Compound Topographic Index and Specific Catchment Area for the Alaska Perhumid Coastal Temperate Rainforest

These files include a derived 50 meter spatial resolution Compound Topographic (or Wetness) Index (CTI or TWI) and Flow Accumulation (as represented by specific catchment area, SCA) calculated from a continuous, transboundary DEM developed across the Alaska perhumid coastal temperate rainforest (AKPCTR). The extent of this dataset includes all of the Alaska and Canada watersheds that discharge into southeast Alaska coastal waters, which covers essentially the northern half of the full PCTR.

The transboundary DEM used to calculate the CTI can be here: link

Flow accumulation is represented by a grid of specific catchment area (SCA), which is the contributing area per unit contour length using the multiple flow direction D-infinity approach. Unit contour length is equal to the DEM resolution of 50 meters.

There are two versions of both the CTI and SCA provided:

CTI_AKPCTR_NoFlowMask.tif and SCA_AKPCTR_NoFlowMask.tif
This is the resulting CTI file when no masks are applied to the study area before the CTI procedure is run. After the CTI procedure is run, all glacial areas are masked out, as CTI is not meaningful over glaciers.

CTI_AKPCTR_FlowMask.tif and SCA_AKPCTR_FlowMask.tif
This is the resulting CTI file where prior to running the CTI procedure, we apply a mask across all active glaciers and all downslope cells receiving flow from glacially affected cells. These masked cells are excluded from the CTI procedure. This results in fewer CTI values on the landscape. This dataset is provided to identify cells in the direct downslope path of glaciers because CTI values for cells receiving upslope accumulation from glaciers may not be reliable due to uncertainties in surface water flowpaths in glaciated areas.

Processing Steps:

  1. Using the pitremove function from TAUDEM, filled sinks in the DEM following the method of Planchon and Darboux (2001).

  2. (For *_FlowMask versions only) Mask out glacial areas and associated downslope cells. Prior to calculating flow direction and flow accumulation, change cells in the filled DEM to NoData where glaciers exist, using the Randolph Glacier Inventory version 5.0 raster dataset to identify the presence of glaciers. Cells downslope from the masked glaciers will then also be identified as NoData in subsequent processing and the final CTI raster. Compute D-infinity slopes and flow direction and flow accumulation (as specific catchment area) rasters from the filled DEM using TAUDEM D-infinity method. As part of the D-infinity routine, TauDEM uses the method of Garbrecht & Martz (1997) to resolve flats.

  3. The D-infinity slope raster was modified in the following way: if slope = 0, then change slope value to 0.0001; otherwise leave slope value unchanged. This was done to avoid dividing by zero when calculating CTI.

  4. Compute Compound Topographic Index (CTI) using these steps:

    Dsca = D-infinity specific catchment area raster
    Dslp = D-infinity slope raster

    CTI = Ln(Dsca/Dslp)

  5. Clip the CTI raster to the Alaska perhumid coastal temperate rainforest (AKPCTR) watershed boundary.

Data and Resources

Additional Info

Spatial resolution

50 meters


The following people worked on producing this data set:

  • Julien Schroder
  • Frances Biles
Field Value
Maintainer Tom Kurkowski
Version 1.0.0
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