Ready To Use Watershed tool versus Spatial Analyst Watershed tool?

[ShelbyMyers]

Hello, I'm trying to dilineate watersheds as accurately as possible for a compost facility. There are two basins on site which I've chosen as my pour points for the watershed analysis. I orginally went the route of using the spatial analyst watershed tool after getting a DEM, filling it, getting flow then accumulation, and then running the tool with the pour points. I changed the distance for my pour points to see if anything would change, but the outputs wound up the same, as can be seen in the first image. It essentially just shows the area of the basins, which I guess could be seen as a good thing, as we want the drainage for this site to be contained. However, when I use the ready to use watershed tool, there is quite a big difference in output. Which should I trust more? Why are these outputs so different? I'm essentially trying to see what area is draining into these basins, and if drainage is contained to this site. Thank you for any input!

Spatial Analyst Watershed with Pour Points

Ready to Use Watershed with Pour Points

Read to Use Watershed with Flow Accumulation Shown

[TaniaLopezCantu]

Hi Shelby,

I’ll address each of your questions separately, and then finish with one question that will help us determine the most suitable solution for your situation.

Why are these outputs (Spatial Analyst Watershed tool and Ready to Use Watershed tool) so different?

The Spatial Analyst Watershed tool has two required inputs:

• a flow direction raster
• a pour points feature layer or raster.

The flow direction raster is typically generated using the Flow Direction tool, which is derived from a Digital Elevation Model (DEM), either raw or hydroconditioned using the Fill tool or other techniques. DEMs have varying resolutions (e.g., 1 meter or 30 meters), which can significantly affect results.

The Ready to Use Watershed tool has only one required input:

• a pour points feature layer.

However, it also uses pre-calculated flow direction and flow accumulation rasters, which it retrieves for the analysis area. These rasters are created using a similar process to the Spatial Analyst workflow: starting with a DEM, filling it, and then running flow direction and accumulation analysis. 

The DEM resolution for the Ready-to-Use tool varies by location. In the U.S., it is typically 30 meters, but this can differ depending on the study area. DEM resolution plays a substantial role in hydrologic analysis results. Additionally, the Ready-to-Use Watershed tool automatically snaps the pour point to a location with high flow accumulation to ensure the delineated watershed is realistic.

The differences between the two tools are most likely the reason why you obtain different results from the tools.

Which should I trust more?

If the inputs are identical, meaning same DEM resolution, same filling method, and pour points snapped to high flow accumulation locations, the two tools should produce the same output. In your case, most likely the differences in one or more of these inputs are causing the variation in results.

Next Steps 

To recommend the best approach, I’d like to better understand your data and your expected outcome. Could you please share:

1. The resolution of your DEM
2. Details about the two basins associated with the compost facility you’re trying to identify

Looking forward to your reply. 

Thank you,