Abstract
Optical remote sensing of aquatic vegetation in shallow water is an essential aid to ecosystem protection, but it is difficult because the spectral characteristics of the vegetation are sensitive to external features such as water background effects, atmospheric effects, and the structural properties of the canopy. A global sensitivity analysis of an aquatic vegetation radiative transfer model provides invaluable background for algorithm development for use in optical remote sensing. Here, we use the extended Fourier Amplitude Sensitivity Test (EFAST) method for the modelling. Four different cases were identified by subdividing the ranges of water depth and leaf area index (LAI) involved. The results indicate that the reflectance of emergent vegetation is affected mainly by the concentrations of chlorophyll a + b in leaves (Cab), leaf inclination distribution function parameter (LIDFa) and LAI. The parameter LAI is influential in sparse vegetation cases whereas Cab and LIDFa are influential in dense vegetation cases. Canopy reflectance for submerged vegetation is dominated by water parameters. Relatively, LAI and Cab are highly sensitive vegetation parameters. The analysis is extended to vegetation index as well, which takes the Sentinel-2A as the reference sensor. It shows that NDAVI (Normalized Difference Aquatic Vegetation Index) is suitable for retrieving LAI in all cases except deep-sparse for emergent vegetation, whereas NDVI (Normalized Difference Vegetation Index) would be better in the deep-sparse case. NDVI, NDAVI and WAVI (Water Adjusted Vegetation Index), respectively, are suitable for retrieving Cab, Car and LIDFa in dense cases. For submerged vegetation, the sensitivity of LAI to NDAVI is relatively high only in the shallow-sparse case. The adjustment factor L in SAVI and WAVI fails to suppress the sensitivity to water constituent parameters. The sensitivity of LAI and Cab to NDVI in deep cases is relatively higher than that to the other indices, which may provide clues for the construction of inversion algorithms in macrophyte remote sensing in the aquatic environment using spectral signatures in the visible and near infrared regions.
Highlights
Aquatic vegetation is a key element in the sustainability of wetland ecosystems [1,2]
Its canopy reflectance is dominated by vegetation parameters, which is similar to the case for terrestrial vegetation
A global sensitivity analysis was carried out by the extended Fourier Amplitude Sensitivity Test (EFAST) method with the complex Aquatic Vegetation Radiative Transfer model (AVRT) model to explore a broad range of aquatic vegetation and water body and observation conditions
Summary
Aquatic vegetation is a key element in the sustainability of wetland ecosystems [1,2]. Aquatic vegetation can be divided into three groups: emergent vegetation, whose roots grow in the sediment underwater but whose leaves and stems emerge out of water; floating vegetation, which has leaves floating just above the water surface; and submerged vegetation, which lives entirely below the water surface [3]. These plants serve to absorb nitrogen and phosphorus, reduce turbidity and inhibit eutrophication, and can be used as indicators of the trophic status of a water body. The technology of remote sensing has the advantage of a broad coverage and frequent data acquisitions, which makes it a valuable tool for the assessment of macrophyte distribution, vegetation status and related biophysical parameters
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