How SWAT understand the "subbasin"?

[Eduardo Jauch]

Hi to all,

I think that this is the first time that I send a message to the group.
I apologize if my questions are "stupid", but I start to study this model only recently and have a few questions to ask you...

The first question that assult me is how the SWAT model see the "subbasin".

I fon't think that SWAT calculates the "path" of water on the subbasin ultil it reach the channel, but instead, uses some type of "simplification", like transform the subbasin in a single "hillslope", based on the mean declivity of the subbasin, for example.

Is it true? How the SWAT "see" the subbasin for the different calculations, like runoff, infiltration, lateral flow, ...?

Thanks for any help :)

[Jim Almendinger]

The SWAT theory manual does a good job of explaining most of the basic concepts and many of the details -- you have a lot of reading ahead of you.  

But briefly, SWAT uses the HRU concept.  In each subbasin, SWAT lumps all ares of the same land use, soil type, and slope into a single aggregate unit called a "hydrologic response unit" (HRU).  I envision an HRU as a very large field, bordered on its downstream edge by the stream channel.  SWAT does all its calculations of infiltration, runoff, sediment transport, nutrient transport, plant growth, etc., for each HRU.  A subbasin may have any number of HRUs -- it depends on how many unique combinations of land use, soil, and slope there are in the subbasin, and how much detail you allow SWAT to see (you may tell SWAT to ignore combinations that are very small, for example).  

     So -- if your subbasin has 5 HRUs, I would envision it as 5 large fields, each sloping uniformly down to a stream channel, which receives runoff, sediment, and nutrients from each HRU.  These components are then fully mixed in the channel and passed to the next downstream subbasin.  

The HRU concept allows SWAT to use fairly mechanistic equations to calculate runoff and transport of sediment and nutrients.  But of course the real world does not (usually) consist of uniformly sloping fields leading directly to stream channels.  So models should be calibrated to real monitoring data to "scale" (or "fudge" as some would say) the model output to match the measured data, to try and account for all factors not explicitly modeled.  

I tend to think that understanding the HRU concept is key to understanding how to interpret results from SWAT -- and should serve as a constant reminder to not over-interpret results.  

Cheers,

-- 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/

[Eduardo Jauch]

Jim,

This is the exact information that I was in need to start to understand how SWAT works.
I've already read about the HRU, but couldn't (until now), understand how the model work with it.

Thank's for explain this to me :)

Eduardo.

2009/2/13 Jim Almendinger <din...@smm.org>