An important part of the classic version of TOPMODEL is the topographic index given by
Ln (a / tan β) where a is the upslope area per
unit contour length and tan β is the surface slope.
(e.g. Lane et al., 2004 for
a modification).
Recently, Ambroise et al. (1996) and others have proposed generalisations of this to deal with different catchment characteristics (e.g. parabolic and linear transmissivity profiles).
Ambroise
B., Beven K., Freer J. 1996. Toward a generalization of the TOPMODEL
concepts: Topographic indices of hydrological similarity, Water
Resources Research 32, 2135-2145.
Lane
SN, Brookes CJ, Kirkby MJ, Holden J. 2004. A network-index based version
of TOPMODEL for use with high-resolution digital topographic data.
Hydrological Processes 18, 191-201.