Tool Name	Index of Connectivity
Tool ID	34
Library ID	ta_hydrology
Version	1.0
Author(s)	V. Wichmann (c) 2026

Description

The tool allows one to calculate the 'Index of Connectivity' (IC) as first proposed by Borselli et al. (2008) and later adapted by Cavalli et al. (2012). The IC can be used to assess the spatial sediment connectivity in a catchment, i.e. how efficiently hillslopes and channels (or other sinks) are coupled. The index represents the potential connectivity between the different parts of a watershed. For each cell the upslope (D_up) and downslope (D_dn) components of connectivity are evaluated:
IC = log10(D_up / D_dn)
Please refer to the original publications for further details and formulas.
When no weights and slope grids are provided, the tool calculates the IC as proposed by Cavalli et al. (2012): weights are estimated from local surface roughness and are limited to the range [0.001;1], local slope is estimated as the maximum slope (D8) and limited to the range [0.005 m/m;1 m/m].
The provided flow routing options are those of the 'Flow Accumulation (Top-Down)' tool (for references see there).

References

Borselli, L., Cassi, P. & Torri, D. (2008): Prolegomena to sediment and flow connectivity in the landscape: A GIS and field numerical assessment. Catena, 75, 3: 268-277. doi:10.1016/j.catena.2008.07.006

Cavalli, M., Trevisani, S., Comiti, F.. & Marchi, L. (2012): Geomorphometric assessment of spatial sediment connectivity in small Alpine catchments. Geomorphology, 188: 31-41. doi:10.1016/j.geomorph.2012.05.007

Parameters

Name	Type	Identifier	Description	Constraints
Input
Elevation	grid, input	ELEVATION	The digital terrain model (DTM). Usually a hydologically sound DTM is used.
Sinks	grid, input, optional	SINKS	A grid indicating sink (terminal) cells, e.g. the channel network. Cells with valid values (i.e. not NoData) act as sinks that capture all incoming flow and sediment and do not route anything downslope.
Weights	grid, input, optional	WEIGHTS	Pre-computed weights [-], greater than zero. If not given, weights are estimated from local surface roughness within the specified kernel size.
Slope	grid, input, optional	SLOPE	Pre-computed slope [m/m], greater than zero. If not given, slope is estimated as the maximum slope (D8).
Output
Index of Connectivity	grid, output	IC	The index of connectivity.
Upslope Component	grid, output, optional	UPSLOPE	The upslope component.
Downslope Component	grid, output, optional	DOWNSLOPE	The downslope component.
Options
Radius Smoothing	integer number	KERNEL_RADIUS	The kernel radius [cells] used for DTM smoothing in local surface roughness / weighting computation.	Minimum: 1 Default: 2
Flow Routing	choice	FLOW_ROUTING	Choose the flow routing method.	Available Choices: [0] Deterministic 8 [1] Rho 8 [2] Braunschweiger Reliefmodell [3] Deterministic Infinity [4] Multiple Flow Direction [5] Multiple Triangular Flow Direction [6] Multiple Maximum Downslope Gradient Based Flow Direction Default: 4
Thresholded Linear Flow	boolean	LINEAR_FLOW	Apply linear flow routing (D8) to all cells that have a flow accumulation greater than the specified threshold.	Default: 0
Linear Flow Threshold	integer number	LINEAR_MIN	The flow accumulation threshold [cells] for linear flow routing.	Minimum: 0 Default: 500
Convergence	floating point number	CONVERGENCE	The convergence factor for the 'Multiple Flow Direction' and 'Multiple Triangular Flow Direction' algorithms.	Minimum: 0.001000 Default: 1.100000
Contour Length	boolean	MFD_CONTOUR	Include a (pseudo) contour length as additional weighting factor in multiple flow direction routing, reduces flow to diagonal neighbour cells by a factor of 0.71 (see Quinn et al. 1991 for details).	Default: 0