Lucas Rabins

Hydrology Toolset Model Manual

The hydrology toolset available in AcrGIS is a set of tools built for modeling the flow and accumulation of

water across a surface. Modeling water flow and accumulation is a valuable tool used in a wide variety

of fields such as agriculture, city planning, and land management. Once stream networks and

watersheds are modeled, they can be used in conjunction with the geometric network feature available

in the ArcGIS feature dataset, so simulate flow direction and connectivity. To Use this toolset efficiently,

I have created a model which combines many of the tools in this toolbox. This manual will demonstrate

how to model hydrologic features using this model to combining many of the tools in the hydrology

toolset. For my demonstration purposes I will be modeling the South Fork watershed of the Nooksack

River in Western Washington. Figures 3-10 describe specific outputs and inputs for this model.

Figure 1. Model used for combining many parts of the Hydrology toolset to model stream flow for an individual watershed.

Data Preprocessing and Inputs

Before using the hydrology toolset model, you must first clip all of your data to the watershed of

interest and project all your data to the same projection. The only inputs you will need to run this model

are a DEM of your watershed and a point file of intersections of your two highest order streams. The

latter can be obtained by an outside source or by manually creating the points in an editing session

using hydrology data. These two files are listed as DEM and Outlet on the dialog box of the model.

(Figure 2.)

Figure 2. Dialogue Box for the Hydrology Model highlighting the inputs required to run the model in red. All other file

names not outlined in red specify output locations.

Filling sinks and calculating flow Direction

Figure 3. Flow direction. The first two tools in the model are used to create a raster depicting the flow direction of water

from each cell to its closest downstream neighbor (Figure 3). In order to create this, we must first use the fill tool to fill in

sinks in the DEM that would not allow the model to run correctly. The output of these two tools are DEM fill and Flow

direction. Each of the 8 colors in the image represents a different direction of flow in the watershed creating the

appearance of aspect.

Flow Accumulation

Figure 4. Flow Accumulation. The flow accumulation tool uses the flow direction tool as an input and creates a raster layer

with cell values representing the total cells that flow into a given cell. The resulting higher cell values represent higher

order streams, giving a detailed representation of the stream networks in the watershed. Conversely the lower cell values

represent ridged and high points within your watershed.

Stream Segments

Figure 5. Stream Segments. To convert the output of the flow accumulation raster to the STREAMS binary stream segment

raster you need to decide the cutoff flow accumulation cell value that will be classified as a stream. For this exercise I set my

cutoff value at 900. This is done using the raster calculator to reclassify everything below your cutoff value as NULL and

everything above your cutoff value as a value of 1 using the following expression: SetNull(" Flow Accumulation Raster" < (YOUR

CUTOFF VALUE, 1).

Stream Order

Figure 6. Stream Order. Using the STREAM and flow direction rasters as inputs, this tool numbers the streams according to

stream orders 1-5. The higher order a stream is; the more streams eventually flow into it. For example, a third order stream in

this watershed represents the water flow downstream of the junction of two second order streams.

Flow Length

Figure 7. Flow Length. Using the Flow direction raster as an input, this tool creates a raster by calculating the total downstream

distance following the flow direction path specified in by the flow direction. The raster cells with the shortest distance are those

near the outlet/sink for the whole watershed which I have symbolized in green.

Pour Points and The Stream Network.

Figure 8. Pour Points, and Watersheds. The Snap Pour Point tool uses the flow accumulation raster to specify a cell with the

greatest flow accumulation within a specified distance to a defined point. By using the outlets file as these defined points, this

tool creates a raster layer with cells defining pour points for each 4th order watershed. These cells are depicted as point markers

even though they are created as single raster cells. We can then use the watershed tool with these pour points and the flow

direction raster to define these watersheds in the watersheds output. Colored polygons represent sub watershed delineation.

Total Watershed Area: 183 Miles

Catchments

Figure 9. Catchments. Similar to the previous step in which we defined sub watersheds for each 4th order stream, the

catchments file created from the watershed tool creates sub watersheds for each 1st order stream. The flow direction input for

this watershed is the same as the previous figure, however the pour point input for his file the output of the Stream link tool,

which created a raster cell for each intersection of two waterways. Each separate polygon represents a sub watershed. Streams

are shown in blue for context.

Geometric Network

Figure 10. Geometric Networks. Once all tools in the model have been run, you can use your streams layer and outlets layer to

create a geometric network for your streams. By setting your outlet as the sink for all of your watershed, you can simulate the

flow direction of all the streams in your watershed!