Large-scale And Long-time Water Quality Remote Sensing Monitoring Over Lakes Based On Water Color Index

Widely recognised as sentinels of environmental change,lakes and reservoirs around the world have been widely impacted by the drivers of land use and climate change.The use of remote sensing?RS?technology to obtain water quality information in lakes over large area and long time scales is of considerable scientific value in exploring the trend of lake changes and responses to global changes.However,the challenge brought about by the optical complexity of inland waters and overlying atmosphere,and interference due to adjacency effects,have presented additional challenges to the development of valid RS approaches to water quality monitoring over inland waters globally.Forel-Ule index?FUI?is one of the oldest and easiest measurements of water optical properties based on visual determinations,and it divides natural water colour into21 classes from dark blue to yellowish-brown based on the Forel-Ule water colour scale.FUI was promoted as a water quality parameter given its relations to water quality parameters and high capacity of indicating natural events and the change of water constitutes at large scale.In this paper,the FUI colour index is taken as an entry point for the study of water quality RS in lakes and reservoirs at a large and long time scale.The relationship between the water colour change and the optical properties and water quality over lakes of different types around the world was analysed and tapped to explore the FUI‘s capability in inland waters monitoring.Based on the theme of FUI-based RS retrieval of large-scale water quality,including trophic state assessment,water clarity retrieval and inherent optical properties?IOPs?inversion,mainly five aspects of research were carried out in this paper:1)A MODIS?Moderate Resolution Imaging Spectroradiometer?data preprocessing scheme for remote sensing product production over global inland waters was proposed,and it addressed the key issues in the preprossing,including water-leaving reflectance correction,water area extraction,cloud/ice/snow-cover removal,shallow water excluding.2)The FUI and hue angle???extraction algorithms for RS images with different spectral resolutions were built,which fully considered the water-leaving reflectance(R_rs???)characteristics in the global inland water database.3)Through obtaining the measured and simulated water quality and optical parameters of different types of lakes around the world and analysing the relationships between FUI and Trophic State Index?TSI?,a FUI-based trophic state assessment method for global large lakes and reservoirs was built.For the first time around the world,the trophic states of global large lakes and reservoirs were assessed and the spatial pattern was analyzed globally.4)Trough the relationship analysis between water transparency(Z_sd)and FUI based on a 26-lakes dataset,a Z_sd retrieval model was established using MODIS FUI and hue angle?for Chinese large lakes.The long time series analysis of water clarity in more than 200 Chinese large lakes between 2000 and 2014 was completed.5)By further exploring the relations between FUI and water inherent optical properties?IOP?,a system to derive IOP from FUI and Z_sd was proposed.It demonstrates that we may confidently derive water‘s IOP with the combination of FUI and Z_sd,which further opens the door to extend our knowledge of water‘s IOP of global seas and lakes to at least a century ago.This paper has the following four main conclusions:1)FUI color index is transferable in different satellite sensors,and is highly immune to observation geometry and aerosol perturbations.2)FUI color index is capable to be used to derive trophic state and water transparency over lakes and reservoirs in long time and large scales,with relatively high and stable performances.3)Through the trophic state assessment of global large lakes,it presents that oligotrophic large lakes were found to concentrate in plateau regions in Central Asia and southern South America,while eutrophic large lakes concentrated in central Africa,eastern Asia,mid-northern and southeast North America.4)Through the analysis of water clarity change in Chinese large lakes,it shows that lake clarity in the east areas was systematically and spatially lower than that in the west areas in China,while lake clarity was decreasing overall in China between 2000 and 2014.The most obvious decreases were found in Yunnan-Guizhou Plateau,which might be related to the obvious lake eutrophication and air temperature rising in this area in the past two decades.The water clarity of large lakes in Qinghai-Tibet Plateau changed drastically with partly significantly decrease and partly significantly increase,which might be influenced by the climate change and temperature rising in Qinghai-Tibet Plateau.