Geography Reference
In-Depth Information
Basic Surface Hydrology Concepts
The concepts are described in terms of the tools you can use to examine surface hydrology.
The FlowDirection calculation determines the direction of flow from each cell of a surface raster. The
raster generated by FlowDirection must be well behaved. The sort of analysis I am describing specifi-
cally excludes land areas that contain lakes or ponds. The assumption is that all the water placed on
the raster will ultimately exit the raster at one or more low points on its edge.
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Assuming that the study area involved does not contain lakes or ponds, one of the ways the raster
can be ill behaved is to contain a cell that is lower than its surrounding neighbors; such a cell is
called a sink. Sinks distort the analysis; to find them, you will use the Sink calculation. (Editing ras-
ters with sinks is beyond the scope of this lesson.)
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Another requirement of the raster is that the cells of primary interest—for example, the mouth of a
river near a town that might flood—must include all the “uphill” cells. That is, all the cells that con-
stitute the drainage basin for the cells of interest must be considered. The FlowAccumulation calcula-
tion may be configured to compute the amount of water that flows into each cell from all of the cells
that are uphill from it.
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Stream networks are characterized by small creeks flowing into larger ones, these flowing into small
streams, and so on. It is useful to speak of the “order,” or relative size, of such water entities. The
smallest creeks are labeled order 1. Larger entities have larger integer numbers. The StreamOrder
calculation handles the process of assigning order numbers to streams. Both of the two principle
methods for numbering streams (Strahler and Shreve) are available.
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The Mississippi River has a watershed consisting of all the land that supplies water to it. The small-
est creek also has a watershed that consists of all the land that supplies water to it. The creek's water-
shed may be contained in the Mississippi's watershed, so the delineation of watersheds (or drainage
basins, catchment areas, and contributing areas, as they are also called) is not trivial, either in con-
cept or calculation. The WaterShed calculation assigns cells to such areas.
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In addition to these calculations, an important operation that precedes surface hydrology analysis is the
generation of a surface raster that gives the elevation at every cell. There are several ways to do this, as
previously discussed.
Calculating Flow Direction
The primary data source for hydrologic operations in ArcGIS is a raster of flow direction. This raster is
formed by the FlowDirection calculation based on a surface of elevation; for our discussion here, we will
call the resulting raster “DirOfFlow.” Each cell in the DirOfFlow raster contains an integer number; these
numbers are powers of 2: 1, 2, 4, 8, 16, 32, 64, and 128. (Just why these numbers were chosen, rather than
1, 2, 3, etc., has a historical and computer component, which will be discussed.) Each number indicates a
direction, as shown by Figure 8-21.
The idea is, simply, that the precipitation that falls, or otherwise appears, on a given cell flows
immediately to a single adjacent cell. To which of the eight adjacent cells? The one indicated by the
number and the arrow in Figure 8-21, which points in the direction of the steepest descending slope.
For example, consider the simple raster shown in Figure 8-22. The numbers in the cells indicate elevation.
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