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Module 5: Percentile Ranking System

Understanding Comparative Environmental Assessment

Why Percentiles Matter More Than Absolute Values

The single most important concept in Nimpact reports is percentile ranking. A temperature of 18°C means completely different things at different latitudes—Nimpact solves this by comparing your beach to similar waterbodies in similar climates.

The Fundamental Problem with Absolute Thresholds

Traditional water quality assessment uses fixed thresholds (e.g., "good water quality = < 20°C"). This fails across geographic regions:

Example: 18°C Water Temperature
  • At 60°N (Northern Canada): Peak summer warmth, prime algae bloom temperature
  • At 45°N (Great Lakes): Comfortable swimming temperature, moderate conditions
  • At 30°N (Florida): Abnormally cold, might indicate upwelling or unusual conditions
  • At 10°N (Caribbean): Extremely cold, would trigger investigation of coral bleaching risk

Percentile ranking automatically accounts for these regional differences by comparing "apples to apples"—your beach versus similar beaches in similar climates.

How Percentile Ranking Works

# Percentile Calculation Example Your beach temperature: 16.8°C Regional comparison group: 127 lakes within ±250km Temperatures sorted (lowest to highest): Rank 1: 8.2°C (coldest) Rank 32: 14.5°C (25th percentile) Rank 64: 17.1°C (50th percentile - median) Rank 96: 19.8°C (75th percentile) Rank 127: 23.5°C (warmest) Your beach: 16.8°C = Rank 62 out of 127 Percentile: (62/127) × 100 = 49th percentile Interpretation: "Near-average temperature for regional lakes"

Interpretation Guide by Percentile Range

Critical Insight: High percentile isn't always "bad" and low percentile isn't always "good." Context matters:
  • Water Clarity: 90th percentile = very clear (excellent)
  • Temperature: 90th percentile = very warm (good for swimming, high algae risk)
  • Shoreline Risk: 90th percentile = very unstable (needs engineering review)

Regional Grouping: The ±250km Rule

Nimpact compares your beach to all beaches within a 250km radius having the same water body type (tidal/lake/river). This distance represents:

Water Body Type Stratification

Percentiles are calculated separately for each water body type because they function differently:

Why Separate Percentiles by Water Type?

  • Tidal/Coastal: Wave energy, salinity, tidal flushing create unique dynamics
  • Lakes: Stratification, thermal inertia, enclosed systems with long residence times
  • Rivers: Flowing water, upstream influences, continuous flushing, high sediment loads

Comparing a river's temperature to a lake's temperature would be meaningless—they have fundamentally different thermal regimes.

Statistical Confidence and Sample Size

Percentile reliability depends on comparison group size:

# Sample Size Interpretation n > 100: Excellent - highly reliable percentiles n 50-100: Good - reliable for most purposes n 20-50: Adequate - percentiles approximate n < 20: Poor - interpret with caution

Nimpact reports always show the comparison group size (e.g., "compared to 127 regional lakes") so you can assess statistical confidence.

Percentiles vs. Standard Deviations

Why use percentiles instead of standard deviations (σ)?

Action Thresholds Based on Percentiles

Nimpact uses percentile-based decision rules for flagging conditions:

# Example: Algae Risk Flagging if chlorophyll_percentile > 75: flag = "Elevated - above regional average" action = "Consider monitoring program" if chlorophyll_percentile > 90: flag = "High - top 10% regionally" action = "Water quality testing recommended"

These thresholds are water-type and region-specific, automatically adjusting for local conditions while identifying truly exceptional values that warrant attention.

How Percentiles Apply Across Different Water Body Types

The percentile approach is especially powerful because each water body type has fundamentally different environmental characteristics:

Lakes: Standing Water Bodies

Key Characteristics:

  • Thermal Stratification: Lakes develop distinct temperature layers (epilimnion/thermocline/hypolimnion) in summer. Percentile rankings account for this seasonal pattern.
  • Oligotrophic vs. Eutrophic: Nutrient-poor lakes naturally have low algae (10th-30th percentile chlorophyll), while nutrient-rich lakes may have high algae (70th-90th percentile) without being "impaired"—it's their natural state.
  • Long Residence Times: Water stays in lakes for months to years, so pollution accumulates. High turbidity percentiles often indicate watershed erosion problems.

Percentile Interpretation Example: A northern oligotrophic lake at 95th percentile for clarity (very clear) and 15th percentile for temperature (cool) is perfectly healthy—those extremes define its natural character.

Rivers: Flowing Water Systems

Key Characteristics:

  • Flow-Dependent Clarity: River turbidity naturally varies with discharge. A river at 80th percentile turbidity during spring snowmelt is normal; the same value in summer low-flow indicates erosion problems.
  • Continuous Sediment Transport: Rivers constantly move sediment. Percentile rankings for suspended solids are compared only to other rivers in similar flow conditions, not to static lakes.
  • Upstream Influences: River conditions reflect everything happening in the entire watershed. High percentile rankings for nutrients often trace to agricultural or urban sources kilometers upstream.

Percentile Interpretation Example: A river at 65th percentile for temperature but 35th percentile for clarity isn't "inconsistent"—it might have warm, sediment-laden water from prairie agricultural runoff, which is typical for that region.

Tidal/Coastal Waters: Saltwater Systems

Key Characteristics:

  • Tidal Resuspension: Twice-daily tides stir up bottom sediments. Turbidity percentiles are calculated separately for flood vs. ebb tide conditions to avoid false "poor clarity" flags.
  • Wave Action: Coastal waters experience far more wave energy than lakes. High suspended sediment percentiles near beaches are often natural from wave-driven resuspension, not pollution.
  • Estuarine Mixing: Where rivers meet the ocean, salinity gradients create complex layering. Percentile rankings account for whether the site is freshwater-dominated, saltwater-dominated, or mixed.
  • Salinity Effects: Marine algae species differ completely from freshwater. A "high chlorophyll" percentile in coastal water might indicate healthy phytoplankton blooms supporting fisheries, not harmful cyanobacteria.

Percentile Interpretation Example: A coastal beach at 75th percentile for turbidity but 40th percentile for chlorophyll reflects high wave energy (natural sediment) with moderate biological productivity—typical for exposed coasts.

Critical Insight for Comparisons: Nimpact NEVER compares lakes to rivers to oceans directly. A "50th percentile temperature" lake and a "50th percentile temperature" ocean are compared only within their respective water types, ensuring meaningful regional context. The same absolute temperature (e.g., 15°C) might be 90th percentile for a northern lake but 20th percentile for a tropical coast.
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