How to Create a Catchment Area of a Polygon (Wetland) using DEM in QGIS or Whitebox GAT

Question

I am looking for a way to calculate the upstream catchment area where water would flow into a given wetland.

I have delineated the boundary of a wetland (creating a polygon) and would like to determine the upslope catchment area and downslope area of the wetland polygon. My wetland polygon is not on a stream network.

In QGIS, I used the SAGA Rasterize Tool to rasterize my polygon - making sure that my new rasterized wetland has the same projection, resolution, and extent as my DEM. Then I proceeded to use the SAGA Upslope Area tool using my rasterized wetland as the "target area," and my DEM for the Elevation. However, I keep getting the error "Input layers do not have the same grid extent."

As an alternative, I tried using Whitebox GAT using the process described in here: How to create a catchment from a point using a DTM raster in QGIS?

However, this process is for a point vector, whereas, I am looking for something that can calculate the catchment for a specific polygon or area.

I'm fairly new to using QGIS and very new to using Whitebox. I have some experience with ArcGIS as well, but don't have access to a licence.

Answer 1

If you have a vector polygon representing a wetland, then in Whitebox GAT, you just need to convert that polygon to a raster using the Vector Polygons to Raster tool. Use the DEM as the 'Base Raster File' to ensure that output polygon raster has the same dimensions as the DEM. Then you simply need to input this image as the 'Input Pour Point' file to the Watershed tool. So long as each wetland polygon is assigned a unique ID, each associated watershed will be delineated in the output watershed raster.