TWI: Predicting Where the Landscape Stays Wet
Water does not distribute evenly across terrain — it accumulates where large upslope areas funnel into gentle ground. The Topographic Wetness Index predicts those persistently wet zones from the shape of the land alone.
How it works
TWI takes the log of the upslope contributing area divided by the tangent of the local slope. Large catchments feeding flat ground yield high TWI (wet); small catchments on steep ground yield low TWI (dry). It is a steady-state proxy for soil-moisture potential.
The formula
Reading the values
Low on well-drained ridges · high in valley bottoms and convergence zones prone to saturation.
The data it needs
| Derived from | Digital Elevation Model (SRTM / Copernicus, 30 m) — not an optical band index. |
In practice
It informs soil mapping, wetland and hydric-soil prediction, digital soil-carbon modelling, and siting for agriculture and drainage.
Where it struggles
TWI assumes steady-state hydrology and breaks down on flat terrain (where slope approaches zero and the log blows up) and on coarse DEMs. Your flow-accumulation algorithm materially changes the result, so document it.
Compute TWI on your own study area
Skip the code. Draw or upload a boundary and Spatial Research Suite runs TWI on live data — with cloud masking, exports and citations built in.
Run this analysis in GISforus →Frequently asked
What does a high TWI value indicate?
A location likely to stay wet — valley bottoms and convergence zones with large upslope area and gentle slope; low TWI marks well-drained ridges.
Why does TWI misbehave on flat terrain?
As slope approaches zero the tangent in the denominator does too, sending TWI toward infinity, so nearly flat cells need special handling.