The topographic wetness index (TWI), also known as the compound topographic index (CTI), is a steady state wetness index. It is commonly used to quantify topographic control on

hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...

processes. The index is a function of both the

slope In mathematics, the slope or gradient of a Line (mathematics), line is a number that describes the direction (geometry), direction of the line on a plane (geometry), plane. Often denoted by the letter ''m'', slope is calculated as the ratio of t ...

and the upstream contributing area per unit width

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

to the flow direction. The index was designed for hillslope catenas. Accumulation numbers in flat areas will be very large, so TWI will not be a relevant variable. The index is highly correlated with several soil attributes such as

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

depth,

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

percentage, organic matter content, and

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

. Methods of computing this index differ primarily in the way the upslope contributing area is calculated.

Definition

The topographic wetness index is defined as:

\ln

where

a

is the local upslope area draining through a certain point per unit contour length and

b

is the local

radian The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. It is defined such that one radian is the angle subtended at ...

s. The TWI has been used to study spatial scale effects on hydrological processes. The topographic wetness index (TWI) was developed by Beven and Kirkby 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. Rather, it should be interpreted that areas with similar TWIs become saturated under similar moisture conditions, as described by Dr John Lindsay of the University of Guelph.

Uses

The TWI has been used to study spatial scale effects on

processes and to identify hydrological flow paths for

geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...

modelling, as well as to characterize biological processes such as annual net primary production,

vegetation Vegetation is an assemblage of plants and the ground cover they provide. It is a general term, without specific reference to particular Taxon, taxa, life forms, structure, Spatial ecology, spatial extent, or any other specific Botany, botanic ...

patterns, and forest site quality.

References

{{reflist Topography Hydrology