Three Factors Influencing Watershed Analysis: DEM Resolution and Flow Accumulation Threshold and Direction Method (Especially for GATE-Geospatial 2022)

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Factors influencing the outcome to watershed analysis including a stream network and watersheds can be influenced by DEM resolution, flow direction method, and flow accumulation threshold. The choice of algorithm is another important factor in watershed analysis output.

Diagram Shows Three Factors Influencing Watershed Analysis

DEM Resolution

A higher-resolution DEM can better define the topological features and produce a more detailed stream network than a lower- resolution DEM. This is illustrated in both figures, comparing a 30-meter DEM with a 10-meter DEM. In a study involving field verification, Murphy et al. (2008) report that a 1-meter LiDAR DEM can predict more first-order streams and produce flow channels that extend further upslope into the landscape than a 10-meter DEM. A 30-meter DEM likely to be too coarse to provide detailed topographic features for geomorphic and hydrologic modeling

Image Shows 30-Meter and 10-Meter Resoultion at DEMs
Image Shows Stream Networks Derived from the DEMs

Flow Direction Method

Commercial GIS package, including ArcGIS, use the D8 method mainly because it is simple and can produce good results in mountains topography with convergent flows. But it tends to produce flow in parallel lines along principal directions. And it cannot represent adequately divergent flow over convex slopes and ridges and does not do well in highly variable topography with floodplains and wetlands. As an example, figure shows that the D8 method performs well in well-defined valleys but poorly in relatively flat areas.

Picture Shows Flow Direction Method Using D8 Method

In figure, the gray raster lines represent stream segments derived using the D8 method. The thin black lines are stream from the 1: 24,00 – scale DLG. The two types of lines correspond well in well-defined valleys but poorly on the bottomlands.

Flow Accumulation Threshold

Given the same flow accumulation raster, a higher threshold value will result in a less dense stream network and fewer internal watersheds than a lower threshold value. Figure illustrates the effect of the threshold value. Given figure-a shows the flow accumulation raster, figure-b the stream network based on a threshold of 500 cells, and figure-c the stream network based on 100 cells. Ideally, the resulting stream network from a threshold value should correspond to a network obtained from traditional methods such as from high-resolution topographic maps or field mapping.

Picture Shows Flow Accumulation Threshold

In figure, (a) A flow accumulation raster; (b) a stream network based on a threshold value of 500 cells; and (c) a stream network based on a threshold value of 200 cells.

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