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I have two pour points in a stream network created using the flow accumulation methodology in ArcMap.

When I use the watershed tool with this pour point shapefile, I get a watershed between these two points. What exactly is this watershed?

I've tried searching on the ESRI site, but I can't seem to pinpoint the exact answer.

Edit: I don't want two catchments here. I want an area of everything that flows from the "upper pour point" to the "lower point." Picture included.

enter image description here

enter image description here

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    Change your extent to union of inputs and you'll get 2 catchments – FelixIP Feb 19 '16 at 1:35
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    perhaps you could add some images to demonstrate your result – Adam Feb 19 '16 at 2:00
  • @FelixIP I don't want two catchments here. I want an area of everything that flows from the "upper pour point" to the "lower point." – Samtheham Feb 19 '16 at 2:30
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    Using the D8 model for water flow, "everything that flows from the "upper pour point" to the "lower point" doesn't make sense, maybe you want the area that flows into the lower point, but not the higher point? – Devdatta Tengshe Feb 19 '16 at 11:45
  • @Devdatta Yeah, I don't know if the watershed tool is the right one for me to be using. Essentially what I'm trying to do is find an specified area that is "downstream" of the upper pour point. – Samtheham Feb 19 '16 at 18:12
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Watershed doesn't really work that well with only two points. As its name implies, it is supposed to delineate watersheds. To answer your question, the watershed you created is a subbasin. That is, it is the contributing area above the bottom spill point and below the top spill point. The tool is designed to have enough spill points as input so that it can merge together subbasins into watersheds based on a tolerance that you give it.

That all said, if all you want to see is that small area, maybe that is all you need. Assuming the initial raster that you created the spill points from covered a big enough area to provide accurate enough spill points for your purposes, and you included the value field (since you are using features for spill points), what you did should have given you what you wanted, i.e. a map of what cells flow to the bottom spill point from the top spill point.

Just keep in mind that if you didn't include a very large area while determining your spill points, that the small area you used may not be correct. Things could happen in nearby areas off your raster that would affect the accumulation pattern.

The help file has a good picture. http://pro.arcgis.com/en/pro-app/tool-reference/spatial-analyst/how-watershed-works.htm

  • Thank you very much for your answer. I am actually interested in finding subbasins as you described for a set of 200 pairs of pour points. The image that I posted is ideal for me. Unfortunately, for some of the other pairs, I cannot get subbasins that "touch" both points. As in, it only extends from one point and does not reach the other. Is there any way I could change this? Thanks! – Samtheham Feb 19 '16 at 18:19
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    What are your pour points based on? What I mean is, most people place them at the base of dams or where they have monitoring equipment. What do your pour points represent? I'm wondering if they could possibly be located in different watersheds? You might want to post a picture of some areas that you can't get to join into watersheds. – RHB Feb 19 '16 at 18:27
  • I'll post a picture in about an hour when I get back to the lab. I actually have one pour point at a dam location, but I'm interested in finding the dams "command area" downstream. I know how big it should be, the problem is finding the appropriate location and shape. I'm approaching this by placing another pour point further downstream and using the watershed feature to find the subbasins as you described. – Samtheham Feb 19 '16 at 18:30
  • Is there a reason why you are not using the NHD watersheds? If nothing else, you could use them to get you started, even if you need to create original data. – RHB Feb 19 '16 at 18:39
  • My problem is that I need the subbasin to begin exactly at the dam point, which the watersheds to not at my knowledge. I'm studying economic effects of dam construction, and I need a clear delineation between what flows into a dam and what "flows out" or is downstream in the command area. – Samtheham Feb 19 '16 at 18:49

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