Abstract
The possibilities of chlorophyll a (Chl a) and total suspended matter (TSM) retrieval using Sentinel-2/MSI imagery and in situ measurements in the Gorky Reservoir are investigated. This water body is an inland freshwater ecosystem within the territory of the Russian Federation. During the algal bloom period, the optical properties of water are extremely heterogeneous and vary on scales of tens of meters. Additionally, they vary in time under the influence of currents and wind forcing. In this case, the usage of the traditional station-based sampling to describe the state of the reservoir may be uninformative and not rational. Therefore, we proposed an original approach based on simultaneous in situ measurements of the remote sensing reflectance by a single radiometer and the concentration of water constituents by an ultraviolet fluorescence LiDAR from a high-speed gliding motorboat. This approach provided fast data collection including 4087 synchronized LiDAR and radiometric measurements with high spatial resolutions of 8 m for two hours. A part of the dataset was coincided with Sentinel-2 overpass and used for the development of regional algorithms for the retrieval of Chl a and TSM concentrations. For inland waters of the Russian Federation, such research was performed for the first time. The proposed algorithms can be used for regular environmental monitoring of the Gorky Reservoir using ship measurements or Sentinel-2 images. Additionally, they can be adapted for neighboring reservoirs, for example, for other seven reservoirs on the Volga River. Moreover, the proposed ship measurement approach can be useful in the practice of limnological monitoring of inland freshwater ecosystems with high spatiotemporal variability of the optical properties.
Highlights
The largest cities, industrial centers, and agricultural lands are often located on the banks of inland waters
To develop an empirical chlorophyll a (Chl a) model for the Gorky reservoir, we considered the semi-empirical NIR-red edge algorithms that are popular for optically complex waters: two-band (2B) and three-band (3B) algorithms [24], the Normalized Difference Chlorophyll Index (NDCI) algorithm [25], and the peak height algorithm (PH) [21]
We proposed the approach of remote rapid in situ measurements with the resolution of 8 m suitable for inland waters with spatial heterogeneity and temporal variation of its optical properties
Summary
The largest cities, industrial centers, and agricultural lands are often located on the banks of inland waters. In the Russian Federation, unlike Europe, Asia or America, such algorithms were developed for significantly fewer water bodies: the Ivankovo [7], the Krasnoyarsk [8], and the Gorky [9] Reservoirs, and the Lakes Baikal [10,11] and Ladoga [12] These studies were limited by retrieval of Chl a concentration by regression to Level 2 (L2) satellite data. The presence of regular channel currents and variable wind forcing leads to significant spatial-temporal variability of their water optical properties on scales of tens of meters and some hours These factors limit the usage of the MODIS and Landsat-7,8 data due to insufficient resolution and low revisit interval (16 days), respectively
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