Free TWI Calculator: Topographic Wetness Index for Any Location

The Topographic Wetness Index (TWI) is one of the most widely used terrain metrics in hydrology, ecology, and land-use planning. It quantifies how likely a given point in the landscape is to accumulate water based purely on topography. Despite its importance, calculating TWI traditionally requires GIS software, a high-resolution DEM, and knowledge of flow accumulation algorithms. PixelGust makes TWI data accessible to anyone — click a location and get instant results.

What Is the Topographic Wetness Index?

TWI was introduced by Beven and Kirkby in 1979 as part of the TOPMODEL hydrological framework. The formula is simple but powerful:

TWI = ln(a / tan β)

Where:

• a = upslope contributing area per unit contour length (m²/m). This measures how much terrain drains through a given point.
• β = local slope angle in radians. Steeper slopes shed water faster, reducing wetness.

The result is a dimensionless index where higher values indicate greater tendency for water accumulation. TWI effectively identifies valley floors, depressions, and flat areas that receive runoff from large upslope catchments.

Why TWI Matters

Flood Risk Assessment

TWI is the foundation of terrain-based flood risk assessment. While hydrodynamic flood models require complex inputs and expert calibration, TWI provides a first-pass identification of flood-prone areas from topography alone. High TWI zones are where water accumulates first and drains last — exactly where flood damage is most likely.

Real Estate and Site Selection

For real estate climate risk assessment, TWI reveals drainage problems that may not be obvious from site visits. A flat building plot may look ideal, but if it sits at the bottom of a large catchment (high TWI), it faces chronic waterlogging, basement flooding, and foundation moisture issues.

Agriculture and Soil Moisture

In precision agriculture, TWI maps help farmers understand field-scale moisture variability. High-TWI areas retain moisture longer, affecting crop health, equipment access, and soil erosion patterns. Low-TWI ridges may suffer drought stress first.

Ecological Mapping

Wetland delineation, habitat modeling, and species distribution studies frequently use TWI as a predictor variable. Many plant and animal species show strong associations with terrain wetness, making TWI an essential input for environmental assessments.

How PixelGust Calculates TWI

PixelGust computes TWI using the following pipeline:

1. DEM source: Copernicus GLO-30 at 30-meter resolution — the highest freely available global elevation dataset.
2. Pit filling: The DEM is hydrologically conditioned to remove artificial depressions that would trap flow.
3. Flow direction: A D8 (deterministic eight-neighbor) algorithm determines the steepest downslope direction from each cell.
4. Flow accumulation: The number of upslope cells draining through each cell is computed, then multiplied by cell area to get contributing area.
5. Slope calculation: Local slope is derived from the DEM using a 3×3 neighborhood gradient.
6. TWI computation: The natural log of (contributing area / tan(slope)) is calculated for each cell.

The result feeds directly into the flood susceptibility rating shown in the Hazards panel. You can also view the underlying elevation and slope data in the Terrain panel.

Interpreting TWI Values

TWI values depend on DEM resolution and landscape type, but general guidelines:

• TWI < 6: Well-drained ridges and steep slopes. Low flood risk. Good drainage for construction.
• TWI 6–8: Moderate hillslopes. Normal drainage conditions.
• TWI 8–10: Lower slopes and gentle terrain. Some waterlogging possible during heavy rainfall.
• TWI 10–14: Valley floors, floodplains, and convergent terrain. Expect seasonal saturation and elevated flood risk.
• TWI > 14: Permanent wetlands, river channels, lakeshores. Chronic saturation. Not suitable for construction without major drainage engineering.

TWI vs. Other Flood Risk Methods

TWI is a static, topography-only indicator. It does not account for rainfall intensity, soil permeability, or engineered drainage. For comprehensive flood risk, combine TWI with:

• Historical precipitation data: Areas with high TWI AND high rainfall are at the greatest flood risk.
• Land cover data: Impervious surfaces (urban areas) amplify runoff and increase effective TWI.
• Infrastructure proximity: Distance to rivers and water bodies provides additional flood exposure context.
• Soil type: Clay soils with low infiltration rates behave as if TWI were higher than the pure terrain calculation indicates.

Use Cases

Property Due Diligence

Before purchasing land, check TWI to identify hidden flood risks. Many properties appear dry during summer visits but sit in high-TWI zones that flood during winter storms. Combine with the flood risk panel for a complete picture.

Urban Planning

Municipalities use TWI maps to identify areas where new development should require stormwater management infrastructure. High-TWI parcels may need retention ponds, permeable paving, or green infrastructure.

Insurance Underwriting

Insurers increasingly use terrain-based flood indicators like TWI to supplement traditional flood zone maps. TWI captures pluvial (rainfall) flood risk that may not appear in river-based flood maps. Read more about weather data in insurance.

Renewable Energy Siting

Solar and wind farm developers check TWI to avoid waterlogged areas that complicate construction and access road maintenance. For solar projects, combine TWI with solar radiation data for optimal site selection.

Data Sources

PixelGust's TWI calculation uses the Copernicus GLO-30 DEM (30m resolution) from the European Space Agency. Flow accumulation and slope are computed server-side using hydrologically conditioned terrain data. The resulting flood susceptibility classification is displayed in the Hazards panel alongside RUSLE soil erosion and fire weather index data.