The diffuse attenuation coefficient of downwelling irradiance at 490 nm, Kd_490, quantifies how quickly blue-green light diminishes with depth—an indicator of water clarity and the depth to which sunlight penetrates. Kd_490 is generated from ocean-color reflectance using the KD2 algorithm. Lower values indicate clearer water (deeper light penetration); users commonly apply Kd_490 to estimate euphotic depth, track coastal water quality and turbidity, interpret bloom/light environments, and supply inputs to biogeochemical and ecosystem models. As with all ocean-color products, uncertainties can be larger in optically complex coastal or inland waters; refer to the file metadata and algorithm documentation for quality flags and usage notes.

Geophysical variables in this suite include:
• Kd_490 — Diffuse attenuation coefficient at 490 nm, KD2 algorithm (m⁻¹)

This activity was informed by the Satellite Needs Working Group (SNWG), an interagency effort of the U.S. Government dedicated to identifying and addressing Earth observation needs across U.S. civilian federal agencies.