Hydrologic modeling begins with an ordinary DEM, which is used to
 model flow directions (equivalent to aspect)
 identify the basins that flow to different outlet points on the map edge
 calculate down-gradient flow accumulations
 infer stream networks from flow accumulations
 identify the sub-basins feeding individual segments of the stream network.
ArcToolbox includes a suite of raster hydrologic modeling tools to perform each of these
functions. The following steps illustrate principal steps in the hydrologic modeling process.
Start with a 30-meter resolution DEM covering the upper Nanticoke watershed in Sussex
County, Delaware. This is a particularly challenging watershed to analyze, since the terrain is
very flat.
Using the HILLSHADE tool in the Surface Analysis raster functions, you can create a hillshade
map and overlay the DEM with 50% transparency to get a better sense of the terrain to be
analyzed.
Most DEM’s include local sinks or depressions—cells completely surrounded by higherelevation cells--which interrupt the calculation of off-map flow directions. Use the FILL tool to
create a “filled” DEM with these sinks eliminated. Use the SINK tool (or just subtract the
source DEM from the filled DEM) to identify the sinks.
Use the FLOWDIRECTION tool on the filled DEM to calculate the direction of flow for each
cell. The flow direction map provides the input data for most of the watershed analysis and
stream inference tools discussed below.
Use the BASIN tool on the flow direction map to identify the basins that flow to each outlet
point on the map edge. Use the Identify button to determine the value of the particular basin you
want to analyze, and use the raster calculator to create a 0-1 raster map of just that one basin.
In the Spatial Analyst Options you can specify this single-basin map as a mask for further
analysis. To convert this basin to a vector polygon feature, select the basin cell record in the
raster attribute table and then use Spatial Analyst’s Convert—Raster-to-Features. You can
narrow the map extent in Spatial Analyst’s Options to cover just this basin. (Alternately, you
could simply multiply the flow direction map by the 0-1 single-basin map to mask out flow
direction values outside the basin.
Inferring the Stream Network
Use the FLOWLENGTH tool on the flow direction map to determine the total flow distance
from each cell to the outlet point of the basin.
Use the FLOWACCUMULATION tool on the flow direction map to calculate the number of
up-gradient cells that drain to each cell in the basin. You may be able to get a clearer idea of the
flow accumulation if you use the raster calculator to take the natural logarithm of the flow
accumulation map.
The inferred stream network can be derived from the flow accumulation map by using the Raster
Calculator to extract the cells that exceed some threshold accumulation value. This map shows
inferred principal streams where the natural logarithm of accumulation is 10 or greater. You can
include progressively smaller permanent or ephemeral streams by reducing the threshold. The
selected stream cells were then converted to polylines (using the Generalize Lines option).
Use the STREAMORDER tool on the stream raster map with the flow direction map to
calculate the order of each segment or link in the stream network. Each stream junction
combines two upstream tributary links. The links near the basin divide have no upstream
tributaries, and are designated as first-order. Proceeding downstream, each successive link can
be ordered as the sum of the orders of its two tributary links (Shreve method). Alternately, if
both its tributaries have the same order n, the link is order n+1; otherwise if the two tributaries
have orders m and n with n > m, the link is also order n (Strahler method)—this better
distinguishes principal flows in the stream network. This map shows polylines converted from
the Strahler method output.
Delineating Sub-Basins
Use the STREAMLINK tool on the stream raster map with the flow direction map to assign a
unique ID number to each stream segment.
The stream link map can be used as input pour point targets for the WATERSHED tool. The
WATERSHED tool uses the flow direction map and pour point targets to determine the subbasins within the basin that flow to each stream link. The sub-basins can be converted to
polygon features. To obtain more detailed sub-basin delineations, use a lower flow accumulation
threshold to extract a more detailed stream network, create a new stream link map (with many
more links), and rerun the WATERSHED tool.