Geography Reference
In-Depth Information
each given cell by the value in the geographically equivalent cell in the weight raster. The weight raster
provides the cost or impedance for water to flow through each cell. Thus, you could simulate the fact that
water flowing through forested land takes longer to cover a given distance than water flowing over rock.
You can use the output of the Flow Length tool to find the length of the longest flow path in a given
basin. This is one of the values needed to calculate a more sophisticated hydrologic quantity, “time
of concentration” for a basin. You can use flow length rasters to create distance-area diagrams of
hypothetical rainfall/runoff events, using the optional weight raster as an impediment to downslope
movement.
Assigning Identities to Streams
The most basic hydrologic unit (outside of the individual cell) is the stream segment. Generally, streams
segments (also called links) run between intersections in the linear network. A segment consists of all the
cells between the junctions of two or more streams or between junctions and the pour points. (The cell
that is the junction is considered to belong to one of the streams.) Figure 8-32 illustrates this numbering.
The integer number is only a nominal, identifying value.
On a raster, ArcGIS places the same unique identifying number in all the cells of a given stream segment.
In the discussion of the Flow Direction and Flow Accumulation calculations, every cell was considered a
contributor to the creeks, streams, and rivers that developed (“Into each cell some rain must fall.”). But
you do not want to define all the cells in the study area as part of the water network. Instead, the software
will delineate specific stream channels, running from intersection to intersection. In other words, all of
the study area contributes to the total amount of water to be dealt with, but only a small part of the study
area carries most of that water. That area is known variously as the water network or the stream channels.
This area is defined by including only those cells with flow accumulations greater than a chosen value;
that value is called the cell threshold. Figure 8-33, an illustration of how stream segments are numbered,
3
5
6
4
1
7
8
11
2
10
9
12
13
Stream linking
FIGURE 8-32 Numbered stream links
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