Slope criteria in SWAT

[Pan]

Hello Everyone,

in SWAT HRU defination part,as we need to define slope classes,single or multiple.how can we be sure that the slope classes are correct,for example we may have a mountainous area which may have 10,15,20% ...slope ,so in giving the slope thresholds do we have to find the slope of terrain in ArcGIS first or we can just simply input an approximate percentage for slope classes?

and finally does the slope class significantly affect the flow stream output in SWAT or just a little bit.

Thank you

[Natalja C.]

Hello,

To answer your questions:

-  how can we be sure that the slope classes are correct ?  - the slope classes will be based on GIS processing. You, as a modeller, have to make sure that your watershed and the land processes are represented satisfactory. I personally don't like the simplification and prefer to go for "all/full hru". 

-  do we have to find the slope of terrain in ArcGIS first or we can just simply input an approximate percentage for slope classes?  - It is wise to perform an analysis of your study area :) See which LU, soil, slope classes dominate, which are very small and can be neglected, etc. You can always "simply input an approximate percentage for anything", but how you justify this decision for yourself and for others?

-  does the slope class significantly affect the flow stream output in SWAT  - it does, but "significance" is TBD. :) I'm not fully aware of how arcswat incorporates slope class information into the swat setup files, but most parameters with "slope" are usually sensitive for my studies (HRU_SLP (or SLSUBBSN), SLSOIL, USLE_P (which is based on slope),  USLE_LS, etc.). Even if classes are used for only creating the HRUs, this will effect your final model: how many HRUs will you have, hence, the runtime of the model and the detailness.

Best,

Natalja

[Steven Jepsen]

Pan,

An approach that worked pretty well for me was to first define the number of slope classes, realizing that increasing the # of classes will increase the # of HRUs and therefore run times. I chose 3 classes. Generate the slopes in the your DEM using the ArcGIS raster tool that creates a slope raster. Then determine the slope values at the 33.3-percentile and 66.6-percentile of the slope distribution assuming you're going with 3 slope classes. You could use these slope thresholds for your ArcSWAT slope class settings. I've obtained the slope thresholds in the past by exporting to text file the slope raster masked to watershed, and then analyzing that text file. But there's probably a raster tool in ArcGIS that will do that for you.

Steve

[summera khan]

hello all

please help me in classifying the slopes

i have a mountainous watershed

i have calculated slopes in degrees and in percent rise (plz see attached fig)

but i m not clear how to assign upper limit to the slope class

thanking in anticipation