Manually delineating a watershed by analyzing contour lines or interpreting topographic maps is a time-consuming process that requires advanced knowledge in hydrology and topography. This method involves identifying ridge lines and tracing the watershed boundaries, which can lead to approximate results depending on the analyst’s skill level.
To address this challenge, ArcGIS Pro provides a set of hydrology tools that enable the automatic and precise delineation of watersheds. These tools rely on digital elevation models (DEMs) to compute various hydrological parameters, such as flow direction and flow accumulation. Using these parameters, it is possible to build a drainage network and delineate watershed boundaries efficiently.
Automatically delimited watershed using ArcGIS Pro.
Step 1: Filling Depressions in the DEM
The first step involves using the “Fill” tool to correct imperfections or empty cells in the DEM. This step is important because depressions in the terrain can obstruct water flow paths. The “Fill” tool processes the DEM by eliminating pits and ensuring a continuous surface for flow analysis.
To use the tool, load the DEM as the input raster and specify the output location where the corrected raster will be saved. This tool is located at:
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Fill
Configuration of the “Fill” tool.
Step 2: Defining Flow Direction
After filling the DEM, the next step is to determine the direction in which water will flow across the terrain. The “Flow Direction” tool calculates the direction of flow for each cell in the raster based on elevation values. The most commonly used method is the D8 algorithm, which allows flow to one of eight neighboring cells.
To configure the tool, select the filled DEM as the input raster and choose the D8 method for calculating flow direction. This tool is accessed from:
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Flow Direction
Configuration of the “Flow Direction” tool.
Step 3: Flow Accumulation
To determine the accumulation of flow in each cell, the “Flow Accumulation” tool is used. This tool receives the flow direction raster as input and calculates the total amount of water flowing into each cell, highlighting the zones of higher accumulation within the watershed.
This tool can be found at:
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Flow Accumulation
Configuration of the “Flow Accumulation” tool.
Step 4: Creating a Drainage Network with the Con Tool
With the flow accumulation raster ready, a drainage network can be created using the “Con” tool. This tool classifies cells with accumulation values above a user-defined threshold, producing a binary raster representing the network.
Geoprocessing > Toolboxes > Spatial Analyst Tools > Conditional > Con
Configuration of the “Con” tool.
The tool is configured by selecting the accumulation raster as input and setting an expression such as VALUE > 5000. The result will depend on the raster resolution and the value used. A value of 1 is assigned to the cells that meet the condition.
Using different expressions in the “Con” tool.
Step 5: Linking Streams with the Stream Link Tool
The “Stream Link” tool connects the segments of the drainage network to form continuous flow paths. It requires the conditional raster and the flow direction raster as inputs.
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Stream Link
Configuration of the “Stream Link” tool.
Step 6: Assigning Stream Order
The “Stream Order” tool assigns a hierarchy to each stream segment. It can use either the Strahler or Shreve method, depending on the type of analysis required.
It uses the rasters from the “Stream Link” and “Flow Direction” tools as inputs.
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Stream Order
Configuration of the “Stream Order” tool.
Step 7: Converting Raster Streams to Vectors
The raster drainage network can be converted into vector format using the “Stream to Feature” tool. This step simplifies the visualization and further analysis.
Required inputs are the stream order raster and the flow direction raster.
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Stream to Feature
Configuration of the “Stream to Feature” tool.
Step 8: Assigning the Watershed Outlet
To assign the discharge point of the watershed, the “Snap Pour Point” tool is used. A manually digitized point is required and must be placed directly over the outlet.
Inputs include the accumulation raster and the pour point feature. The snap distance should be zero if the point lies exactly on the stream.
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Snap Pour Point
Configuration of the “Snap Pour Point” tool.
Step 9: Delineating the Watershed
The “Watershed” tool is used to delineate the watershed area. It requires the flow direction raster and the snapped pour point raster as inputs. This tool outlines the watershed boundaries and extracts useful hydrological metrics.
Geoprocessing > Toolboxes > Spatial Analyst Tools > Hydrology > Watershed
Configuration of the “Watershed” tool.
Step 10: Converting Watershed Raster to Polygon
To work with vector data, the watershed raster must be converted to a polygon using the “Raster to Polygon” tool. This step is essential for high-quality mapping and geoprocessing.
Geoprocessing > Toolboxes > Conversion Tools > From Raster > Raster to Polygon
Step 11: Generating Contour Lines and Final Map Enhancements
Contour lines can be generated from the DEM using the “Contour” tool. To refine the output, use the “Clip” tool to limit both the contour lines and stream network to the watershed boundaries. For improved visualization, apply a hillshade layer and appropriate labeling.
These final steps help produce a complete and informative map of the watershed area.
Automatically delineated watershed along with its water network.
