Water Optics – Colour shift of turbid (muddy) water with increasing turbidity As turbidity increases, the peak of upwelling radiance in muddy water shifts towards which region of the visible spectrum, often making the water appear brown?

Introduction / Context:
Water colour in remote sensing reflects the balance of absorption and scattering by water molecules, suspended sediments, coloured dissolved organic matter, and phytoplankton. Turbid (muddy) waters dominated by mineral particles show characteristic spectral shifts valuable for sediment mapping and dredging monitoring.

Given Data / Assumptions:

• Increasing turbidity due to suspended mineral sediments.
• Observation in the visible spectrum under daylight.
• Open surface (no strong bottom reflectance).

Concept / Approach:
Mineral particles enhance backscattering, particularly at longer visible wavelengths, while pure water absorbs more strongly in the red but much less than it absorbs in the blue/green when particle load is high. As particle concentration rises, backscattering dominates, shifting the apparent reflectance peak toward longer wavelengths, giving a yellowish-brown to reddish tone.

Step-by-Step Solution:
Low turbidity: peak reflectance often in blue-green.Higher turbidity: particle backscatter increases with wavelength within VIS ⇒ spectral peak drifts toward red.Perceptual outcome: brownish appearance driven by elevated red/yellow reflectance.

Verification / Alternative check:
Empirical red-band turbidity indices and suspended sediment retrievals rely on the monotonic rise of reflectance into the red with particle load.

Why Other Options Are Wrong:

• Green/Yellow: may increase transiently, but the dominant peak for very muddy water shifts further toward red.
• Blue/Violet: typically suppressed by absorption and less backscatter under high turbidity.

Common Pitfalls:
Confusing phytoplankton-driven green peaks (biological) with mineral turbidity; mechanisms differ and produce different colour trends.

Final Answer:
Red