Nimpact Academy - Module 3: Water Clarity & Turbidity

Multispectral Water Clarity Measurement

Water clarity is measured using Sentinel-2 multispectral imagery, which captures light reflectance across 13 spectral bands from visible to near-infrared wavelengths. Unlike temperature (which uses thermal infrared), clarity assessment analyzes how visible light penetrates and scatters through water.

The Science of Light Penetration

When sunlight enters water, three processes occur:

Absorption: Water molecules absorb light energy, converting it to heat (strongest in red/infrared wavelengths)
Scattering: Suspended particles deflect light in all directions (creates turbidity)
Transmission: Light that passes through without interaction (determines clarity depth)

Sentinel-2 Key Bands for Water Quality

Band 2 (Blue, 490nm): Deepest penetration, used for bathymetry and clarity estimation
Band 3 (Green, 560nm): Chlorophyll absorption feature, algae detection
Band 4 (Red, 665nm): Sediment and suspended solids
Band 8 (NIR, 842nm): Water vs. land discrimination, algae scum detection

Secchi Depth: The Universal Clarity Standard

Secchi depth (named after Italian astronomer Pietro Angelo Secchi, 1818-1878) is the depth at which a standardized 20cm diameter black-and-white disk disappears from view when lowered into water. This simple measurement has been the gold standard for water clarity since the 1800s.

# Secchi Depth Interpretation 0-1m: Very turbid (rivers, harbors, algae blooms) 1-2m: Turbid (typical coastal waters) 2-5m: Moderate clarity (most recreational lakes) 5-10m: Clear (oligotrophic lakes, coral reefs) 10m+: Exceptionally clear (crater lakes, deep ocean)

Nimpact uses machine learning algorithms trained on thousands of simultaneous Sentinel-2 images and in-water Secchi disk measurements to predict Secchi depth from satellite reflectance values. The algorithm achieves R² = 0.78 accuracy compared to field measurements.

Turbidity: Particle Concentration Measurement

Turbidity measures the concentration of suspended particles in water, reported in Nephelometric Turbidity Units (NTU). While Secchi depth tells you "how far can you see," turbidity tells you "how much stuff is floating in the water."

    Turbidity Sources:
    Algae: Living phytoplankton cells (green to brown tint)
Sediment: Clay, silt, sand from erosion (brown/tan tint)
Organic Matter: Dissolved tannins from vegetation (tea-colored tint)
Industrial Discharge: Various pollutants (depends on source)

Color Analysis: CDOM and Chlorophyll

By analyzing the ratios of different spectral bands, Nimpact can determine what's causing low clarity:

High Band2/Band3 Ratio: Colored Dissolved Organic Matter (CDOM)—tannins from wetlands
High Band3/Band4 Ratio: Chlorophyll-a from algae—indicates biological productivity
High Band4/Band8 Ratio: Suspended sediments—indicates erosion or resuspension

Interpretation Nuance: Low clarity isn't always "bad." Tea-colored lakes from natural wetlands are ecologically healthy despite low Secchi depth. Conversely, crystal-clear water can be "too clean" (oligotrophic), supporting little aquatic life.

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Seasonal Clarity Patterns and Regional Comparisons

Seasonal Clarity Variations

Water clarity shows dramatic seasonal patterns driven by biological and physical processes:

The Annual Clarity Cycle

Winter (Dec-Feb): Maximum clarity—algae dormant, cold water settles sediments, ice cover stops wind mixing
Spring (Mar-May): Rapid clarity decrease—snowmelt brings sediments, nutrients trigger spring algae bloom
Summer (Jun-Aug): Minimum clarity—peak algae growth, wave action resuspends sediments
Fall (Sep-Nov): Clarity recovery—algae die-off, cooling water, reduced recreational activity

This is why Nimpact presents clarity data in seasonal tables rather than annual averages—a beach with 8m winter clarity but 2m summer clarity needs very different management than one with consistent 5m year-round.

Percentile Ranking System

Nimpact compares your beach's clarity to regional waterbodies (±250km) of the same type:

# Example Clarity Percentile Your Lake: 3.2m Secchi depth Regional Lakes (n=67): 90th percentile: 8.5m (clearest 10%) 50th percentile: 4.1m (median) 10th percentile: 1.8m (cloudiest 10%) Result: 45th percentile Interpretation: "Slightly below average clarity"

Water Body Type Differences

Clarity expectations vary dramatically by water body type. Understanding these differences is critical for proper interpretation:

Lakes: Naturally Clearer Waters

Standing Water: Longer residence time allows sediment to settle naturally
Thermal Stratification: Summer stratification traps nutrients in deep water, limiting surface algae growth
Deep Lakes (Oligotrophic): Secchi 5-10m+ typical, crystal clear blue water
Shallow Prairie Lakes: Secchi 1-3m due to wind mixing resuspending bottom sediments
Seasonal Patterns: Clearest in winter (ice cover stops mixing), murkiest in summer (algae blooms)

Rivers: Highly Variable Clarity

Flow-Dependent: High flow = turbid (suspended sediment), low flow = clearer water
Continuous Sediment Transport: Natural erosion constantly adds clay and silt particles
Prairie Rivers: Secchi 0.5-2m typical due to fine clay particles that stay suspended
Mountain Rivers: Secchi 2-4m typical, coarser sediments settle quickly
Flood Events: Can reduce clarity to <0.5m for days or weeks after rainfall
Benthic Algae: Bottom-attached algae don't affect clarity as much as planktonic algae

Tidal/Coastal Waters: Naturally Turbid

Tidal Resuspension: Twice-daily tides constantly resuspend bottom sediments
Wave Action: Surf zone creates high turbidity from sediment mixing
Sediment Plumes: River discharge creates brown/green plumes extending offshore
Typical Secchi: 1-3m is normal and healthy for coastal waters
Open Ocean: Clearer water offshore (Secchi 5-20m) where sediment settles
Estuaries: Mixing of fresh and salt water creates variable clarity (0.5-3m typical)

    Critical Insight: Never compare a river's clarity to a lake's clarity or a tidal beach to a lake. Nimpact's percentile system automatically compares apples-to-apples by grouping similar water body types within the same climate region. A "fair" rating for a prairie river (Secchi 1.5m) would be "excellent" for coastal waters but "poor" for a deep lake.

Regional and Climate Context

Even within water body types, clarity varies by region:

Tropical Waters: Warmer = more biological activity = more particles = lower clarity
Cold Temperate Waters: Less biological activity = clearer water year-round
Glacial Lakes: "Glacier milk" from rock flour creates turquoise but low-clarity water
Wetland-Fed Waters: Natural tannins create tea-colored water (healthy but low clarity)

Clarity and Ecology

Water clarity has profound ecological implications:

Submerged Aquatic Vegetation (SAV): Requires light penetration to ~2x Secchi depth for photosynthesis
Fish Visual Predation: Pike, bass, and walleye hunting efficiency correlates with clarity
Zooplankton Behavior: Clear water forces zooplankton deeper to avoid visual predators
Human Recreation: Swimming comfort strongly influenced by seeing bottom at 1-2m depth

Limitations of Satellite Clarity Measurement

Sentinel-2 clarity estimates work best in open water. Accuracy decreases in:

Shallow water (< 1m): Bottom reflectance contaminates signal
Heavy cloud cover: Fewer usable images reduce statistical confidence
Sun glint: Direct reflection of sunlight creates false readings
Narrow rivers (< 30m): Mixed pixels from shoreline vegetation

Quality Check: Nimpact reports "images used" for each season. 10+ images = reliable, 5-9 = adequate, < 5 = interpret with caution.

Actionable Clarity Thresholds

Based on water quality standards and recreational guidelines:

# Recreation and Ecology Thresholds Secchi > 4m: Excellent - suitable for all recreation Secchi 2-4m: Good - typical for most lakes Secchi 1-2m: Fair - acceptable for swimming, limited visibility Secchi < 1m: Poor - investigate sources (algae, sediment, pollution) Turbidity > 5 NTU: Elevated - monitor for sources Turbidity > 10 NTU: High - likely impairment

If Nimpact detects chronic low clarity (< 1.5m across all seasons) or sudden clarity decline (> 50% drop from historical baseline), it flags for investigation.

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Module 3: Water Clarity & Turbidity