QGIS Parcel Watershed Delineation - How to use

1. Load Your Data.

- Add your DEM (GeoTIFF) to QGIS
- Add your parcel shapefile to QGIS

2. Parameters.

- Digital Elevation Models (DEMs): Select one or MORE DEM raster layers. DEMs will be automatically mosaicked/merged

- Parcel layer (uses selected features if any). If you have features selected, you can limit the watershed delineation to selected parcels only.
If nothing is selected, all features in the layer will be used.

- Number of boundary points: (default: 24). Higher = more accurate but slower

- Simplify watershed boundaries: ✓ Checked (recommended). Smooths jagged edges

- Watershed polygons: Choose output location (or use temporary layer)

3. Run command.

The tool will:

a. Merge multiple DEMs if provided
b. Reproject parcels if needed (the DEM CRS will be used)
c. Condition the DEM
d. Calculate flow direction and accumulation (the D8 method is chosen by default)
e. Delineate the watershed for the entire area
f. Output a single watershed polygon

4. Output attributes

The output shapefile includes:

[Area_ac] = Watershed area in acres
[Outlet_X] = X-coordinate of outlet point
[Outlet_Y] = Y-coordinate of outlet point
[Outlet_Elv] = Elevation at outlet (meters)
[Parcel_Cnt] = Number of parcels in this watershed



QGIS Parcel Watershed Delineation - Troubleshooting


Issue: "Required library not installed: pysheds"

Cause: PySheds is not installed. 
Solution: Follow Step 1 again. Make sure you restarted QGIS after installation.


Issue: "Invalid input layers"

Cause: The selected files are not compatible for a watershed delineation
Solution: Check that your DEM is a proper grid raster layer. Check that your parcels are polygon features with no geometry errors.


Issue: Script takes a long time

Cause: Large DEMs (>500MB) can take several minutes
Solution: Reduce DEM extent to your area of interest first. Use Raster > Extraction > Clip Raster by Extent to crop the DEM


Issue: Areas are incorrect

Cause: DEM and parcels have custom CRS and cannot be converted properly
Solution: Check that your DEM and parcels have proper CRS defined. This tool automatically handles CRS conversion for most common CRSs. This tool outputs acres regardless of input CRS.


Issue: Watershed doesn't look right

Cause: DEM extent or resolution is too low.
Solution: Check DEM quality and resolution. Verify DEM covers the entire upstream drainage area. Increase boundary points for complex parcels.

