Topographic Wetness Index derived from 1" SRTM DEM-H

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Topographic Wetness Index derived from 1" SRTM DEM-H

Topographic Wetness Index (TWI) is calculated as log_e(specific catchment area / slope) and estimates the relative wetness within a catchment. The TWI product was derived from the partial contributing area product (CA_MFD_PARTIAL), which was computed from the Hydrologically enforced Digital Elevation Model (DEM-H; ANZCW0703014615), and from the pe... more

Environmental Management Land Capability and Soil Degradation Landscape Ecology Natural Resource Management Soil Sciences not elsewhere classified

11 Feb 2000

22 Feb 2000

CSIRO Enquiries
1300 363 400

Topographic Wetness Index LAND Topography Models ECOLOGY Landscape TERN_Soils Land Surface Australia

Source data 1. 1 arcsecond resolution partial contributing area derived from the DEM-H (ANZCW0703014615). 2. 1 arcsecond resolution slope percent derived from DEM-S (ANZCW0703014016) 3. 3 arcsecond resolution SRTM water body and ocean mask datasets TWI calculation TWI was calculated from DEM-H following the methods described in Gallant and Wilson (2000). The program uses a slope-weighted multiple flow algorithm for flow accumulation, but uses the flow directions derived from the interpolation (ANUDEM) where they exist. In this case, they are the ANUDEM-derived flow directions only on the enforced stream lines, so the flow accumulation will follow the streams. The different spacing in the E-W and N-S directions due to the geographic projection of the data was accounted for by using the actual spacing in metres of the grid points calculated from the latitude. Contributing area was converted to specific catchment area using the square root of cell area as the best estimate of cell width on the approximately rectangular cells. The contributing area value was also reduced by half of one grid cell to provide better estimates at tops of hills. Slope was converted from percent to ratio, as required by the TWI calculation, by dividing by 100. A minimum slope of 0.1% was imposed to prevent division by zero. The TWI calculation was performed on 1° x 1° tiles, with overlaps to ensure correct values at tile edges. The 3 arcsecond resolution version was generated from the 1 arcsecond TWI product. This was done by aggregating the 1” data over a 3 x 3 grid cell window and taking the mean of the nine values that contributed to each 3” output grid cell. The 3” TWI data were then masked using the SRTM 3” ocean and water body datasets. Note that the limitation of partial contributing area due to tiled processing, so that catchment areas extending beyond about 5 km from a tile edge are not captured, has little impact on topographic wetness index. TWI is useful as a measure of position in the landscape on hillslopes (not river channels) and all hillslope areas will be accurately represented by the partial contributing area calculations. Some typical values for TWI in different positions on the landscape are: Position Specific catch. Slope (%) TWI area (m) Upper slope 50 20 5.5 Mid slope 150 10 7.3 Convergent lower 3000 3 11.5 slope In channels, some typical values would be (using flow width of 30 m): Contributing Specific catch. Slope (%) TWI area (km2) area (103 m) 1 33 1 15.0 25 833 0.5 18.9 1000 33,333 0.1 24.2 Values of TWI larger than about 12 are most likely in channels or extremely flat areas where the physical concepts behind TWI are invalid and probably are not useful for measuring relative wetness, topographic position or any other geomorphic property. Contributing area (for channels) and MrVBF are more likely to be useful indicators of geomorphic properties in these areas. See, for example, McKenzie, Gallant and Gregory (2003) where soil depth is estimated using TWI on hillslopes and MrVBF in flat valley floors: the range of validity for TWI in that example was approximately 4.8 to somewhat beyond 8.5. Hence the omission of contributing areas larger than about 25 km2 has no effect on the practical applications of TWI. Gallant, J.C. and Wilson, J.P. (2000) Primary topographic attributes, chapter 3 in Wilson, J.P. and Gallant, J.C. Terrain Analysis: Principles and Applications, John Wiley and Sons, New York. McKenzie, N.J., Gallant, J.C. and Gregory, L. (2003) Estimating water storage capacities in soil at catchment scales. Cooperative Research Centre for Catchment Hydrology Technical Report 03/3.

Access to this data has been made possible by the Terrestrial Ecosystem Research Network (TERN), supported by the Australian Government through the National Collaborative Research Infrastructure Strategy and the Super Science Initiative.

Creative Commons Attribution 4.0 International Licence

CSIRO (Australia)

Gallant, John; Austin, Jenet (2012): Topographic Wetness Index derived from 1" SRTM DEM-H. v2. CSIRO. Data Collection.

All Rights (including copyright) CSIRO 2012.

The metadata and files (if any) are available to the public.

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Location Details

10°0′0″ S

44°0′0″ S

154°0′0″ E

113°0′0″ E


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John Gallant

Terrain Analysis Research Team Leader

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About this Project

1181.2 TERN Facility No9 InfoGrid GRUNDY

The Soil and Landscape Grid of Australia is a comprehensive fine spatial resolution grid of functional soil attributes and key landscape features across Australia. The landscape attributes are derived from the data collected by the Shuttle Radar Topography Mission, whilst the soil attribute surfaces are modelled from existing soils information. The... more

John Gallant

National Elevation and Terrain Datasets

SRTM-derived elevation and terrain covariate datasets at 1 second or 3 second resolution


John Gallant

Jenet Austin

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