How to Detect Floods with Satellite Radar
Learn how to visually interpret flood extent in Sentinel-1 SAR imagery. This guide explains why water appears dark in radar and how to compare before/after scenes.
Quick answer — how SAR flood mapping works
SAR detects floods through clouds because water appears dark (specular reflection). Use a Sentinel-1 before/after comparison: VV gives the strongest open-water contrast, while VH helps where vegetation is flooded. 6-day repeat cycle. Sentinel-1 is a plan-gated layer in Off-Nadir Delta (see Pricing).
Works Through Storms
Radar penetrates clouds, rain, and darkness - exactly when floods occur.
Rapid Assessment
Work from the scene as soon as the provider publishes it — how quickly that happens depends on the acquisition and the distribution pipeline.
Wide Area Coverage
Single Sentinel-1 scene covers 250km swath for regional assessment.
How SAR Detects Water
SAR sends microwave pulses and measures the reflected signal. Calm water surfaces act like mirrors, reflecting the radar away from the satellite, making water appear very dark in SAR imagery.
By comparing pre-flood and during-flood imagery, new dark areas indicate flood extent. This works even when the area is covered by clouds or at night.
SAR Flood Interpretation Guide
What Appears Dark
- • Open Water: Calm surfaces reflect radar away (specular reflection)
- • Flooded Fields: Agricultural areas covered by water
- • Smooth Surfaces: Roads, runways, calm lakes
What Appears Bright
- • Urban Areas: Buildings cause corner reflections
- • Flooded Vegetation: Double-bounce from water under trees
- • Rough Surfaces: Waves, rough terrain, forests
Polarization Tips
Best for open water detection. Strong contrast between water and dry land.
Better for flooded vegetation. Cross-polarization sensitive to volume scattering.
Applications
Emergency Response
Prioritize rescue and relief efforts based on flood extent
Damage Assessment
Document affected areas for insurance and recovery planning
Floodplain Mapping
Historical flood extent for risk assessment and planning
Flood Monitoring
Track flood progression and recession over time
How It Works
Identify Affected Area
Navigate to the flooded region on the map.
Get Pre-Flood Baseline
Load Sentinel-1 SAR imagery from before the flood event.
Load Current SAR Data
Search for the most recent Sentinel-1 imagery during/after the flood.
Compare & Map Extent
Use difference analysis to identify flooded areas (water appears dark in SAR).
Frequently Asked Questions
Why use SAR for flood mapping?▼
How do I identify water in SAR?▼
How quickly is new data available?▼
Can I map floods in urban areas?▼
What about flooded vegetation?▼
Related Resources
Sentinel-1 SAR Viewer
View SAR imagery in the browser
Learn moreChange Detection
Compare before & after
Learn moreShip Detection Guide
Detect ships with SAR
Learn moreUse Cases
Disaster response applications
Learn moreFlood Mapping with SAR Guide
Practical step-by-step workflow
Learn moreSAR Time Series for Floods
Monitor flood dynamics over time
Learn moreView SAR Imagery
Apply what you've learned to Sentinel-1 scenes in the workspace — see Pricing for the plans that include SAR. Or see live flood and disaster events prioritized on the Watchfloor.
From headline to satellite evidence
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