[1] The index is a function of both the slope and the upstream contributing area per unit width orthogonal to the flow direction.
The index is highly correlated with several soil attributes such as horizon depth, silt percentage, organic matter content, and phosphorus.
The topographic wetness index (TWI) was developed by Beven and Kirkby[3] within the runoff model TOPMODEL.
Although the topographic wetness index is not a unitless number, it is sufficiently approximate that its interpretation doesn't rely on its physical units.
[4] The TWI has been used to study spatial scale effects on hydrological processes and to identify hydrological flow paths for geochemical modelling, as well as to characterize biological processes such as annual net primary production, vegetation patterns, and forest site quality.