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in the grid. Based on the flow accumulation grid, a raster-based stream network can be developed based on a user-defined cell threshold (Olivera et al., 2000).

Threshold, a term used to describe the number of cells that drain to a point, also defines the number of cells that constitute the beginning of a stream. The selected threshold creates a drainage (stream) network based on all the cells with a catchment area greater than the threshold. The choice of threshold is complicated. Two methods for determining a threshold are the constant area threshold method and the slope-dependent critical support area method.

The constant threshold area method represents the change in sediment transport from sheet flow to concentrated flow, rather than a spatial transition in

longitudinal slope profiles. method the drainage density

Using the slope-dependent critical is greater in steeper portions of the

support area catchment, as

found in natural landscapes (Tarboton et al., 1991).

The constant threshold area

method critical

is considered more support area method,

practical and is the

in application than the only method considered

slope-dependent in this report. In

this study, both CRWR-PrePro (Garbrecht et al., 1999)

and

WMS

use

the

constant

threshold

area

method.

Once the DEM has been processed by filling pits, flow direction and flow accumulation grids have been computed and a threshold chosen, the user may add streams and watershed outlets to the model. The final step for DEM processing is to vectorize the streams and watersheds. While a grid is convenient for the development of hydrologic data, it is inefficient for data storage. This is because with a grid there is a one-to-many relationship for most basin parameters. A more

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