I am trying to delineate just the catchment area of some lakes, and am wondering the correct way to do this. I already have a filled DEM, along with a flow direction and a flow accumulation raster. Is there a way to accomplish this task without the use of pour points? Or do I have to use them. If that is the case, can a pour point be located in the middle of a lake (assuming that the middle of a lake is the highest area of flow accumulation). Whenever I do try to run this data, whether with or without a pour point, I get something that does not look completely right. The catchment area does not encompass the entire lake, and the actual watershed itself does not accurately represent the flow accumulation arrows. Am I doing something wrong, or is there a simple cause to my problem. Maybe if someone could take a quick second to step through the watershed toolset, that would be helpful.
Am using ArcGIS 10 SP1, with Spatial Analyst, ArcView license.
[GIS] Watershed Delineation
arcgis-10.0demgeoprocessingraster
Related Solutions
In a conversation following the question, we refined the objective. It is to obtain
the watershed area of each point. Some of these points are close to each other, but they are all unique and we would like to determine the watershed area for each point. We will use the watershed area in a predictive model to distinguish a score for each point.
The proposed solution, which worked in this case (and was far faster than the batch watershed delineation), is to run a FlowAccumulation calculation using a constant grid as input. (Set its values to the area of a single cell). Then simply read off its values at each of the 200,000 points.
To understand this, note that the total area draining to a point is (obviously) equivalent to "total number of cells draining to the point" times "area of a cell." The former is precisely what FlowAccumulation calculates when it is given a unit grid as input.
An important insight here is that sometimes (frequently, in my experience) we can accomplish a compute-intensive task more efficiently by focusing on our ultimate objective, rather than on each specific technical step in a workflow, and attempting to identify an accurate and fast algorithm to reach that objective. Adding more processors, getting speedier disks, etc., can speed up a long operation, but only by a limited amount. Changing the algorithm altogether can improve it by orders of magnitude.
To do this I think you need to use:
- Snap Pour Point
- Watershed
- Feature Set parameter (to let you indicate a point on your map) for a model or script that runs the above
UPDATE
I do not have a Spatial Analyst license to test the Snap Pour Point and Watershed parts, but I just tested the Feature Set parameter to ask a user to input a point using the cursor on a map, and then buffer it. This demonstrates the remaining bit that seems to be causing you problems.
The Python script (test.py) is:
import arcpy
geom = arcpy.GetParameter(0)
arcpy.Buffer_analysis(geom,"C:/Support/Prasad/test.gdb/testPntBuffer","1 DecimalDegrees","FULL","ROUND","NONE","#")
I then created a point feature class with the correct coordinate system to act as the schema for the point I want to Buffer (or in your case to Snap Pour Point then Watershed).
When I used Add Script to create a tool from test.py I just gave it one parameter:
With that done I run the tool, which gives me the chance to click on the map to create a point geometry that gets passed to the Python script to Buffer it.
Best Answer
The way that ArcGIS computes watersheds probably doesn't match with your intuition: only areas upland of a specific pourpoint will drain to that spot, so if you locate the pour point at the center of the lake, 'lower' elevation spots drain into other basins. As Dan mentioned, instead you want to locate it at the top of the outflow stream, where the water from the lake heads to rivers below.
Another option to produce this effect is to continue locating the pour point in the center of the lake, but build an artificial 'dam' at the bottom of the watershed by 'burning': create a new line feature, convert it to a raster with a large value (say, 300). Now the DEM should be forced to empty at your desired pour, with the only artifacts being introduced by the manually produced dam itself.
If you're looking for something more sophisticated, you may want to look into the ArcHydro tools (beta version for ArcGIS 10), which provide a more detailed set of tools for working with hydrologic features -- see this tutorial for details.