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 processes.
The index is a function of both the
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is used ...
and the upstream contributing area per unit width
orthogonal
In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''.
By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
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 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 with water. Silt usually has a floury feel wh ...
percentage, organic matter content, and
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
.
Methods of computing this index differ primarily in the way the upslope contributing area is calculated.
Definition
The topographic wetness index is defined as:
where
is the local upslope area draining through a certain point per unit
contour length and
is the local
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is used ...
in
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. The unit was formerly an SI supplementary unit (before tha ...
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. The topographic wetness index is unitless.
Uses
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
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
,
vegetation
Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic charact ...
patterns, and forest site quality.
References
{{reflist
Topography
Hydrology