a (Chl-a). However, the 17 use of RS techniques for assessment of the trophic state of inland waters on a global 18 scale is hindered by the performance of retrieval algorithms over highly dynamic and 19 complex optical properties that characterize many of these systems. In this study, we 20 developed a new RS approach to assess the trophic state of global inland water bodies 21 based on Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and the 22Forel-Ule index (FUI). First, the FUI was calculated from MODIS data by dividing 23 2 natural water colour into 21 indices from dark blue to yellowish-brown. Then the 24 relationship between FUI and the trophic state index (TSI) was established based on in-25 situ measurements and MODIS products. The water-leaving reflectance at 645 nm band 26 was employed to distinguish coloured dissolved organic matter (CDOM)-dominated 27 systems in the FUI-based trophic state assessment. Based on the analysis, the FUI-based 28 trophic state assessment method was developed and applied to assess the trophic states 29 of 2058 large inland water bodies (surface area > 25 km 2 ) distributed around the world 30 using MODIS data from the austral and boreal summers of 2012. Our results showed 31 that FUI can be retrieved from MODIS with a considerable accuracy (92.5%, R 2 =0.92) 32 by comparing with concurrent in situ measurements over a wide range of lakes, and the 33 overall accuracy of the FUI-based trophic state assessment method is 80.0% (R 2 = 0.75) 34 validated by an independent dataset. Of the global large water bodies considered, 35 oligotrophic large lakes were found to be concentrated in plateau regions in central Asia 36 and southern South America, while eutrophic large lakes were concentrated in central 37 Africa, eastern Asia, and mid-northern and southeast North America. 38

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