Index of Connectivity
- Author: V. Wichmann (c) 2026
- Menu: Terrain Analysis | Hydrology | Topographic Indices
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 | Grid System | grid system | PARAMETERS_GRID_SYSTEM | - | - |
| 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 |
Command Line
Usage: saga_cmd ta_hydrology 34 [-ELEVATION ] [-SINKS ] [-WEIGHTS ] [-SLOPE ] [-IC ] [-UPSLOPE ] [-DOWNSLOPE ] [-KERNEL_RADIUS ] [-FLOW_ROUTING ] [-LINEAR_FLOW ] [-LINEAR_MIN ] [-CONVERGENCE ] [-MFD_CONTOUR ]
-ELEVATION: Elevation
grid, input
-SINKS: Sinks
grid, input, optional
-WEIGHTS: Weights
grid, input, optional
-SLOPE: Slope
grid, input, optional
-IC: Index of Connectivity
grid, output
-UPSLOPE: Upslope Component
grid, output, optional
-DOWNSLOPE: Downslope Component
grid, output, optional
-KERNEL_RADIUS: Radius Smoothing
integer number
Minimum: 1
Default: 2
-FLOW_ROUTING:
Flow Routing
choice
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
-LINEAR_FLOW: Thresholded Linear Flow
boolean
Default: 0
-LINEAR_MIN: Linear Flow Threshold
integer number
Minimum: 0
Default: 500
-CONVERGENCE: Convergence
floating point number
Minimum: 0.001000
Default: 1.100000
-MFD_CONTOUR: Contour Length
boolean
Default: 0