User defined watershed delineation

[Jamil]

Dear ArcSWAT users,

I'm defining the subbasin in my watershed and have  important questions about it: In the manual it is said that there should be only one stream feature per subbasin.

1. Is it possible to have tributary channels of the main channel in the watershed?

2. Is it possible to have situations where two main channel enter the same subbasin, merge and exit as one stream?

3. If there's only one main channel with no tributary channels per subbasin, how to input the rest of the hydrographic network of the watershed? I guess the "Burn in" operation in the DEM setup is used only when a DEM-based stream definition is chosen?

Finally, I know Srini posted a document regarding user defined watersheds but couldn't find it, if anyone has it it would be very helpful as well!

I would be very grateful is someone could help!

Jamil

[Jim Almendinger]

Technically yes, you can lump subbasins at a confluence by manually deleting the twin outlets created by ArcSWAT at the confluence during stream channel delineation.  Instead of 3 subbasins around the confluence (1 downstream and 2 upstream), you would end up with a single large subbasin that encompasses the confluence.  

The only time I do this routinely is when I have several streams entering a large lake or reservoir -- these streams conceptually link inside the lake, but their watersheds actually terminate at the lake boundary.  In this case, I manually add outlets where the streams enter the lake, and delete the outlets inside the lake.  ArcSWAT then delineates distinct subbasins for each tributary, and the remaining area around the lake becomes the directly contributing area to the lake (technnically -- to that lake's subbasin's "reach" which then conceptually enters the lake).  

As for burning in stream networks -- I've found this to be very useful to improve the accuracy of the stream network, especially if I'm using 30-m DEMs.  A good, dense network that is burned in will greatly improve subbasin delineation, even with a coarse DEM.  This is a pretty easy way to get  a good subbasin delineation.  I haven't tried using an imported user-defined set of subbasin boundaries -- but it seems like it could be difficult and time consuming.  Others who have actually done this may have a recommendation here (and a method they could share).  

-- Jim

Dr. James E. Almendinger, Senior Scientist
St. Croix Watershed Research Station
Science Museum of Minnesota
16910  152nd St. N
Marine on St. Croix, MN  55047
tel: 651-433-5953 X 19
fax: 651-433-5924
email: din...@smm.org
web: www.smm.org/SCWRS/

[Jamil]

Thanks for your helpfull answer! I did the user-based watershed delineation and it seems to work well although I have not run SWAT yet. It did not take too long as I'm working in a very small catchment. 

I have another question for the people who have already used SWAT for a catchment of less than 100km²: are there recommendations or specific rules to follow when using SWAT in such a small catchment? The size and numbers of the subbasins or HRU?

Thanks for helping,

Jamil