Hydrology toolbox

1   Simple toolbox first

Let’s automate the following hydrology analyses:

• Flow accumulation
• Stream network delineation
• Watershed delineation

Download sample data: nc_spm_08_grass7_exercise.zip

2   How to define parameters

Use arcpy.Parameter and data types.

All parameters in arcpy.Parameter have a default value, but it doesn’t really make sense to omit the first five parameters*.

arcpy.Parameter(
    name=,          #*Parameter name with no spaces (?)
    displayName=,   #*Parameter label for the user interface
    direction=      #*Input, Output
    datatype=,      #*Data type
    parameterType=, #*Required, Optional, Derived (always output with no user value; use SetParameterAsText to define its output)
    enabled=,       # False to make it unavailable (optional, None by default)
    category=,      # Parameter category (optional, None by default)
    symbology=,     # Path to a layer file for drawing the output (optional, None by default)
    multiValue=,    # True if the parameter has multiple values (optional, None by default)
)

Oops! That trailing comma after multiValue=? Don’t worry. Python can handle that for you.

3   Data types for today’s exercise

There are a lot of different data types, but for now, let’s focus on text, numbers, raster and feature layers.

4   Syntax for geoprocessing tools

We will combine built-in geoprocessing tools, but how do we know how to invoke those tools programmatically?

• Fill
• FlowDirection
• FlowAccumulation
• Raster Calculator’s Con
• RasterToPolyline
• Watershed
• RasterToPolygon

5   Flow accumulation

    def getParameterInfo(self):
        elev = arcpy.Parameter(
                datatype="GPRasterLayer",
        fill = arcpy.Parameter(
                datatype="DERasterDataset",
        fdir = arcpy.Parameter(
                datatype="DERasterDataset",
        facc = arcpy.Parameter(
                datatype="DERasterDataset",

    def execute(self, parameters, messages):
        outFill = arcpy.sa.Fill(elev)
        outFill.save(fill)

        outFlowDirection = arcpy.sa.FlowDirection(fill)
        outFlowDirection.save(fdir)

        outFlowAccumulation = arcpy.sa.FlowAccumulation(fdir)
        outFlowAccumulation.save(facc)

6   Stream network delineation

    def getParameterInfo(self):
        facc = arcpy.Parameter(
                datatype="GPRasterLayer",
        thresh = arcpy.Parameter(
                datatype="GPLong",
        streamRast = arcpy.Parameter(
                datatype="DERasterDataset",
        streamVect = arcpy.Parameter(
                datatype="DEFeatureClass",

    def execute(self, parameters, messages):
        outStreamRast = arcpy.sa.Con(arcpy.Raster(facc) >= thresh, 1, 0)
        outStreamRast.save(streamRast)

        arcpy.RasterToPolyline_conversion(streamRast, streamVect)

        prj = arcpy.mp.ArcGISProject("CURRENT")
        m = prj.activeMap
        m.addDataFromPath(streamVect)

7   Watershed delineation

    def getParameterInfo(self):
        fdir = arcpy.Parameter(
                datatype="GPRasterLayer",
        outlets = arcpy.Parameter(
                datatype="GPFeatureRecordSetLayer",
        watershedRast = arcpy.Parameter(
                datatype="DERasterDataset",
        watershedVect = arcpy.Parameter(
                datatype="DEFeatureClass",

    def execute(self, parameters, messages):
        arcpy.env.extent = "MAXOF"
        outletsOidFieldName = arcpy.Describe(outlets).OIDFieldName
        outWatershed = arcpy.sa.Watershed(fdir, outlets, outletsOidFieldName)
        outWatershed.save(watershedRast)

        arcpy.RasterToPolygon_conversion(watershedRast, watershedVect)

        prj = arcpy.mp.ArcGISProject("CURRENT")
        m = prj.activeMap
        m.addDataFromPath(watershedVect)

8   Homework: Longest flow path

Develop a tool that calculates the longest flow path in raster and add it to the Hydrology toolbox.

Hints:

1. Run the Flow Length tool with the Upstream direction
2. Run the Flow Length tool with the Downstream direction
3. Add outputs from steps 1 and 2