ian.anderson
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Description
This wetness indicator (WI) is a means of measuring drainage or drainage potential in a landscape, identifying areas where water will either pool or runoff will slow significantly. The process identifies where elevation changes are less than surrounding areas and where flow routes may be somewhat inhibited. WI areas are exclusive of permanent water bodies and wetlands but may provide significant storage and contribution to these areas. This model should not be confused with the Total Wetness Index (TWI) in that it is predicated on deriving two variables from the DEM – Elevation variation and Aspect variation.
The model was tested with DEMs ranging from 6m to 100m in spatial resolution in gently rolling (eastern Alabama) and mountainous (Grand Canyon, AZ) terrains and with a DSM with a spatial resolution of 6". The DSM was over a re-forested area and demonstrated that the model was not applicable to this data set. Tests on bare earth DEMs indicated that the model could detect collection spots and areas that are more likely to hold moisture. The model overestimated Most Probable Wet Areas at 6 meter resolution and under estimated Most Probable Wet Areas at 100 m resolution.
| Bare Earth DEM | Results at 6 m resolution |
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| Results at 10 m resolution | Results at 30 m resolution |
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| Results at 60 m resolution | Results at 100 m resolution |
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| TerrainWetnessIndicator-v15-1-5.gmdx |
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Assumptions and Data Requirements:
- Bare earth elevation model (DEM) as opposed to a Digital Surface Model (DSM);
- Medium to coarse DEM spatial resolution, i.e., roughly between 6 and 60 meters (20 to 200 feet). Finer resolution demonstrates too much noise that is misinterpreted;
- NoData values are correctly represented in the data file. Values used to represent NoData in Floating point format may (and often do) vary from pixel to pixel. Therefore it is encumbant upon the user to redefine these values to a constant, for example -99999.0, so that the processor correctly identifies NoData values;
- X and Y cell dimensions are equal, i.e., square pixels. Longitudinal ground measurements vary as one proceeds away from the equator. This is illustrated by longitudinal lines of convergence as one approaches the poles. All geographic coordinate system graticules are trapezoids.
- X, Y, and Z units are equal, i.e., a projected coordinate system. If the projection units are expressed as meters, then the elevation units must also be expressed in meters.
