Force water to one cell

[Anonymous User]

I have a polygon and a 3-meter DEM.  I want to clip the DEM to this polygon, and ensure all water would flow to one cell (the lowest) in the basin.  Is there a method that would work best for this?  For the most part, the natural drainage of the basin goes to this cell, but how do I make the water along the clipped edges flow the right way?  Thanks.

[MarkBoucher]

You can draw a polygon along the known watershed. Give the polygon a field with a large number in it, say 1000. Call this the "wall". Then convert the wall polygon feature to a raster. Make sure the grid size and grid coordinates match the DEM during the conversion. Add the created raster to the DEM (walledDEM). This gives you and artificially high ridge that will force the flow direction. You will also need to create a "streams" polyline layer. Make a field with a very low value (-2000) and convert that polyline to a raster and add it to the walledDEM.** The stream feature must "cut through" the wall. After this, using the Spatial Analyst hydrology tools: fill sinks, flow direction, flow accumulation, etc...  (see http://forums.arcgis.com/threads/100210-Deliniating-Drainage-Areas?p=356888&viewfull=1#post356888).

This is the "manual" method for using "outerwalls" as Arc Hydro does in its tools.

I can't guarantee that the resulting watershed will fall exactly on the cells you think are the watershed boundary. Some of the grids may flow away from the watershed.

Clipping the DEM using "Spatial Analyst Tools.tbx\Extraction\Extract by Mask" is one way to simply eliminate the grid cells outside of the watershed. You still have to make the "wall" polygon layer, but you don't have to convert it to a raster, etc. and you most likely don't need a stream layer.

**Footnote: If you create the "wall" using a polyline instead, and don't  cross the outlet, you can likely avoid the need for the stream layer  and process following.