correction of the Multi-Spectral Instrument (MSI)-SENTINEL-2 imagery over inland and sea waters from SWIR bands. Remote Sensing of Environment, Elsevier, 2018, 204, pp.308-321 A B S T R A C TRemote sensing of inland and sea waters depends on the quality of the retrieval of the water-leaving radiance from the top-of-atmosphere measurements. The water-leaving radiance can be difficult to observe due to the reflection of direct sunlight on the air-water interface (sunglint) in the direction of the satellite field of view. The viewing geometry of Sentinel-2 satellite (European Space Agency) makes it vulnerable to sunglint contamination. In this paper, an original method is proposed to correct Sentinel-2-like imagery for sunglint contamination. The sunglint contribution is first estimated from the shortwave-infrared (SWIR) part of the spectrum and then extrapolated toward the near-infrared and visible bands. The spectral variation of the sunglint signal is thus revisited for a wide spectral range (from 350 to 2500 nm). The bidirectional reflectance distribution function related to the sunglint is shown to vary by > 28% from the SWIR to the blue bands of Sentinel-2. The application of the proposed algorithm on actual Sentinel-2 data demonstrates that sunglint patterns are satisfactorily removed over the entire images whatever the altitude of the observed target. Comparison with in situ data of water-leaving radiances (AERONET-OC) showed that our proposed algorithm significantly improves the correlation between satellite and in situ data by 55% (i.e., from R 2 = 0.56 to R 2 = 0.87). In addition, the discrepancies between satellite and in situ measurements are reduced by 60%. It is also shown that the aerosol data provided by the Copernicus Atmosphere Monitoring Service (CAMS) can be safely used within the proposed algorithm to correct the Sentinel-2-like satellite data for both sunglint and atmospheric radiances. Improvements of the proposed method potentially rely on simultaneous retrievals of the aerosol optical properties. The proposed method is applicable to any satellite sensor which is able to measure in SWIR spectral bands over aquatic environments.
Damming is one of the most widespread impairments of river systems around the world. The formulation of scientifically justified guidelines for restoration and remediation of impairments requires better understanding of the relationships between alteration and stream condition. The present study examines relationships between the presence and density of dams and biological metrics of river health in the context of a variety of environmental drivers over the Loire river basin. We hypothesized that dam density measured at supra-reach level would show more significantinfluence on river health than the local level density, and further that the impact of dams is best estimated with measures for the functional traits of biotic assemblages. An extensive dataset of fish (169 sites) and invertebrate (211 sites) communities in the Loire river basin,described with metrics of density of ecological guilds, taxonomic richness and life history traits,and coupled with reachhydromorphology and catchment characteristics was constructed.Generalized linear modellingwas performed in order to quantify dam impact and investigate the importance of regional-and local-scale measures of dam density to the structure of biotic communities.The analysis showed that community structure at the basin scale responded significant to dam presence and confirmed that the strongest relationships were observed for specific functional trait-based metrics. For the macroinvertebrates the observed impact counts up to 25% of the variance in the trait-based quality indices, whereas for fish communities the dam density only explains up to 12%. Macroinvertebrate responses were stronger at higher scale level, and especially the upstream context explained on its own 70% of the observed impairment. For fish communities, the local context prevails and explained up to 70% of the dam impact. These results can be explained by the biotic processes ruling community assembly in the specific groups, passive dispersal for the invertebrates and migrations between habitats for fish. The geographic context furthermore explains the differentiation in these responses, reflecting the metacommunity structure of invertebrate assembly over the river basin. We conclude that for upstream parts of the river basin, locally based management actions can be successful in restoring biotic integrity, whereas more downstream, dam removal actions require more integrated measures at regional rather than local scale. Highlights ► Dam density is proposed as a multi-scale indicator distinguishing local and network scale impact. ► Responses in macroinvertebrate and fish communities are strongest for functional trait metrics ►Macroinvertebrate communities respond strongest to upstream dam density whereas for fish the local context prevails. ► Invertebrate assembly and responses reflect metacommunity structure. ► Upstream-downstream contexts and responses differ and demand for different restoration strategies.
Abstract. The spatial and temporal coverage of the Landsat satellite imagery make it an ideal resource for the monitoring of water temperature over large territories at a moderate spatial and temporal scale at a low cost. We used Landsat 5 and Landsat 7 archive images to create the Lake Skin Surface Temperature (LakeSST) data set, which contains skin water surface temperature data for 442 French water bodies (natural lakes, reservoirs, ponds, gravel pit lakes and quarry lakes) for the period 1999-2016. We assessed the quality of the satellite temperature measurements by comparing them to in situ measurements and taking into account the cool skin and warm layer effects. To estimate these effects and to investigate the theoretical differences between the freshwater and seawater cases, we adapted the COARE 3.0 algorithm to the freshwater environment. We also estimated the warm layer effect using in situ data. At the reservoir of Bimont, the estimated cool skin effect was about −0.3 and −0.6 • C most of time, while the warm layer effect at 0.55 m was negligible on average, but could occasionally attain several degrees, and a cool layer was often observed in the night. The overall RMSE of the satellite-derived temperature measurements was about 1.2 • C, similar to other applications of satellite images to estimate freshwater surface temperatures. The LakeSST data can be used for studies on the temporal evolution of lake water temperature and for geographical studies of temperature patterns. The LakeSST data are available at https://doi.org/10.5281/zenodo.1193745.
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