iNFRAWORKS oversizes drainage areas

[MKWeb2]

I have an area in Iowa that is fairly flat (most of central Iowa is that way). and when I try to create a watershed for a small site the watershed that gets returned is massive.

Even though in the topo you can see ridge lines and channels between the limits of the watershed and the outlet point I am trying to generate the watershed for.

[Matt.Anderson]

The tool was not intended for small sites.  It uses Raster data to locate streams, and the general area that would likely be tributary to that.  

[MKWeb2]

Thank you for the reply @Matt,

Even so, it should be able to determine the ridge lines and low spots. This site has ten feet of fall from the ridge to the west and the point I placed the outlet which has me concerned about the validity of the model that gets generated.

[fcernst]

According to the Help the Watershed threshold should be able to go down to 10x10x50 = 5,000 m2 = 6.17 acres

It's interesting the current minimum Stream Threshold has now been raised up to 50, instead of the 20 shown in the Help.

Another odd thing, is that even though I have the model units set to Imperial, the Watershed settings stay in Metric.

[Matt.Anderson]

Nice catch Fred.  Bug. 

Word of warning - if you are tightening the grid space to a level below the imported data - the results will likely be bogus.  It's something not preventable.

As for the metric - the grid exports to a UTM coordinate system (all metric) and the calculations perform in metric.