Calculating watershed associated with polygon using DEM and QGIS

Question

I have found instructions that outline how to determine the watershed uphill of a selected point using a DEM in QGIS (https://youtu.be/CE7zZCkFbg4).

I am starting with a polygon representing property boundaries that lie somewhere in the midst of a watershed however - and I need to determine what the area of the entire watershed that drains water into and receives water runoff from that polygon is.

Is there a function in QGIS that will do this using a DEM to calculate the entire watershed associated with a selected polygon?

If not, then is there a function that will determine the highest single point downhill of a property boundary where all of the stormwater runoff that is generated from the property will converge - a "reverse" watershed analysis?

If so, I could then use that point and the method outlined in the youtube video linked above to get the result I'm looking for.

Answer 1

Your workflow will probably be something like this:

1. Create a layer of streams and basins using r.watershed and your dem layer. Choose a threshold size that is a bit smaller than the property.
2. Find all points where streams intersect with the property boundary. THese are the drainage entry points.
3. Use the r.water.outlet module (probably what you saw on youtube) to get the upstream basin size for each of the drainage entry points, and add them together to find total upstream contributing area.
4. Find the lowest point on the property boundary - the drainage outlet. This point should intersect with the streams layer.
5. Run r.water.outlet using this point.
6. That result will be the total drainage area at the property outlet.