Satellite observations of water transparency from VIIRS in global aquatic ecosystems

This study presents a new satellite ocean color data record of Secchi depth ( Z SD ) observations from the Visible Infrared Imaging Radiometer Suite (VIIRS). As part of the NOAA enterprise satellite data processing, the decade-long Z SD data are derived from the visible and near-infrared reflectance measurements over oceanic, coastal, and inland waters. Based on in situ data, the model is excellent in generating low-uncertainty Z SD data with an absolute percentage difference (APD) of 15%–29%. The satellite and in situ matchups confirm reliable satellite retrievals with APD = 19%–26% over the Z SD range of 0.1–60 m. Although the product uncertainties are dependent on optical water types, assessments show that the satellite Z SD estimations are very reliable, especially where Z SD ≥ 1 m. This new satellite product has enabled the ability to access Level-2 daily Z SD imagery and information as well as Level-3 data aggregated on daily to monthly scales. Our examination indicates that the ocean transparent windows are situated at 443 and 486 nm in the vast open oceans and 551 nm for most coastal waters. They shift to the red band at 671 nm for extremely turbid environments, such as large river estuaries. From a global perspective, the Z SD data extend from less than half a meter in nearshore environments to >70 m in the South Pacific Gyre, while demonstrating a strong dependency on the optical water types. Short-term fluctuations over time are registered in the satellite Z SD daily and monthly data from almost every aquatic environment. Trend analyses reveal significant increases in water transparency over many regions, especially the open ocean. We stress the necessity of normalizing satellite Z SD estimations to eliminate the uncertainties induced by different solar-zenith angles. The present satellite products can be further improved by accounting for the limitations imposed by the multispectral reflectance data with hyperspectral ocean color spectra. • Global Secchi depth ( Z SD ) measurements are obtained from NOAA satellite observations • The data are processed with hybrid algorithms and are normalized to the zenith sun • Data quality is rigorous in clear and turbid waters • Spatial contrast and daily-seasonal variability are recorded across various environments • Decade-long satellite data provide a new tool for operational water quality monitoring