Watershed Analysis Issues

Anonymous User — Tue, 21 Dec 2010 01:32:25 GMT

Original User: Jvhowube

Dear GIS Community,

I am having some trouble with watershed analysis. I know I'm not the first one, but I've searched the internet/forums, and I can't seem to find any exact workthroughs for the issues I'm having.

The eventual goal of this analysis is to create a layer that shows the result of installing hydroelectric in a certain area. If this is not what watershed analysis is used for, please let me know. I'm going by a user-guide that I was given here in a developing country, so I'm working with very little. The before and after pictures show the kind of output that is desired - i.e. if you choke off a part of the watershed with a dam, what is the resulting reservoir? At least that's what it looks like to me. Again, I'm new to this watershed stuff so bear with me.

Basically, I understand the toolset, and how it's a step-by-step process to eventually delineate watersheds from a DEM. My trouble starts when I go to set a pour point (what the heck is a pour point anyway), and then use the Watershed tool. My understanding of a pour point is as follows: the point in a watershed or catchment where all the water flows out of said watershed or catchment. When I set this manually by just creating a point file, it's never the "right" point, and any watershed analysis thereafter results in a very small 20-30 raster-cell-sized area around my pour point.

I've tried to remedy this by using the Snap Pour Point tool, and searching the entire area for a "better" pour point - if this isn't what that tool does, then I'm using it wrong. I can't figure out what it does, if not that. Anyway, this usually results in an area slightly larger, but still quite small, and it fails to delineate any reasonable watershed-esque area.

Is my issue possibly with DEM resolution? I've been using Fill to correct the DEM from any Sinks (although I am using a Z of 100, I'm not sure this is correct). The Flow accumulation raster never actually looks like a river/stream network (i.e. the one that's shown in ArcHelp) - more like a bunch of stringy confetti thrown on the map - there's minimal connectivity, which makes me think there's a problem with the original DEM data.

The guide also has me using a Flow_Accumulation layer that has been reclassed in the watershed analysis (instead of the flow direction raster that the watershed tool asks for). This results in basically nothing. But maybe I do have to use an accumulation layer of some sort instead?

I've tried analyzing both a large area (1800 sqkm) (too large?), and then also a catchment section of that larger area. Both turn out the same broken watershed analysis.

This has served as a source of frustration for the past few weeks. If you can lend any help, it'd be much appreciated. Thank you very much.

Best,
Jordan

Before and After .jpgs show the guides depiction of what I should get from this analysis. I also attached flow_direction layer and flow_acc_with_watershed.

Re: Watershed Analysis Issues

Anonymous User — Wed, 22 Dec 2010 17:48:53 GMT

Original User: Lady_Jane

Hi Jordan,
First of all, have a look at a document called "Comprehensive Terrain Preprocessing Using Arc Hydro Tools" that can be downloaded at the following link: http://resources.arcgis.com/content/hydro-data-model

It seems a little complicated to read through at first, but after a couple of reads it starts to make a lot of sense.

You are right about what a pour point is.

What are the values of your flow direction grid? Do they go all the way up to 255? If they do, that means that your DEM still has sinks which could be why your watershed delineations are producing weird results.

I'm not sure what the pictures you attached are supposed to show. What do the polygons in the first two, and the red area on the other represent?

The document above should help, but feel free to post any other questions you might have.

Re: Watershed Analysis Issues

ZacharyMetz — Tue, 25 Jan 2011 12:04:23 GMT

I have a similar result to the initial post. The question then is how do we remove the sinks from the flow direction raster?

Re: Watershed Analysis Issues

Anonymous User — Wed, 26 Jan 2011 12:17:29 GMT

Original User: Lady_Jane

Look at section 4.2, in the document I attached; "Identifying sinks".
You don't remove sinks from the flow direction grid, you remove them from the DEM and then use the "corrected" DEM to create a new flow direction grid.

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