Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS - page 476

One conjecture is that, in the early days of hydrologic analysis, which correspond to the early days of
computers, central processing unit speeds were slow and storage space in memory was at a premium.

It was efficient to indicate direction with a single bit (a 1 or 0) in a position in a computer byte. Those
positions correspond to columns in the base 2 number system. Those columns are designated 1, 2, 4, 8,
and so on. Eight bits in a byte; eight neighbors for each cell. It may be that the precedent set in the early
days endures in the hydrologic modeling field today.

Flow Accumulation: Drainage Delineation and
Rainfall Volume

Once you have a raster that indicates flow direction, a number of other interesting and useful calculations
are possible. In particular, you can determine the locations of all the linear bodies of water, and you can
determine, from slope and elevation, those areas where water may accumulate during times of intense
precipitation. This is accomplished with the ArcToolbox Flow Accumulation tool.

Basically, the value in each cell in the resulting raster contains the sum of the amount of water that has
fallen on all the raster cells upstream from it. The intent is to simulate the flow, or potential flow, of water
to form creeks, streams, and rivers. If each cell is presumed to have one unit of water (say, an inch in
depth) to contribute—under a condition of “uniform rainfall,” you can think of the number in a given cell
as the number of cells upstream from that cell. To illustrate, examine the elevation surface in Figure 8-25.
Note that the low points are in the middle of the south edge (elevation 1) and the west edge (elevation 3).
All around the rest of the raster the elevations are 9 or somewhat less.

From this, you can produce a raster showing the direction of flow, using the ArcToolbox Flow Direction
tool. Some arrows have been scattered on the raster to show flow direction. See Figure 8-26.

Now, applying the Flow Accumulation tool to the flow direction raster produces a raster that shows,
for each cell, the water that accumulates due to adding up the accumulations from the cells “above” it.
Figure 8-27 depicts some of these accumulation values.

FIGURE 8-25 Elevations
of a raster

FIGURE 8-26 Some
directions of flow, given
the elevations of the
raster of Figure 8-25